Download Inside the atom - Oxford University Press

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Inside the atom
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All matter is made of atoms. Everything we see around us is made of atoms. When we feel
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something, we are touching atoms. When we smell something, atoms have entered our nose and
made contact with particular cells that absorb some of the atoms and then send messages to
our brain. We ourselves are made from atoms. Atoms are continuously recycled. When we take
a breath of air we are breathing in at least some of the same atoms that were breathed in and
out by people who lived on our planet thousands of years ago. So how does it make you feel to be
breathing in some oxygen atoms that were once breathed in and out by Beethoven or Einstein, or
by a dinosaur?
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Development of the atomic model
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Describing the structure and properties of atoms is extremely difficult because we do
not have the technology to see them, and yet atoms were first described thousands of
years ago. Our understanding of what atoms are and their structure are described using
theories and models. These models have changed considerably over time as scientists
have discovered more evidence.
Students:
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»» identify that all matter is made up of atoms
»» outline the developments to the atomic theories and models as a
process of refinement and review of the scientific community
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Subatomic particles
1.2
Students:
»» describe the structure of atoms in terms of protons, neutrons and electrons
»» use models to describe the arrangement of subatomic particles in
common elements (additional)
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As our understanding of the structure of atoms improved, the existence of subatomic particles
was discovered. Subatomic particles, their numbers, proportions and arrangements define
the atom in terms of the element, its mass, the overall charge and its properties.
Radioactivity
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Not all atoms are stable. Unstable atoms will decay to form different, but more stable atoms.
This decay results in the emission of energy and particles. These emissions are called
radiation.
Students:
»» identify that radioactivity arises from the decay of nuclei in the form
of particles and energy
»» evaluate the benefits and limitations of the medical and industrial
use of nuclear energy

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Experimental evidence is a key driver of science. As Richard Feynman
(American Nobel Prize winner) once said, ‘If it disagrees with experiment
it must be wrong’. But is there evidence that atoms actually exist? They
are too small for us to see in detail, even with modern technology. So
just imagine the problems that earlier scientists encountered trying to
piece together ideas that explained the many and varied properties of
substances. The discovery of atoms has been a journey involving many
different scientists working in different ways, all gathering evidence and
seeking answers to these questions – not just about whether atoms exist,
but what atoms actually are.
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1.1
Development of the atomic
model
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The Atomic Theory of Matter
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The idea of the atom has been around for
thousands of years. But the explanations
of exactly what atoms are, what they
themselves are made of and how they behave
have changed over time as new scientific
discoveries have been made. The technology
that enables us to actually see individual
atoms is only just being developed, but we
still cannot see the particles within them,
and so we use models to describe their
structure and behaviour. It is these models
that have been modified, and contribute to
the Atomic Theory of Matter.
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Democritus and ‘atomos’
Democritus, an Ancient Greek philosopher,
was the first person recorded to have
considered the presence of atoms. He
suggested that if you took any substance
and continued to cut it in half, you would
eventually get to a particle that was too
small to be divided any further. He called
these indivisible particles atomos – atoms.
He also suggested that all matter, even
invisible gases, is made up of atoms.
Dalton’s atomic theory
In the early 19th century, over a thousand
years later, English chemist John Dalton
built on Democritus’s idea of indivisible
particles. He also suggested that different
substances were made up of different
particles that had specific masses and
properties – elements. In other words, the
particles that made up gold were different
to the particles that made up water. He
used the term ‘atom’ to describe these
tiny particles. Dalton also suggested that
these different atoms could combine in
regular ratios to make new substances –
compounds.
Dalton did not have overwhelming
evidence for his hypothesis at this time.
Not all scientists agreed with him and a
long time passed before his ideas became
recognised as an important theory.
Imagine for a moment that you had
never heard of atoms, chemical symbols or
formulas. Nobody had ever called oxygen O2,
water H2O or carbon dioxide CO2. This was
the situation in the world of the chemical
sciences in the early 19th century.
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It was also known that another
compound containing carbon and oxygen
existed. This compound always contained
1.33 times as much oxygen as carbon when
the masses of the elements were compared.
This compound is now known as carbon
monoxide (CO).
Evidence such as this led Dalton to
propose the law of simple multiple
proportions. It means that when elements
combine, they combine in simple ratios, like
2:1 as in water (H2O), 1:4 as in methane
(CH4) or 2:3 as in aluminium oxide (Al2O3).
This might seem obvious to us now, but we
only know this due to Dalton’s pioneering
atomic theory. Dalton’s theory gave scientists
a way to explain the evidence about atoms.
Figure 1.1 Antoine
Lavoisier.
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Antoine Lavoisier, a French chemist,
made accurate measurements of the
composition of chemical compounds in
the 1780s, about 30 years before Dalton’s
theory. From this work, it was found that
compounds containing more than one
element always had the same relative
amounts of each element. For example, the
compound now known as carbon dioxide
used to be called ‘fixed air’. This was because
it was heavier than air and did not allow
other substances to burn in it. It was
discovered that the mass of oxygen in fixed
air was always 2.66 times the mass of carbon
in the compound. This is an example of
what is now called the law of constant
composition.
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Activity 1.1.1: The case for atoms
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Imagine you are acting as a lawyer in a courtroom trial and your task is to convince the
jury that atoms are real. You have the following pieces of evidence:
• Element can join together to form compounds.
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Figure 1.2 An oxygen
molecule (right) is formed
when one oxygen atom
(left) joins with another.
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• Water always seems to contain twice as much hydrogen and oxygen.
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• When chemicals react with each other, the mass of the chemicals overall does not
change.
• Pure oxygen has the same properties wherever it is found on the Earth, and even in
space.
• Gases (some of which are invisible) have mass, and different gases have different
masses.
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• Under the microscope, tiny particles of pollen in water move in strange ways as if
bumping into invisible objects.
1 Do you think any of these pieces of evidence would be useful in persuading the
jury? If so, why?
Figure 1.3 A water
molecule is made up of
one oxygen atom and two
hydrogen atoms, always
in the ratio 2:1.
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Figure 1.4 A carbon
dioxide molecule is made
up of one carbon atom
and two oxygen atoms,
always in the ratio 1:2.
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A scientific theory is written to explain existing evidence and observations. A good theory
supported by a range of evidence can be used to make testable predictions. Ever since Dalton
first proposed his atomic theory it was used to make predictions, and evidence that was not
even available in Dalton’s time still supports the theory. However, theories can be amended
as new and more accurate information is discovered, just like models can be amended.
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2 Choose three of the above points and explain why you think they could be used
as evidence to support the existence of atoms.
Scientific theory
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Figure 1.5 Carbon
monoxide is a compound
made up of one carbon
atom and one oxygen
atom, always in the
ratio 1:1.
H
H
H
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Figure 1.6 Methane is a
compound made up of
one carbon atom and four
hydrogen atoms, always
in the ratio 1:4.
1.1 Development of the atomic model 5
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filed past his coffin prior to his burial.
Dalton’s last act was to allow his eyes
to be used for scientific research after
his death. Ironically, his theory about the
cause of his own colour blindness was
found to be incorrect. DNA tests carried
out on his preserved eyes 150 years after
his death showed that his colour blindness
was caused by a genetic disorder.
Summary of Dalton’s atomic theory of
matter
• Elements are made up of atoms, which
are extremely small particles.
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John Dalton was born in 1766 in
Cumberland, England. His older brother
Jonathon ran a Quaker school in Kendall,
near the English Lake District. Because
Dalton was part of the Quaker tradition
(Quakers did not follow the teachings of
the ‘establishment’ Church of England),
he was not able to attend or teach at an
English university. Instead, when he was
27, he taught mathematics and natural
philosophy at a college in Manchester. His
earlier scientific work was not particularly
successful. His main scientific focus was
meteorology, and he also produced work
on colour blindness, inspired no doubt by
the fact that he was colour-blind.
Dalton’s first chemistry-related work,
when he was 35, was the study of the
behaviour of gases in the atmosphere.
This work led him to consider the idea of
atoms, and it is this theory for which he is
now most remembered. The work of many
other scientists has been built on John
Dalton’s atomic theory. When he died at
the age of 77, after a series of strokes, he
was extremely famous and 40 000 people
• Atoms of each element are different
from those of other elements, including
their masses.
• Atoms of a given element are identical
to each other.
• Chemical compounds are formed when
atoms of one element combine with
atoms of other elements in the same
fixed proportions.
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Figure 1.7 John Dalton
John Dalton
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Deeper
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• Atoms cannot be created or destroyed
in a chemical reaction.
Questions 1.1.1: The atomic theory of matter
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1 Identify the philosopher who first described the concept of atoms.
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2 Suggest a reason why Dalton’s ideas about atoms were not initially accepted.
3 Define the law of simple multiple proportions.
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4 List the formulas of:
a carbon dioxide
b carbon monoxide
c methane
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5 The formula of water is H2O. Describe what the ‘2’ in the formula tells you.
6 Explain the difference between an element and a compound, and give an example
of each.
7 Explain the difference between an atom and a molecule, and give an example
of each.
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Discovering more about atoms
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Figure 1.8 Thomson’s
Through even more experiments,
cathode ray tubes. The
Thomson also showed that the atom
electron gun is on the left
contained positively charged material,
hand side of the diagram,
and the parallel plates
although it was not yet clear what this
produce the electric field.
material was. From this evidence, and
As the electric field is
turned on, the electron
knowing that oppositely charged objects
stream (blue line) is
attract each other and move towards each
deflected upwards.
other, Thomson suggested that the atom
was like a plum pudding, in which the
positively charged material is the ‘pudding’
and the electrons are the fruit. This was
called the Thomson plum pudding
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model of the atom.
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In 1911, Thomson’s former
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student, Ernest Rutherford,
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performed an experiment to
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test the plum pudding theory.
The results caused Rutherford
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to conclude that the atom is
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actually mostly empty space. These
observations led to the model of the
Figure 1.9 Thomson’s
atom known as the Rutherford nuclear
plum pudding model of
model. You will find out more about this
the atom.
model in the next section.
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In the early 20th century, a century after
Dalton proposed his theory, physicist
Joseph John Thomson (known as ‘JJ’ by
his colleagues) discovered that atoms were
actually divisible and made up of even
smaller particles.
Thomson experimented with cathode
ray tubes – a glass tube with electrical wires
in either end and the air within the tube
removed to form a vacuum. Passing an
electric current through the tube created a
fluorescent glow. It was a very old-fashioned
version of a florescent tube light globe.
Cathode rays containing electrons could
be fired out of the end of the tube, so it
also became known as an electron gun.
These cathode rays could be focused to
form a glowing dot on fluorescent material.
However, the electrons could not bend
around or pass through solid material and
so Thomson concluded that electrons must
be tiny particles.
Thomson fired the electron gun near an
electromagnetic field, and discovered that
the rays of electrons were bent away from
the magnetic field. From this information,
Thomson concluded that electrons must be
negatively charged.
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The Thomson plum
pudding model
Through a series of further experiments,
Thomson discovered the negatively
charged particles, electrons, were too
small to calculate their individual mass.
But, based on how much the rays bent in
electromagnetic fields of varying strengths,
he deduced that electrons must be about
a thousand times smaller than a hydrogen
ion, and must be coming from the atoms
within the cathode ray tube itself.
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Dalton only had access to a small amount
of evidence compared to the information
we have about atoms today. As scientific
methods improved and technology
advanced, more and more was discovered
about the atom. This work further developed
our understanding of the atom, and helped
provide more evidence for the existence of
atoms as proposed by Dalton.
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Activity 1.1.2: Brownian motion using diluted milk
Predict what you might expect to observe during an experiment to observe Brownian
motion. Use your prediction to formulate a hypothesis to test for the experiment you
are about to conduct.
Read the full method and consider any possible risks, and determine how you can
minimise or eliminate them.
What you need: microscope with objective of at least 20 × and eyepiece 10 ×,
microscope slides and cover slips, full-cream milk, needle or fine wire, distilled water,
petroleum jelly
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1 Place a small drop of distilled water in the centre of a microscope slide. (Note: This
drop must be very small so that no water escapes when the cover slip is added.)
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3 Using the needle, carefully stir the milk into the water drop.
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2 Dip the needle in the milk, and then quickly dip the tip of the needle with the milk
into the drop of water.
4 Using the needle again, carefully line the cover slip edges with the petroleum jelly.
5 Gently lower the cover slip onto the drop of water containing the milk.
6 Place the microscope slide under the microscope and bring it into focus. You
should be able to see the tiny oil droplets in the milk.
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7 Wait for the sideways movement of the oil droplets to stop and look for the ‘jiggling’
motion of the droplets. If you observe this, you are seeing the direct action of water
molecules on these oil droplets.
• Did your observations support your hypothesis?
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• Identify some ways of improving your experimental technique that might make
your observations of Brownian motion easier.
• What do you think causes this random motion?
Questions 1.1.2: Discovering more about atoms
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1 Outline why Dalton’s atomic theory was so highly regarded.
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2 Describe Thomson’s plum pudding model of the atom.
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3 Describe where else you have experienced opposite charges attracting.
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4 John Dalton said the atom is the smallest particle that an element is made from.
He said atoms could not be divided into smaller parts. This theory fitted the
evidence available at the time but new evidence came to hand after this. Using your
previous knowledge, outline what we now know about the atom.
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1.1
Development of the
atomic model
Critical and creative thinking
1 Identify the ratio of carbon to hydrogen
in the compound methane. [1 mark]
11A Year 5 primary school class were
learning about solids, liquids and
gases. Their teacher told them that
everything around them was made
of particles they could not see. One
student responded that this was silly,
because if you can’t see it, it can’t be
there. Write a paragraph to the student
to persuade her that the teacher was
correct. [2 marks]
5 Describe why it would be impossible to
have a chemical formula with a fraction
in it. [2 marks]
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6 Explain what causes the motion of tiny
droplets of oil in diluted milk. [2 marks]
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Making connections
12Use your understanding of atoms and
elements to suggest reasons for the
following:
a carbon dioxide is a heavier gas than
oxygen [1 mark]
b hydrogen and oxygen can be
produced from water [1 mark]
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7 Outline why the molecules of water are
impossible to see, even with powerful
microscopes. [2 marks]
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4 Recall what the ‘2’ in the formula CO2
represents. [1 mark]
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3 Recall the formula for the compound
carbon monoxide. [1 mark]
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2 Identify which chemical law describes
how the composition of compounds is
always the same. [2 marks]
Checkpoint
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Remember and understand
Analyse and evaluate
8 Explain the difference between a theory
and an observation. [2 marks]
d at room temperature, hydrogen is a
gas but water is a liquid [2 marks]
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9 Describe why it was important for
chemists such as Lavoisier to be able
to take accurate measurements of
the mass of elements within chemical
compounds. [2 marks]
c when methane is burned in oxygen,
the gases carbon dioxide and water
vapour are produced [2 marks]
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10Describe the differences between a
scientist and a natural philosopher.
[2 marks]
TOTAL MARKS
[ /25]
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As technology develops, scientists can explore deeper and deeper into
areas once unknown. By experimenting on smaller scales with increased
accuracy, it has been possible to discover information about the inside
of atoms – the subatomic world. Here we encounter protons, neutrons
and electrons, and realise that atoms are tiny systems made up of these
subatomic particles, all interacting with each other. These interactions
are predictable, based on knowledge about the mass and electrical charge
of protons, neutrons and electrons. The properties of atoms are entirely
dependent on subatomic particles, including their number and how they
are arranged. Although we cannot observe the atoms themselves, the
properties of these atoms determine the behaviour of the elements that
we encounter every day.
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1.2
Subatomic particles
deflected (made to change course) by the
gold atoms in the thin sheet of gold foil.
Two aspects of the results surprised
the scientists. The first evidence was that
most of the alpha particles passed straight
through the gold foil with hardly any
deflection at all. Somehow they seemed to
have passed through the gold, and therefore
through the atoms that made up the
gold, without ‘touching’ anything. More
amazing was the second piece of evidence:
some alpha particles travelling with a high
amount of energy bounced straight back in
the direction they had come from.
From this evidence, Rutherford
concluded that the gold atoms must
contain a lot of space, but some areas of the
atom contained a relatively large amount
of positive charge to repel the positively
charged alpha particles so strongly.
Figures 1.11 and 1.12 show how the
evidence from the gold foil experiment
helped people change from using the plum
pudding model of the atom to using an
alternative model. This new model had the
positive nucleus of the atom surrounded
by a relatively large area of space containing
the negatively charged electrons. It was a
large shift in thinking about the structure of
atoms.
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Figure 1.10 Ernest
Rutherford in one of his
laboratories.
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Models are used in science to help
understand, explain and predict. Modelling
is an essential part of science. They can be
used in a huge range of situations such as
modelling the structure of the Universe,
modelling the flow of energy within our own
bodies and modelling systems too small for
the eye to see, as is the case for atoms.
What scientific models have you seen,
or used yourself, to help explain things that
happen around us?
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Looking inside the atom
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Rutherford’s
experiments on atoms
Ernest Rutherford was born in New Zealand
in 1871. His experiments changed the
way people thought about the inside of
the atom. At the age of 37, he supervised
Hans Geiger and Ernest Marsden, who
carried out what is known as the ‘gold foil’
experiment. They set up a very thin layer of
gold and fired a stream of alpha particles
at it. An alpha particle is made up of 2
protons and 2 neutrons, and has a positive
charge. Detectors to record the alpha
particles were set up around the gold foil
to identify whether the alpha particles had
gone straight through the foil or had been
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Figure 1.11 If the ‘plum
pudding’ model of the
atom is correct, it would
be expected that most
of the high-energy alpha
particles would move
through the gold with only
minimal or no deflection.
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Figure 1.12 The gold
foil experiment showed
that high-energy alpha
particles were deflected.
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Activity 1.2.1: Modelling Rutherford’s experiment
Rutherford’s gold foil experiment can be easily modelled.
What you need: A ping pong ball, a hula hoop, string, sticky tape, straws, grains
of rice
It is best to complete this experiment outdoors.
1 Attach the string to the ping pong ball using sticky tape. Tie the other end of the
string to the hula hoop so that when help upright, the ball is hanging in the centre
of the hoop.
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2 Get one person to hold the hula hoop vertically upright. Other students to stand
approximately 1 metre away from the hula hoop.
3 Take some grains of rice and blow them through the straw at the hula hoop.
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4 Record your observations about when:
a the rice grains didn’t hit anything
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b the rice grains hit the ping pong ball
c How often did the rice grains hit the ping pong balls? Try to express this as
a percentage.
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d You have just modelled Rutherford’s gold foil experiment. What did each
component of your model represent?
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The new model of the
atom
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e How successful was this model in representing Rutherford’s experiment? What
were some inaccuracies in your model?
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Ernest Rutherford changed the way we
think about atoms. People had to get used
to the idea that atoms were mainly empty
space. How difficult would that have been,
considering atoms make up substances
such as solid steel? It certainly does not
feel like space when you touch a steel bar.
Rutherford’s model of the atom described
the structure of an atom as containing three
different subatomic (smaller than an atom)
particles.
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Rutherford’s model of
the atom
Imagine trying to persuade someone that
what he or she thought was solid, like a
plum pudding, was actually mainly space.
You would need some pretty good evidence.
Rutherford’s model was supported by the
evidence, and was backed up with further
studies of the structure of the atom.
Rutherford’s model of the atom proposed
the following:
Figure 1.13 If you expanded one atom to the size of
the Sydney Cricket Ground, the nucleus of that atom
would still be no bigger than a pinhead.
• Atoms are made up of two different
subatomic particles.
• The nucleus of an atom is made up of
protons.
• Protons carry a positive electric charge.
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The nucleus
containing positively
charged protons
Orbiting, negatively
charged electrons
• Electrons have a negative electric charge.
It is important to remember that
particles with opposite charges (one positive
and one negative) will attract each other,
and particles with the same charge (for
example, two negative charges) will repel
each other.
The other important thing to know is
that atoms are neutral – overall they have
no electrical charge. In any atom there is
always the same number of positive protons
as negative electrons. The opposite charges
cancel each other out and result in an
overall neutral atom.
Figure 1.14 A model of an atom of the element
carbon as would have been proposed by Rutherford.
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Electron orbits
• Electrons move around in the space
outside the nucleus.
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• The mass of the atom is almost entirely
due to the mass of the nucleus; electrons
have very little mass in comparison.
Remember
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Questions 1.2.1: Looking inside the atom
1 Explain, using the evidence described, why you think Rutherford concluded that:
a the atom contained a lot of space
b there was a central area of positive charge
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2 Describe an alpha particle. What was the most important new understanding of
the structure of the atom that Rutherford inferred from his experiment with alpha
particles?
3 In his model of the atom, how did Rutherford describe the electrons?
4 Name and describe three types of particle we now know are found inside the atom.
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6 Carry out some research to find out more about Rutherford’s experiment. Identify
and describe the technologies required for this experiment to be successful.
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5 Working with a partner, construct a three-dimensional model of an atom using
modelling clay or other suitable materials. Make sure you label all parts correctly
and identify which model of the atom you are representing.
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Discovery of the neutron
Figure 1.15 The improved
version of Rutherford’s
model of the carbon atom.
The nucleus is composed
of two subatomic
particles: positive protons
and neutral neutrons.
The smaller negative
electrons are found in
orbits around the nucleus.
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Protons are all positive and are found
in the nucleus of the atom. Rutherford
theorised that there must be something to
help the positive charges stay together. We
know that positive charges would typically
repel each other, but if they were held far
enough apart they might just be able to stay
together. Rutherford theorised that because
an atom is neutral overall, these ‘things’
holding the positive protons apart must be
neutral (as the electrons already balance out
the protons).
In 1932 Rutherford’s student, James
Chadwick, proved the existence of the
neutron. Chadwick was awarded the Nobel
Prize in Physics in 1935 for his work in the
discovery of the neutron.
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Activity 1.2.2: How can you tell what is inside?
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We now know that the atom contains three
subatomic particles: neutrons (neutral),
protons (positive charge) and electrons
(negative charge). When it was first
proposed, Rutherford’s model of the atom
only contained protons in the nucleus.
Protons and electrons are both charged
particles, which means that studying them
is somewhat easy as we can look at how they
are affected by other charges. As a proton
is positive, we can study the way it behaves
when subjected to a positive or negative
charge. It will repel positive charges and
attract negative charges. The opposite occurs
with the electron.
But how do scientists study, let alone
discover, a particle that is neutral?
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This kind of investigation uses what scientists call ‘indirect evidence’. Many scientists
have used indirect evidence when trying to work out what is inside the atom.
What you need: ball, 2 nails, wooden block, 3 small boxes
Form two teams (A and B) of three students to work with each other.
1 Team A places the ball in one of the boxes, the wooden block in another, and the
two nails in the last box. The boxes are then closed.
2 Team B has to work out a way of knowing what is inside each of the boxes without
opening or touching them.
3 Team B can then touch and examine the boxes, still without opening them.
O
R
R
• Was team B more successful at identifying what was inside the box when able to
touch and examine the box?
• How might scientists have used indirect evidence to model what is inside an atom?
N
C
• Identify at least one other field of scientific investigation in which the scientists
working in that field would have to use indirect evidence to develop their theories.
U
Questions 1.2.2: Discovery of the neutron
Remember
1 Identify the scientist who discovered the neutron.
2 Explain why it was difficult to discover the neutron.
3 Copy and complete the following table:
Name of subatomic particle
Charge
Location within the atom
overmatter
1.2 Subatomic particles 13
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Atoms and their masses
+
–
neutron
+ proton
– electron
Figure 1.16 A lithium
atom with mass
number 7 and atomic
number 3.
–
–
+
+ + +
+
+ +
+
–
neutron
+ proton
–
–
EC
TE
D
– electron
R
Figure 1.17 An oxygen
atom with mass number
16 and atomic number 8.
Mass number
C
O
R
13
6
N
C
Atomic number
E
U
–
PA
G
–
–
FS
+
O
+
PR
O
–
neutrons in its nucleus. Carbon-13 has a
mass number of 13. You will find out more
about isotopes later.
The mass of an atom is calculated by
added the masses of all the protons and
neutrons in its nucleus (remember that
electrons are too tiny to bother counting).
Protons have very slightly less mass than
neutrons. The difference is so miniscule
that to simplify the process, we assume that
protons and neutron have the same mass on
this scale, a mass of 1 amu. Therefore the
mass of a whole atom in amu can be worked
out by considering how many protons
and neutrons the atom has. For example,
a helium atom that contains 2 protons
and 2 neutrons will have a relative mass
of 4 amu. A carbon atom that contains
6 protons and 6 neutrons will have a
relative mass of 12 amu.
The atomic and mass numbers of an
atom can be used to determine exactly how
many protons, neutrons and electrons are
in that atom. The atomic number is the
number of protons. The mass number is the
combined number of protons and neutrons.
So the number of neutrons in an atom can
be calculated by subtracting the atomic
number from the mass number. Carbon-13
has a mass number of 13 and an atomic
number of 6. This information tells us that
a carbon-13 atom must have 6 protons and
7 neutrons (13 − 6 = 7). Remember that
overall, all atoms are neutral. So for every
positive proton in the nucleus, the atom
must have the same number of negative
electrons circling around the outside.
Carbon-13 has 6 electrons to cancel out the
charge of the 6 protons.
Figure 1.18 A carbon-13 atom. The
atomic number of 6 tells us there are 6
protons, which makes the element carbon.
It is a neutral atom so there must also be 6
electrons. The mass number tells us the total
number of protons and neutrons in the
nucleus, so mass number − atomic number
= number of neutrons. 13 − 6 = 7 neutrons.
E
–
All atoms have a mass. The mass of an
atom is made up mainly of the protons and
neutrons in the nucleus of the atom.
The mass of an electron is extremely
–
difficult to calculate because
electrons are so small. Although
electrons do have mass, they
are so tiny that we don’t bother
counting them when calculating
–
the mass of atoms.
neutron
Chemists have devised a relative
+ proton
mass scale
to use with atoms because
– electron
they are so small. It is called the atomic
mass unit (amu), also known as the dalton
(symbol Da). By definition, 1 amu is equal
to one-twelfth the mass of a carbon-12
atom. This scale is more convenient
–
than using actual units (such
as the gram). The number of
protons in an atom determines
–
which element the atom is. If
the number of protons inside
the nucleus changed, the
–
neutron would also change. The
element
+ proton
number of protons in an atom is
– electron
called the atomic number. All atoms
of the same element have the same atomic
number. However, some atoms of the same
element had different masses. They have the
same number of protons, the same number
of electrons, but different numbers of
neutrons. These atoms are called isotopes.
Carbon-12 is just one of the types of
carbon atoms. It has 6 protons (like all
carbon atoms) and 6 neutrons in the
nucleus. It is the most common isotope
of carbon. By convention, isotopes
are represented with a dash after
the element name (or symbol),
E
followed by the total number of
protons and neutrons combined
in the nucleus of the isotope (in
this case, 12). The total number
of protons and neutrons in an
atom is called the mass number.
For example, carbon-13 represents an
isotope of carbon with 6 protons and 7
E
N
N P P
P N N
N N P
P P N
E
E
E
14 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4
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N u m e r ac y
builder
Calculating numbers of protons, neutrons and electrons
One of the isotopes of iron, iron-56, has an
atomic number of 26 and a mass number
of 56. Calculate the number of protons,
electrons and neutrons present in its
uncharged atoms.
FS
Aluminium has one main isotope. It has
an atomic number of 13 and a mass
Your turn
O
Example
number of 27. This means that it has 13
protons and 14 (27 – 13) neutrons in the
nucleus. Its uncharged atoms will have
13 electrons.
PR
O
For uncharged atoms, the number of
electrons must equal the number of
protons. If we know the atomic number
and mass number of an isotope, we can
calculate the number of protons, neutrons
and electrons present in an uncharged
atom of that isotope.
Size is relative!
PA
G
E
In this case, we don’t really need to know
how far each pace is, but we are able to
compare the lengths of the throws.
When referring to atoms, knowing
the actual mass of atoms is not going
to help us that much, partly because
the mass is so small. But being able to
compare the masses of different atoms
is very important when investigating the
behaviour of different atoms and elements.
O
R
R
EC
TE
D
Relative scales are helpful in many
situations where comparisons are made
between objects or events. These scales
are used when it is more important to
know the differences between objects and
events than the actual measurement (size,
mass, time). The following conversation
uses relative measurements:
‘Mum, Chloe has been in the shower
for twice as long as I was.’
‘I know, but you used three times as
much shampoo as her!’
Another example of a relative scale is
where something is chosen as a reference
point to compare measurements. We can
do this when we use paces to compare
distances, for example, ‘I threw the ball 25
paces but he could only throw it 18 paces’.
Deeper
u n d e r s ta n d i n g
N
C
Representing atoms
U
When it is important to show the number
of particles within each atom, the method
of representation shown in Figure 1.19 can
be used. As a whole, the elements can be
presented in a form called the periodic
table. One of the most common types is
shown in Figure 1.20.
Key things to remember about the
subatomic particles:
• The atomic number of an atom is the
number of positive protons in the nucleus;
it determines the element of the atom.
1 Write down one example of another
area of science or everyday life where
relative units are used.
2 For your example, describe why you
think relative units are used.
3 Outline the problems you think would
arise when using relative units for
measuring quantities.
• Neutrons are also
found in the nucleus
but have no charge.
• The mass number
of an atom is the
combined number of
protons and neutrons.
Mass number
(total number of protons
and neutrons)
A
X
Z
Symbol of
element
Atomic number
(total number of
protons)
• Electrons are
negatively charged.
• An atom has the same number of
electrons as protons.
Figure 1.19 The
conventional
representation of an
element.
1.2 Subatomic particles 15
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1
1
C
H
12.01
1.01
Li
13
III A
5
Be
6.94
B
9.01
Lithium Beryllium
3
12
Na
Mg
22.99
24.31
Sodium Magnesium
19
4
20
K
Ca
39.10
40.08
Transition metals
3
III B
4
IV B
5
VB
6
VI B
7
VII B
8
9
VII BI
10
11
IB
12
II B
21
22
23
24
25
26
27
28
29
30
47.88
50.94
Sc
Ti
44.95
V
Cr
52.00
Mn
Fe
54.95
5
Rb
38
39
87.62
88.91
Sr
85.47
40
Y
Zr
91.22
41
Nb
92.91
42
Mo
6
7
56
Cs
137.33
Cesium
Barium
87
88
Fr
57
to
71
Ba
132.91
Ra
(223)
226.03
Francium
Radium
72
58.93
58.70
63.55
65.39
Cobalt
Nickel
Copper
Zinc
45
46
47
48
95.94
(98)
Ru
101.07
Rh
102.91
Pd
106.4
89
to
103
19.00
20.18
Fluorine
Neon
16
17
28.09
30.97
32.07
Al
26.98
31
Ga
69.72
49
In
114.82
118.71
Indium
Tin
81
82
204.38
Thallium
Unp
(262)
Unh
(263)
Uns
(262)
Uno
(265)
Une
(266)
Uun
(267)
140.12
Lanthanum Cerium
89
Ac
227.03
90
Th
232.04
Actinium Thorium
59
Pr
140.91
60
Nd
144.24
61
Pm
(145)
Praseodymium Neodymium Promethium
91
92
231.04
238.03
Pa
Protactinium
U
62
Sm
150.4
63
Eu
151.97
64
Gd
157.25
65
Tb
Ti
Pb
207.2
Lead
158.93
66
Dy
162.50
67
Ho
164.93
Samarium EuropiumGadolimium Terbium Dysprosium Holmium
93
Np
94
Pu
E
58
Ce
237.05
(244)
95
Am
(243)
S
33
As
74.92
51
Sb
18
Cl
Ar
35.45
39.95
Chlorine
Argon
34
35
Se
36
Br
78.96
Kr
79.90
Selenium Bromine
52
83.80
Krypton
53
Te
I
54
121.74
127.60
Antimony Tellurium
83
Bi
208.98
84
126.90
131.29
Iodine
Xenon
85
Po
At
(209)
Xe
(210)
86
Rn
(222)
Bismuth Polonium Astatine
Radon
Mass numbers in parentheses are
from the most stable of common isotopes.
96
Cm
PA
G
57
La
Rare earth elements
Lanthanoid series 138.91
Sn
80
200.59
(261)
50
112.41
Mercury
108
72.61
Cadmium
Gold
107
Ge
79
196.97
186.21
32
Silver
Hg
P
Callium Germanium Arsenic
110
106
Si
Aluminium Silicon Phosphorus Sulfur
195.08
Unq
Actinoid series
16.00
Oxygen
15
Platinum
183.85
Metals
Figure 1.20 A periodic
table of the elements.
14.01
107.87
Au
Ne
Nitrogen
109
105
Pt
10
F
14
Iridium
180.95
Ir
9
O
12.01
192.22
104
Os
8
Helium
Carbon
190.23
178.49
Re
78
4.00
17
VII A
13
Hafnium Tantalum Tungsten Rhenium Osmium
W
75
Cd
7
N
16
VI A
Boron
77
Ta
74
Ag
6
C
15
VA
He
10.81
76
Hf
73
Zn
44
Rubidium Strontium Yttrium Zirconium NiobiumMolybdenum
TechnetiumRuthenium Rhodium Palladium
55
Cu
Iron
43
Tc
Ni
55.85
Potassium Calcium Scandium Titanium Vanadium ChromiumManganese
37
Co
14
IV A
PR
O
11
2
Non-metals
4
3
2
Carbon
2
II A
Hydrogen
18
VIII A
Atomic number
Chemical symbol
Atomic mass
Name of element
6
FS
New designation
Original designation
O
1
IA
(247)
97
Bk
(247)
98
Cf
(251)
99
Es
(252)
68
Er
69
71
70
Tm
Yb
167.26
168.93
Erbium
Thulium Ytterbium Lutertium
100
Fm
(257)
Uranium NeptuniumPlutonium Americium Curium BerkeliumCaliforniumEinsteinium Fermium
173.04
Lu
101
174.97
102
103
(258)
(259)
(260)
Mendelevium
Nobelium
Lawrencium
Md
No
Lr
Questions 1.2.3: Atoms and their masses
EC
TE
D
Remember
1 Copy and complete the following table, showing the numbers of subatomic
particles in a range of atoms.
Atom name and
symbol
Number of
protons
40
20
Fluorine (F)
9
19
9
Sodium (Na)
11
R
20
O
R
C
N
Mass
number
Calcium (Ca)
Argon (Ar)
U
Atomic
number
11
40
Sulfur (S)
Number of
neutrons
Number of
electrons
20
20
12
18
16
16
a Explain how you were able to calculate the number of neutrons in the
argon atom.
b Explain how you were able to work out the atomic number and the mass
number of the sulfur atom.
2 Identify the subatomic particle not found in the nucleus of the atom.
Apply
3 Imagine all the elements did not have names and were simply identified by
their atomic numbers. Explain if you think this would make chemistry easier to
understand. What problems do you think this would cause? What would be the
advantages?
overmatter
4 Identify how many electrons there are in the nitrogen atom, which has an atomic
number of 7 and a mass number of 14.
16 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4
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Atomic mass and isotopes
+
+
U
N
C
–
+
44.95
47.88
Scandium
Titanium
39
40
50.94
Zr
91.22
Zirconium
57
to
71
Hf
+
++
54.95
42
43
Mo
92.91
Tc
95.94
(98)
27
Fe
73
74
Ta
Re
29
Cu
30
Zn
58.93
58.70
63.55
65.39
Iron
Cobalt
Nickel
Copper
Zinc
44
45
46
47
Ru
Rh
101.07
75
W
Ni
55.85
Pd
102.91
Niobium MolybdenumTechnetium Ruthenium Rhodium
72
28
Co
76
Ir
Ag
48
Cd
106.4
107.87
112.41
Palladium
Silver
Cadmium
77
Os
78
Pt
79
Au
80
Hg
178.49
180.95
183.85
186.21
190.23
192.22
195.08
196.97
200.59
Hafnium
Tantalum
Tungsten
Rhenium
Osmium
Iridium
Platinum
Gold
Mercury
104
105
106
107
108
109
110
111
Rf
Db
(205)
105
Sg
Bh
(271)
Hs
(272)
Rutherfordium Dubnium Seaborgium Bohrium
Mt
(277)
(276)
(281)
Rg
(280)
112
Cn
(285)
Figure 1.21 Some atomic
numbers and atomic
masses in the periodic
table.
PA
G
E
carbon, have the same number of protons,
they may well belong to a group of carbon
atoms that have a different numbers of
neutrons.
Isotopes
Ds
Hassium Meitnerium Darmstadtium Roentgenium Copernicium
PR
O
89
to
103
52.00
Nb
88.91
26
Mn
Vanadium Chromium Manganese
41
Yttrium
25
Cr
FS
Y
24
V
For many of the elements, the number of
neutrons in the atoms can vary. For example,
most carbon atoms have six neutrons in their
nucleus but some have seven and some have
eight. The different forms of the atoms of an
element with different numbers of neutrons
are called isotopes.
Carbon-12 (6 protons and 6 neutrons)
is the most common isotope form of
carbon atoms in the natural world (about
EC
TE
D
O
R
–
6 neutrons
23
Ti
–
R
–
22
21
Sc
O
On the periodic table you will have seen the
atomic masses of the elements listed. These
masses are not whole numbers and are not
the same as the mass numbers of the atoms
(although they are pretty close). They are a
more accurate way of comparing the masses
of the atoms of different elements. But why
are many of them not whole numbers? We
certainly cannot have part of a proton or
part of a neutron in an atom. Electrons do
have some mass, but not really enough to
make much difference to the overall mass of
the atom. So where do these atomic masses
come from?
Generally, not all the atoms within an
element have the same mass. This is because
they are not identical. Why is this? What do
they have in common and what is different?
All the atoms of an element have the
same unique number of protons, their
atomic number, sometimes shown by the
letter Z. The atomic number is used to
identify the element. For example, all carbon
atoms contain 6 protons in their nucleus,
so their atomic number is 6. If you examine
the periodic table of the elements you can
see that the elements are listed in order of
their atomic number. However, although all
the atoms of one particular element, such as
6 Protons
–
8 neutrons
–
+
+
–
6 Protons
+
–
+ + +
–
–
–
–
12
14
carbon-12: C
carbon-14: C
6
6
–
–
7 neutrons
–
6 Protons
+
+
+
–
+ + +
–
–
13
carbon-13: C
Figure 1.22 The three
isotopes of carbon.
6
1.2 Subatomic particles 17
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FS
chemists use the average mass of the
isotopes of the element for calculations. This
average mass is known as the relative atomic
mass of the element. Because almost all
carbon atoms exist as the carbon-12 isotope
and only a very small proportion are present
as the two heavier isotopes, the relative
atomic mass is only just above 12. The
relative atomic masses of the elements are
usually shown in the periodic table, correct
to one or two decimal places. Be careful not
to mix this up with their atomic numbers or
the mass numbers shown in isotopes.
L i t e r ac y
bu i l d e r
PR
O
O
98.9%). Only 1.1% of natural carbon on
the Earth is carbon-13 atoms (6 protons
and 7 neutrons). You may have heard of
carbon-14. This is a very small percentage of
total carbon and is comprised of radioactive
carbon-14 atoms. A carbon-14 atom breaks
down naturally by radioactive decay to
form an atom of the element nitrogen, an
electron and a release of electromagnetic
radiation. You will learn more about
radioactivity in section 1.3 of this chapter.
Most elements have more than one
naturally occurring isotope. In these cases,
Formation of atoms
E
as nuclei of new atoms are created. The
process of atom formation is not a gentle,
well-ordered process. It is therefore not
surprising that the products of these
interactions are not all identical.
In your research into nucleosynthesis,
try to identify atoms that:
• were produced very soon after the Big
Bang
EC
TE
D
PA
G
Why do some elements have atoms with
different numbers of neutrons? Why are
they not all the same?
To answer this question you might want
to investigate the idea of nucleosynthesis,
the process by which atoms are formed.
This may have been in the early days of the
Universe, in massive stars or in supernova
explosions. The processes involved in
the formation of atoms require huge
amounts of energy; they are powerful,
often explosive events where neutrons
and protons undergo violent interactions
• were produced later in the history of
the Universe
• are unstable and rapidly convert into
more stable atoms.
R
Questions 1.2.4: Atomic mass and isotopes
Remember
U
N
C
O
R
1 Explain the meaning of ‘mass number’ and how this name arose. Use an example
to assist your explanation.
2 Explain why the atomic number of an element is always a whole number but the
relative atomic mass of an element is often not a whole number.
Apply
3 A student wrote that all the atoms of an element are identical. Is this correct?
Explain your answer.
4 Using your knowledge of isotopes and a copy of the periodic table, copy and
complete the following table:
Isotope
symbol
Isotope
name
Atomic number
of element
Number of
protons
Number of
neutrons
Number of electrons
in uncharged atom
    ​U
 ​​238
 
92 ​
Oxygen-16
overmatter
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10
20
36
29
34
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Arranging electrons
valence shell. The number of electrons in
the valence shell of an atom makes a big
difference to the chemical properties of the
element, and will affect how the atom will
bond with other atoms.
Oxygen:
Table 1.1 Electron configurations for electron shells
of an atom.
• There are now 18 electrons left to place
in shells. Remember, the second shell
can only hold 8 electrons.
First
2
Second
8
Third
Up to calcium: 8
Above calcium: 18
Fourth
32
O
• Second shell holds the other 6 electrons.
Calcium:
Figure 1.23 Niels Bohr
E
• Atomic number of calcium is 20, so the
uncharged atom contains 20 electrons.
• First shell can only hold 2 electrons.
–
–
–
–
–
–
a
• There are now 10 electrons left to
place in shells. Remember the third
shell can only hold 8 electrons.
–
–
• Fourth shell holds the last 2
electrons.
b
R
–
–
–
–
–
–
–
–
–
–
–
–
–
2, 8, 8, 2
b Calcium
2, 6
a Oxygen
–
–
–
–
–
–
–
–
N
C
O
R
• Calcium’s electron configuration is
written as 2.8.8.2.
–
For the first 20 elements, the third
shell can only hold eight electrons. For
atoms with atomic numbers greater than
20, the third shell can accommodate up to
18 electrons. In Year 9, you only have to
– –
consider electron configurations
for the first
20 elements (up to calcium).
– –
Bohr also stated that the electrons of an
atom are normally located as close to the
nucleus as possible, because this is a lower
energy state and is more stable. Therefore,
the shells are filled from the inside out.
The electron configurations of oxygen
and calcium are compared in Figure 1.24.
– –
2, 6
Electron configurations are often
Oxygen
represented by shelladiagrams,
which
show the electron shells as circles and the
electrons in pairs. The outermost occupied
shell of uncharged atoms is known as the
U
PR
O
• Oxygen’s electron configuration is
written as 2.6.
PA
G
Maximum number of
electrons in shell
• First shell can only hold 2 electrons.
EC
TE
D
Electron shell
• Atomic number of oxygen is 8, so the
uncharged atom contains 8 electrons.
FS
After Rutherford had refined his model of
the atom, another scientist named Niels
Bohr concluded that the electrons in an
atom do not behave quite like the planets
around the Sun. Instead, he proposed that
they move about the nucleus in circular
orbits at certain distances from the nucleus.
The more energy they have, the further
their orbit is from the nucleus. These sets of
orbits are known as electron shells. There
is a limit to the number of electrons that
can be found in any of the shells. This is
called the Bohr model of the atom.
The arrangement of electrons in an
atom is called its electron configuration.
In this book we will consider the electron
configuration of an atom according to the
Bohr model of the atom. Table 1.1 shows the
number of electrons each shell can contain.
Figure 1.24 Electron
configurations for (a)
oxygen and (b) calcium
can be shown as simple
shell diagrams.
1.2 Subatomic particles 19
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–
nucleus
+
electron jumps
up one energy
level
–
FS
PA
G
energy level 1
the ground state
O
absorb
electromagnetic
radiation
PR
O
Many substances give off coloured light
when small samples are introduced into a
flame. When this light is seen through a
spectroscope (an instrument that breaks
the light up into its colours), a pattern of
coloured lines is observed. This pattern is
known as an emission spectrum and is
unique for each element. Bohr proposed
that when atoms of the elements were given
energy in a flame, the electrons jumped from
their normal shell to one further out from the
nucleus. He described the electrons as being
excited. Because this higher energy state was
unstable, the electrons then jumped back to
their normal levels almost instantly. For each
electron jump, a certain amount of energy
was emitted (given out) in the form of light
of a particular wavelength. Each coloured
line in the spectrum represents one of these
wavelengths of light. The possible values of
energy for the electrons present are slightly
different for each element, so each element
produces a different spectrum.
The emission spectrum is like the
‘fingerprint’ for an element as each element
produces a unique spectrum. This method
has been used to find out what elements
are present in stars, millions of kilometres
away from us here on the Earth, because
light from these stars can be analysed using
a spectroscope.
Although we can never see electrons, we
can observe the energy changes caused as
electrons move between the different shells
in an atom. This energy is often in the form
of light, and different atoms can produce
different colours of light. This effect can be
used to distinguish between different atoms,
and therefore identify different elements.
This is how flame tests work.
E
Evidence for electron
shells
EC
TE
D
energy level 2
Figure 1.25 As electrons jump from shell to shell
they emit electromagnetic radiation in the form of
light.
4.58
4.09
3.03
x10-28J
N
C
O
R
R
energy
U
Figure 1.26 The three
electrons found in this
atom will absorb energy
to jump into the excited
state. As they come
back down to the ground
state, they will release
the energy in the form
of a photon, or light. The
‘jumps’ back to their
ground state, depending
on where they were in the
excited state, will release
different bands of light.
The bigger the jump, the
more energy released.
4th–>2nd
3rd–>2nd
5th–>2nd
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Experiment 1.2.1: Flame tests
With your understanding of emission spectra, predict what you think might happen
when you burn samples of different metal salts in a Bunsen burner hot flame. Use
your prediction to formulate a hypothesis for this experiment.
Wear safety goggles.
Ensure hair is tied back and loose clothing is removed or tucked out of the way.
Wire loops and flames are hot. Be careful not to burn yourself.
1 M hydrochloric acid can give a small chemical burn. Wash it off skin with tap
water immediately.
FS
>>
>>
>>
>>
O
WARNING
Aim
To observe the coloured light emitted when certain substances are heated in a flame.
carbonate, barium sulfate, calcium
carbonate and strontium carbonate
• Wire loops
• Hydrochloric acid (1 M)
E
• Bunsen burner
• Heatproof mat
• Solid samples of sodium carbonate,
copper carbonate, potassium
PR
O
Materials
PA
G
Method
1 Set up your Bunsen burner, observing safety instructions, and light your Bunsen
burner on the safety flame.
EC
TE
D
2 Adjust your Bunsen burner to the blue flame. Take a wire loop and dip it in a small
beaker of 1 M hydrochloric acid. Flame the loop (heat it briefly in the blue flame).
This will clean the loop, ready for your solid sample.
3 Take a loop of solid chemical and place it in the flame. Observe the colour of the
flame. Try not to lose the solid down the Bunsen burner barrel. This could block the
burner and contaminate the flame, changing the colour.
4 Once you have finished your observation, dip the loop in the 1 M hydrochloric acid
again and re-flame it. This will clean the loop for the next sample.
Chemical
O
R
R
Results
Include your results in a table.
Flame colour
Sodium carbonate
C
Copper carbonate
N
Potassium carbonate
U
Strontium carbonate
Discussion
1 Explain why the loop was treated with hydrochloric acid before any carbonates were
tested.
2 Explain what you think caused the flame to change colour.
3 Describe the kind of change that caused the flame colour to change. Was it
chemical or physical? Identify the evidence that supports your answer.
4 Discuss why electrons in different elements produce different colours.
5 Does the metal or the carbonate part of the powder cause the colour change?
Identify the evidence that that supports your answer.
6 Using Figure 1.26 to help, explain how the subatomic structure of an atom can
overmatter
1.2 Subatomic particles 21
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from his burner, the glass made the flame
turn yellow. He then started to investigate
other chemicals and recorded the colours
that were produced. This work led other
scientists to develop more accurate
methods of analysis by using coloured
spectra from different substances.
Although the Bunsen burner seems
like a simple piece of equipment, it is a
good example of how developments in
technology can open up possibilities for
scientific discoveries.
FS
The German chemist Robert Bunsen was
very particular about the quality of the
equipment he used. When he took over as
professor of chemistry at the University
of Heidelberg, he insisted on having new
laboratories built. However, he still was
not happy with the burners used at the
time because they all produced yellow
or smoky flames. In 1855 he invented a
burner that produced an almost invisible
flame. When melting glass to make
laboratory equipment, he noticed that
when heated with the colourless flame
O
Figure 1.27 Robert Bunsen
The Bunsen burner
Questions 1.2.5: Arranging electrons
Remember
PR
O
Deeper
u n d e r s ta n d i n g
E
1 In the Bohr model of the atom, identify the maximum number of electrons the
second electron shell can contain.
PA
G
2 Propose a reason the second shell can contain more electrons than the first shell.
Apply
3 A potassium atom contains 19 protons.
EC
TE
D
a How many electrons will be present in a potassium atom? Explain your answer.
b Describe the electron configuration of a potassium atom according to the Bohr
model.
c Identify how many electrons are in the valence shell of a potassium atom.
d Describe what could be done to potassium atoms to make electrons jump into
the fifth shell.
R
4 Copy and complete the table.
O
R
Element
Chemical formula
Atomic number
Electron configuration
Helium
U
N
C
Carbon
6
Neon
2.8
1
Magnesium
17
2.8.3
5 Identify a piece of laboratory equipment (other than the Bunsen burner) and explain
how it has made a significant impact on scientific discoveries.
6 Identify the element you think caused the yellow colour that Bunsen saw when he
was heating glass.
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Atoms and ions
magnesium ion
Mg2+ (2, 8)2+
gains 1
electron
U
N
C
O
R
R
magnesium atom
Mg 2, 8, 2
EC
TE
D
loses 2
electrons
E
PR
O
O
FS
electrons leading to a charged atom can only
be caused by the loss or gain of electrons.
Ionisation may occur when atoms come
together to form chemical bonds. It can also
occur when atoms are exposed to radiation.
When ions are formed, the electrons in the
outer electron shell (the valence shell) are
affected. When ions are formed, normally
the resulting ion has a full outer shell of
electrons because it is a stable arrangement.
In this situation the first three shells are
full, with 2, 8 and 8 electrons respectively.
PA
G
Atoms are neutral. The amount of negative
charge within the atom is the same as the
amount of positive charge. The number of
protons (positive) is always the same as the
number of electrons (negative). However, if
electrons are lost or gained from the outside
of the atom, this balance is disturbed –
there will no longer be the same number of
protons as electrons and the atom becomes
charged. Charged atoms are called ions, and
the formation of ions is called ionisation.
Protons cannot be removed from the
nucleus so an imbalance of protons and
chlorine atom
Cl 2, 8, 7
Figure 1.28 shows the formation
of magnesium and chloride ions. The
magnesium atom originally had 2 electrons
in its valence shell. It will lose both of these
electrons (it is easier to lose 2 than to gain
Figure 1.28 How
magnesium and chloride
ions are formed.
chloride ion
Cl– (2, 8, 8)–
6), so the second shell is now the valence
shell and will be full. The chlorine atom had
7 electrons in its outer shell. It would gain 1
electron to make this outer shell full with 8
electrons (it is easier to gain 1 than to lose 7).
1.2 Subatomic particles 23
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Calculating ion charge
When ions are formed, protons are held in the nucleus and are not affected by changes
occurring outside the nucleus. The number of protons stays the same. Electrons are
negatively charged, so for each extra electron gained the charge on the atom becomes
negative by one. If an electron is lost, the charge on the atom becomes positive by one as
there will now be extra protons compared to the number of electrons. Table 1.2 gives some
examples.
Table 1.2 Some positive and negative ions.
Electron
configuration
of ion
Change
Oxygen (O)
2.6
2.8
gained 2
electrons
Chorine (Cl)
2.8.7
2.8.8
gained 1
electron
Sodium (Na)
2.8.1
2.8
Calcium (Ca)
2.8.8.2
2.8.8
Remember
−2
Name and
formula of ion
Oxide (O2−)
Chloride (Cl−)
lost 1 electron
+1
Sodium (Na+)
lost 2 electrons
+2
Calcium (Ca2+)
E
PR
O
−1
PA
G
Questions 1.2.6: Atoms and ions
Charge of ion
FS
Electron
configuration
of atom
O
Name and
symbol of
atom
1 Refer to Table 1.2 and explain the patterns you notice about the:
a names of the negative ions
EC
TE
D
b electron configuration of the ions
c differences between the metals and non-metals
Apply
2 Predict the charges on the following ions:
a potassium (atomic number 19)
R
b aluminium (atomic number 13)
c nitride (produced from nitrogen atoms with atomic number 7)
U
N
C
O
R
3 The elements neon (atomic number 10) and argon (atomic number 18) do not
normally form ions. Suggest why.
24 Oxford Insight SCIENCE 9 Australian Curriculum for NSW Stage 4
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1.2
Subatomic particles
Remember and understand
1 Identify the location of the following
particles in an atom and state their
charges.
a Identify the atomic number of the
element. [1 mark]
b Identify what element it must be.
[1 mark]
Checkpoint
3 Explain the difference between an atom
and an element. Give an example to
support your answer. [2 marks]
4 Explain why the mass numbers of
isotopes are exact whole numbers but
the relative masses of most elements
are not. [2 marks]
O
9 Tellurium, element 52, has a relative
atomic mass of 127.6. The next
element, iodine, has a relative atomic
mass of 126.9.
a Write the symbol for the isotopes
of tellurium-127 and iodine-127.
[2 marks]
b Explain why the atoms of these two
different elements can have the
same mass number. [2 marks]
10Using the example of atoms, explain
the difference between a model and a
theory. [2 marks]
EC
TE
D
5 Titanium, element 22 in the periodic
table, has five naturally occurring
isotopes. Describe what the isotopes of
titanium have in common and in what
way(s) they are different. [2 marks]
Analyse and evaluate
PR
O
2 When an atom is uncharged, identify
the number of protons and electrons
present. [1 mark]
E
c electron [2 marks]
PA
G
b neutron [2 marks]
FS
c Identify the electron configuration
of the next element on the periodic
table. State your reasoning.
[2 marks]
a proton [2 marks]
Apply
R
    ​U
6 ​​ ​235​
  ​​is a naturally occurring radioactive
92 ​
isotope of uranium used in nuclear
reactors. In an uncharged atom, identify
how many of the following are present:
O
R
a protons [1 mark]
b neutrons [1 mark]
C
c electrons [1 mark]
U
N
7 Only 0.7% of the uranium atoms in
naturally occurring uranium exist
as uranium-235. The other isotopes
present are uranium-234 (0.01%)
and uranium-238 (99.3%). Design
an appropriate table to compare the
symbols, atomic numbers and mass
numbers of these isotopes. [3 marks]
8 According to the Bohr model of the
atom, the uncharged atoms of a
particular element have the electron
configuration of 2.8.8.
11Scientists have used indirect evidence
to infer what it is like inside the atom,
in the same way that astronomers
have worked out the temperature
and composition of stars. List the
advantages and disadvantages of using
indirect evidence to develop theories in
science. [2 marks]
Critical and creative thinking
12Create a poster or digital presentation
to show different models of the atom,
from John Dalton’s original solid
sphere theory to the Bohr model used
today. Use the Internet to find images of
the scientists involved and place copies
onto your poster. Identify the year in
which each model was proposed and
include a scaled timeline. [5 marks]
Making connections
13Apply your understanding of atoms to
suggest reasons for the following:
overmatter
TOTAL MARKS
[ /40]
1.2 Subatomic particles 25
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E
When changes like this happen, the atom
is said to decay. Uranium is an example of
a radioactive material because it sometimes
contains these unstable atoms and will
decay over time.
Radiation can be emitted from the
nucleus of radioactive atoms as they decay.
The most common types of radiation
emitted are alpha (α), beta (β) and gamma
(γ) radiation.
These three types of radiation are
different, but are all caused by the nucleus
of the atom breaking up. Why does this
happen? Some atoms contain nuclei that
become more stable by giving out radiation.
EC
TE
D
PA
G
Atoms are usually around for a long time.
An oxygen atom formed billions of years
ago will still exist today in a piece of iron
ore (Fe2O3), in a glass of water (H2O), or
even as a carbohydrate (such as glucose,
C6H12O6) in your body. However, some
atoms are not so stable and can change
over time. As they change, the subatomic
particles may separate and the combination
of protons, neutrons and electrons that
defined that atom may change. These
changeable atoms are radioactive. This
change can happen quickly, or it might
take tens, thousands or even millions of
years – it all depends on the type of atom.
O
What is radioactivity?
FS
With an understanding of the structure of atoms, the idea of radioactivity
can be explained. Alpha (α), beta (β) and gamma (γ) are different types
of radiation released from atoms. When does this happen? Which atoms
are radioactive? What makes these atoms different from other atoms?
Examining the structure of the nucleus of these atoms can help answer
these questions. In-depth knowledge of atoms has allowed scientists to
understand the risks of radioactivity and use radioactive materials for
areas such as medicine.
PR
O
1.3
Radioactivity
R
Activity 1.3.1: Exploring uranium
U
N
C
O
R
Australia has 23% of the world’s known uranium reserves. Uranium is the main fuel
used in nuclear power stations. The atomic number of uranium is 92, which means
that all uranium atoms contain 92 protons. The most common form is uranium-238
(238U) with more than 99% of natural uranium in the world being this isotope. Most of
the rest is uranium-235 (235U).
1 Using your knowledge from section 1.2, calculate the number of protons and
neutrons in an atom of uranium-238. Repeat the calculation for uranium-235.
Like all uranium isotopes, uranium-235 is an unstable atom and can decay into other
atoms. This contributes to uranium’s classification as a radioactive element. When
uranium-235 decays, it can decay into a different atom, thorium-231. Thorium has an
atomic number of 90.
2 Calculate the number of protons, electrons and neutrons in an atom of
thorium-231.
3 Identify the number of protons a uranium atom loses to become thorium-231.
Figure 1.29 The Ranger
Uranium Mine in Kakadu
National Park, Northern
Territory.
4 Identify the number of neutrons a uranium atom loses to become thorium-231.
5 Identify the atom that contains the number of protons and neutrons in questions
3 and 4 in its nucleus.
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You should have found that as the uranium-235 atoms decay to the thorium-231
atoms, the equivalent of the nucleus of a helium atom is lost. This particle is called an
alpha particle and these particles are released from the original uranium atom with
enough energy to travel away from the atom.
6 Research alpha particles and outline:
a how they can be represented
b what electric charge they possess
c how dangerous they are
FS
d how they can be stopped
Marie Curie
PR
O
Figure 1.30 A Geiger counter
is used to detect radiation.
Deeper
u n d e r s ta n d i n g
U
N
C
O
R
R
EC
TE
D
Marie Curie was an exceptional scientist.
She was born Maria Sklodowska in
Warsaw, Poland. When she married Pierre
Curie, a professor of physics at Sorbonne
University in Paris, she became Marie
Curie.
In 1903 the Nobel Prize in Physics
was awarded to Marie Curie, Pierre Curie
and Henri Becquerel for their work in
discovering radioactivity. Marie Curie
was the first woman to ever receive a
Nobel Prize. After her husband died in a
tragic horse-drawn vehicle accident, she
took over his role in the university as the
chair of physics. In 1911 Curie received
her second Nobel Prize, this time for
Chemistry, making her the first person to
win or share two Nobel Prizes.
The dangers of radiation were not
understood at the time and Curie’s
pioneering work in radiation eventually led
to her own death. She died of a fatal form
of anaemia (a disease involving blood cell
deficiencies) brought on by her exposure to
radiation over years in the laboratory. Even
her original laboratory reports are still
considered too dangerous to handle and
are kept in boxes lined with lead to shield
people from the radiation the reports emit.
One year after Marie Curie’s death in
1934, her daughter, Irene, shared another
PA
G
E
Radiation that exists all around us from various sources is called
background radiation. Radioactive minerals can be found in the
ground, and radiation in the form of cosmic rays comes from the Sun
and space. All this radiation can be measured with a Geiger counter.
O
Detecting radiation
Figure 1.31 Marie Curie.
Nobel Prize in Chemistry for her work,
in collaboration with her husband, on
artificial radioactivity. Her grandchildren
are well-known scientists, too, with her
granddaughter Hélène Langevin-Joliot
being a professor of nuclear physics, and
her grandson Pierre Joliot being a wellknown biochemist.
Marie Curie’s research led to a large
number of other scientific discoveries.
Rutherford’s experiments into the
structure of the atom would not have
been possible without the discovery of
radioactive materials, which Rutherford
used as a source of the alpha particles.
1.3 Radioactivity 27
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Isotopes and radioactive decay
FS
O
Whether a nucleus is stable depends on
the number of neutrons and number of
protons in the nucleus. There is no easy
way to predict the stability of different
atomic nuclei. Some nuclei are very stable,
such as carbon-12 nuclei with 6 neutrons
and 6 protons. However, carbon-14 with 8
neutrons in its nucleus is less stable and
will decay over time, giving out radiation
to form a different atom, nitrogen-14.
Scientists know how long this process
takes, therefore carbon-14 isotope decay
can be used to measure the age of objects,
including matter from living things.
This method is called carbon dating,
and is the most common way of dating
ancient artefacts, and plant and animal
material. Carbon dating was used to
measure the age of the Shroud of Turin,
a linen cloth believed by many to be the
cloth that covered Jesus’s face after his
crucifixion and burial.
The decay of radioactive isotopes is
often a very slow change. One gram of
carbon-14 today would take more than
5000 years until half of it had decayed and
become only 0.5 grams. The remaining
0.5 grams would take more than another
5000 years to get down to 0.25 grams,
and more than another 5000 years to
reduce to 0.125 grams. After ten half lives
(or 50 000 years), a gram of carbon-14
PR
O
Carbon dating
would have decayed to leave about 0.0001
grams of carbon-14. Unless the amount
of carbon-14 is measured over a very
long period, it might seem that no change
is occurring. However, scientists using
extremely sensitive instruments are able
to detect the radiation being released
during the decay process.
Some radioactive atoms decay
incredibly quickly. In a sample of the
isotope lithium-8, half of it decays in less
than 1 second. The problem for lithium-8
would not be trying to detect how it is
U
N
C
O
R
R
EC
TE
D
PA
G
Deeper
u n d e r s ta n d i n g
E
Figure 1.32 Some smoke
detectors contain a
radioactive source.
Earlier in this chapter you learned about isotopes. Hydrogen, for instance, has three isotopes:
hydrogen-1 (1 1 H), hydrogen-2 (2 1 H) and hydrogen-3 (3 1 H). Each isotope of hydrogen
has one proton (the same atomic number) but different numbers of neutrons (different
mass numbers). In some isotopes, when the ratio of neutrons to protons becomes too high,
the nucleus is unstable and it decays or changes into another isotope. This is known as
radioactive decay, and causes the emission of radiation.
Hydrogen-1 and hydrogen-2 are stable, but hydrogen-3 is unstable and emits a
beta particle (a single electron). Hydrogen-3 is therefore a radioactive isotope called a
radionuclide. Radionuclides can occur naturally or they can be produced in a nuclear reactor.
Figure 1.33 Carbon dating has indicated that the
Shroud of Turin is less than 2000 years old.
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1 Suggest a reason why carbon-12 atoms
are more stable than carbon-14 atoms.
2 Describe the dangers of isotopes that
decay very quickly.
3 Identify the dangers of radioactive
isotopes that decay very slowly.
4 When used in medicine, radioactive
isotopes that decay relatively quickly
are used. Explain why this might be.
O
FS
5 Carbon dating works because, as
isotopes decay, the amount of radiation
released reduces over time. Therefore
the measured level of radiation will
indicate the age of the object.
PR
O
a Outline why atoms that decay
extremely slowly are not generally
be used for dating objects.
E
b Outline why atoms that decay very
quickly are not generally used for
dating objects.
PA
G
changing, but actually detecting it at all.
Carbon-14 is by no means one of the slow
‘decayers’. Uranium-235, a radioactive
isotope of uranium, would take 700 million
years to reduce to half of what you started
with. If you had some uranium-238, a nonradioactive isotope, most of it would still
be there four billion years later!
In science, there are many situations
where change takes place over a range of
time scales. Radioactive decay is special
as it is a purely random process. It is
impossible to predict how long a single
particular atom will take to decay, giving
out radiation as it does so. With billions of
atoms in any one sample, the overall rate
of decay can be predicted. Think about a
glass of water evaporating: it is impossible
to predict when one particular water
molecule will escape from the liquid, but
overall we can predict how long the water
will take to evaporate.
Questions 1.3.1: What is radioactivity?
Remember
a ‘isotope’
EC
TE
D
1 Explain the meaning of each of the following terms:
b ‘radioactive decay’
c ‘radionuclide’
2 Recall an isotope of uranium that is radioactive.
3 Explain why radioactive decay occurs.
Research
O
R
R
4 Recall the name of an instrument that can be used to determine the level of
radioactivity.
5 Identify at least two achievements of Marie Curie.
N
C
6 Different parts of the world have different levels of background radiation. Conduct
an Internet search to find out how background radiation is measured and what the
levels are in some typical places.
U
7 Find out information about certain jobs that expose workers to more radiation than
what is considered ‘typical’. How is radiation exposure controlled in these jobs?
8 Research the development of ideas about the nature of radioactivity. Present
your information as an annotated time line. Include the relevant dates, scientists
involved as well as their key ideas. Include any significant technological
developments that may have contributed to the new discoveries.
1.3 Radioactivity 29
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Types of nuclear radiation
O
FS
but the proton remains in the nucleus,
increasing the atomic number of that atom
by one. Changing the number of protons in
the atom changes the element of that atom.
It is now known that another particle, called
an antineutrino, is also released in this
process.
Gamma rays are high-energy
electromagnetic rays similar to X-rays.
They are emitted after alpha particle or
beta particle emission, when the nucleus is
still excited. An example is when cobalt-60
decays to form nickel-60:
PR
O
Alpha (α), beta (β) and gamma (γ)
radiation all originate from an unstable
nucleus. An alpha particle is identical to the
nucleus of a helium nucleus. It contains two
protons and two neutrons. Americium-241
(commonly used in smoke detectors) is
an example of an alpha particle emitter. It
decays to neptunium-237, a more stable
isotope.
The decay of americium-241 to
neptunium-237 can be shown as:
237
93
Am
Np
PA
G
241
95
E
Alpha
particle
The number of protons and neutrons shown in the diagram are not exact.
    ​A
    ​​​p +  ​​ 4
  ​α
 ​​​241
  m​ ​→  ​​ 237
  ​​ ​
95 ​
2​ ​
​93   ​​N
C
O
R
R
EC
TE
D
The mass numbers on each side of the
arrow add to 241, which demonstrates that
the total mass of the particles before and
after the decay is the same.
Beta particles are produced when a
neutron in the nucleus decays into a proton
and an electron. The electron is the beta
particle that leaves the atom. An example
of beta decay is the decay of carbon-14 to
nitrogen-14:
U
N
Beta particle
14
6
C
14
7
N
0  ​​ ​
    ​C​​ ​→  ​​ 14
 
​​ ​14
6​
​ 7   ​​N​​ ​+​  ​​ −1   ​β
​
The beta particle has very little mass, so
the new nucleus formed has a mass very
similar to the original carbon-14 nucleus.
The electron is formed when a neutron
breaks apart into a proton and an electron.
The electron is emitted from the atom,
60
27
Co
60
28
Beta particle +
Ni
Cobalt-60 is an artificially produced
radioisotope used in medical radiotherapy,
sterilisation of medical equipment and
irradiation of food. Because gamma
radiation is an electromagnetic wave, rather
than a particle like alpha and beta radiation,
gamma radiation is highly penetrating and
can cause cell damage deep within the body
if exposure levels are high.
Radioactive half-life
Radioactive decay is a random process and
we cannot predict which radioactive atom
nuclei in a sample will decay at any given
moment. However, the rate of radioactive
decay follows a pattern. As a radioactive
sample decays, less and less of the original
substance is left and the radioactivity drops.
The half-life of a radioactive material is
the time taken for half of the radioactive
nuclei in a sample to decay, resulting in
the radioactivity of the original radioactive
material to drop to half of what it was.
When the radioactivity reaches one-half
of its original level, one half-life has passed.
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Bismuth-213
46 minutes
Technetium-99m
6 hours
Lutetium-177
6.7 days
Iodine-131
8 days
Chromium-51
28 days
Strontium-89
50 days
25
0
0
25
50
75 100 125 150 175
Time (years)
Figure 1.34 A radioactive decay curve for
strontium-90, which has a half-life of 28.8 years.
FS
Half-life
50
O
Radionuclide
75
Activity 1.3.2: Modelling radioactive decay
PR
O
Table 1.3 Half-lives of important medical
radionuclides.
100
Strontium-90 (g)
When it reaches one-quarter of its original
level, two half-lives have passed and the
pattern continues. A graph of radioactive
decay against time gives a characteristic
shape called an exponential decay curve.
EC
TE
D
1 Create a table to record your results.
PA
G
E
You can model the idea of exponential decay and half-life by using M&M’s to represent
the nuclei of atoms.
Predict the outcome of the activity. Will your model accurately predict radioactive
decay?
What you need: pack of M&M’s, A4 plain paper, disposable plastic cup
2 Record the total number of M&M’s, then place them into the plastic cup.
3 Shake the cup and tip all the M&M’s onto the paper.
4 Those that have the ‘M’ facing upwards represent atoms that have decayed. Move
these to one side to form a ‘discard’ pile.
5 Count the remaining ‘nuclei’ and record the number in your table.
6 Place these remaining nuclei back into the cup, shake them and tip out again.
R
7 Remove the decayed nuclei to the discard pile and count those remaining.
O
R
8 Continue until you have three or fewer nuclei.
9 Repeat the whole process two more times so you have a total of three trials.
N
C
10Draw a set of axes with the number of atoms remaining (vertical axis) and the
number of shakes (horizontal axis). Using just this set of axes, plot points and draw
a line or curve of best fit through the points for each of the three trials.
U
• In this activity, the atomic nuclei were represented by M&M’s. What represented
the half-life of the decay process?
• Are the shapes of the three curves similar or different? Comment on your
answer.
• Do you think the overall shape of the curves would be different if you started
with more atomic nuclei? Explain your answer.
• In this activity, could you predict when each individual nucleus would decay?
Why is this similar to the behaviour of real radioactive atoms?
• In this activity, you would eventually end up with no ‘undecayed’ M&M’s. Would
this be the case with a real radionuclide? Explain your answer.
1.3 Radioactivity 31
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N u m e r ac y
bu i l d e r
Calculating half-life
Answer
Your turn
Calculate the count rate of a sample of
iodine-131 after 32 days if the initial count
rate is 2016 counts per minute. The halflife of iodine-131 is 8 days.
FS
The half-life of technetium-99m is 6 hours.
If the initial radioactive count rate is 1088
counts per minute, what will be the count
rate after 30 hours?
O
Example
rate is halved. So:
1st half-life = 544; 2nd half-life = 272; 3rd
half-life = 136; 4th half-life = 68; 5th halflife = 34
Therefore the answer is 34 counts per
minute.
PR
O
If you know the half-life of a material, you
can calculate how much of a material is
left after a specific time. Likewise, if you
know the rate of decay of a material, you
can calculate its half-life.
PA
G
Effects of radiation
E
Over 30 hours, there are five half-lives:
30 ÷ 6 = 5. After each half-life, the count
U
N
C
O
R
R
EC
TE
D
Radiation can be harmful mainly because
it can cause atoms in other substances to
be ionised. The alpha and beta particles
have enough mass and/or energy to remove
electrons from the outside of atoms, which
will change the properties of the atoms. This
process also causes the release of reactive
particles called free radicals. If this occurs
in our bodies, free radicals can go on to
damage other molecules that may have
particular functions in the body.
Damage to DNA in our cells by free
radicals can have serious effects on our
bodies, because DNA is the molecule that
contains instructions for other biochemical
Radiation
Figure 1.35 Radiation can
damage the structure of
DNA molecules.
Figure 1.36 X-rays use radiation to make images of
the bones in the body.
processes. DNA can reproduce itself, so the
effect of one damaged DNA molecule may
be multiplied thousands or even millions
of times as copies of the affected DNA are
created. Many cancers linked to radiation
start this way.
When DNA in sperm or egg cells is
altered, it may not necessarily affect the
individuals themselves but the faulty DNA
could be passed on to their children instead.
This can result in physical deformities,
intellectual disability or genetic diseases in
the children of people exposed to dangerous
levels of radiation.
People who work with radiation take
precautions to ensure their cells are not
harmed. Lead aprons and protection booths
are commonly used by medical practitioners
who operate machinery that emit radiation,
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such as X-ray and MRI machines. Lead
shielding is too dense for the radiation to
pass through.
Dosimeter badges may be worn if
background radiation is likely. These
badges contain a photographic film that is
developed to measure exposure to radiation.
The film darkens in response to radiation –
the darker the film, the more radiation
exposure. Other versions of dosimeter
badges react instantly to radiation, changing
colour to indicate the levels of exposure.
O
FS
Figure 1.37 Overexposure to X-rays
can be harmful.
PA
G
E
Deeper
u n d e r s ta n d i n g
Figure 1.38 The
devastated Chernobyl
nuclear power plant.
R
EC
TE
D
One of the world’s worst nuclear accidents
happened at the Ukrainian Chernobyl
Nuclear Power Plant in 1986. The accident
occurred during a safety check. A surge in
power caused the reactor to operate at up
to ten times its normal level, which caused
a massive explosion and a huge release of
radioactive material into the atmosphere.
Some workers received fatal doses of
radiation in the first few minutes of the
accident and it is thought about 50 workers
died as a direct result of the accident.
Figures for additional deaths linked to
cancers caused by increased levels of
radiation from the Chernobyl accident are
varied, ranging from 5000 to 200 000.
PR
O
Nuclear accidents
O
R
Questions 1.3.2: Types of radiation
Remember
1 Identify the three types of radiation.
C
2 Define the term ‘half-life’.
U
N
3 Write the conventional representation of an isotope for each of the following in the
form AZX. You may need to use the periodic table to find out the atomic number of
the elements.
a iodine-131
c technetium-99
b cobalt-60
d fluorine-18
Apply
4 A scientist was using radioactive substances without proper precautions. Explain
the possible consequences.
5 An unearthed sample of a ceramic vase was analysed and found to contain 50%
carbon-12 and 50% carbon-13. Knowing that the half-life of carbon is 5730, how old
is the vase? Explain your answer.
1.3 Radioactivity 33
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Radiation and medicine
L i t e r ac y
bu i l d e r
O
FS
used as a treatment for cancer. A small
amount of a radioactive substance is placed
inside or close to a tumour. The radiation
from the substance kills the cancerous
cells in the tumour. Many forms of cancer,
including skin and breast cancer, can be
treated in this way. Brachytherapy is often
used in combination with other treatments,
including surgery and chemotherapy.
Radiation therapy can be used to treat
diseases other than cancer, such as in
coronary artery disease (a heart disease) to
reduce the chance of an artery closing.
PR
O
Radiation is used in medicine in a number
of ways. Diagnosis methods that use
radiation include X-rays, scans and the
injection of radioactive material for nuclear
medicine imaging.
During radiation therapy, radiation can
be used to treat or relieve the symptoms
of disease. Radioactive materials can be
injected, swallowed or placed directly inside
the body, or an external beam of radiation
might be used.
Radiation therapy inside the body is
called brachytherapy. It is most commonly
Nuclear medicine technologist: Fran Maestrale
E
scans. These may be done for a variety of
reasons: to diagnose cancer, investigate
the extent of arthritis, look for fractures
not visible a plain X-ray, or look at bone
infection.
In other cases, where the blood rather
than the bone is of interest, the blood of
a patient can be ‘labelled’—mixed with
a small amount of radionuclide, a small
radioactive nucleus. This can be used to
locate the site of an internal bleed. Once
the bleed has been located, these patients
can go to surgery and the surgeon will
know exactly where to begin finding the
haemorrhaging vessel so that it may be
sealed to prevent further blood loss.
A typical day at Maestrale’s work
consists of performing a number of these
different tests, looking at a variety of
different diseases and disorders. Nuclear
medicine technologists must be familiar
with many organs in the body in order to
know whether the images obtained appear
normal or abnormal. There is also the
opportunity to learn about the various
treatments for different conditions patients
can have. Although through practice
they may be able to interpret images and
determine what pathology a person has,
nuclear medicine technologists are not
qualified to do this. They must present the
images to the radiologist, who is responsible
for making a diagnosis. Due to the nature
of their profession, nuclear medicine
U
N
C
R
O
R
Figure 1.39 A technetium99m bisphosphonate
bone scan shows up
abnormalities within
bones.
EC
TE
D
PA
G
Nuclear medicine is a diagnostic
imaging method usually located within
the X-ray department of hospitals or in
private clinics. Most nuclear medicine
departments are relatively small, with
only a few nuclear medicine technologists
(NMTs) at each site. Nuclear medicine is
concerned with the function rather than
the structure or appearance of organs (as
is the case with general X-rays). An NMT
performs many procedures each day.
Before the first patient arrives at her
department, nuclear medicine technologist
Fran Maestrale must measure the
amount of radioactivity delivered to their
department. The isotope,
in liquid form, is drawn up
into the required amounts
and added to ‘cold’ kits, so
that the day’s scans can be
performed. A cold kit is a
vial containing a particular
chemical agent that travels
to a particular organ once
it is introduced to the body.
Each test uses a particular
compound, which travels
to a known organ of the
body based on its chemical
composition and the way it
is introduced into the body.
Most people referred
to nuclear medicine
departments require bone
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technologists have a close working relationship
with radiologists, surgeons and nurses.
Adapted from F. Maestrale, Frankston Hospital,
‘The Job of a Nuclear Medicine Technologist
(NMT)’
1 Outline the training you think a nuclear
medicine technologist might need. Why is this?
2 Explain why you think Fran begins her day
by measuring the amount of radioactivity
delivered to the department.
3 Explain why the nuclear medicine technologist
must give the images to a radiologist to make
a diagnosis.
PR
O
O
FS
Figure 1.40 Radioactive
dye injected into the
blood shows blood flow
in the blood vessels.
Nuclear technology and carbon dating in Australia
PA
G
E
help archaeologists identify the chemicals
used to make the pigments, and enable
the comparison of elemental composition
of the materials to help identify the
origins of different artefacts. Ancient
exchange networks between different
Indigenous groups can be investigated
because of this technique. Dr PopelkaFilcoff was awarded a fellowship from the
Australian Institute of Nuclear Science
and Engineering (AINSE) for this work.
AINSE aims to assist cooperation between
scientists working in nuclear science and
engineering.
In an AINSE project carried out in 2010
and 2011, carbon dating techniques were
used to analyse cultural material from
Mabuiag Island, in the Torres Strait in
northern Australia. Previously, materials
had been dated at 5000 years old, but
recent carbon dating techniques have put
the age of the site as 7500 years old – one
of the earliest known inhabited sites in
N
C
O
R
R
EC
TE
D
Nuclear science is being used in Australia
to uncover some details of both the age
and composition of historical materials. A
technique called neutron activation analysis
has been used at the Australian Nuclear
Science and Technology Organisation
(ANSTO) to study the composition of ochre,
a red pigment used in Indigenous Australian
artworks. Using this technique, a sample is
placed in a beam of neutrons. The atoms in
the sample absorb the extra neutrons and
become radioactive. As the nuclei of these
atoms decay, they emit gamma radiation.
Different elements give off gamma radiation
at different energies, so by measuring
the energy of the radiation produced,
researchers can identify the atoms and
hence the elements in the sample.
In 2010, Dr Rachel Popelka-Filcoff, a
researcher at Flinders University, used
this technique to determine the elemental
make-up of the ochre pigments used in
Aboriginal artefacts. This information will
Deeper
u n d e r s ta n d i n g
overmatter
Figure 1.41 Dr Rachel
Popelka-Filcoff with
some of the Aboriginal
artefacts.
U
Questions 1.3.3: Radiation and medicine
Remember
1 Recall two ways radiation can be used in medicine.
2 Identify the name given to internal radiation therapy.
Apply
3 Investigate one radioactive isotope used in medicine. State the symbol of the
isotope and its uses. Find out how it works, in simple terms. What are the benefits
of using this as an alternative to other treatments, such as chemotherapy alone or
immediate surgery? What precautions must be taken in handling this radioactive
isotope? Does its use have any side effects?
overmatter
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3 Recall four radioisotopes. [4 marks]
4 Explain what it means if a substance is
radioactive. [2 marks]
5 Describe how and where radionuclides
for industry and medicine are produced.
[3 marks]
Making connections
13Apply your understanding of the
structure of atoms and radioactivity
to outline reasons for the following
differences in properties:
PA
G
Apply
12Radiation produced from the decay
of radioactive atoms can be very
dangerous, but it can also be used
in life-saving medical treatments.
Imagine you are a doctor wanting to
reassure a patient about a treatment
based on exposure to radiation. List the
main ideas you would tell your patient.
[2 marks]
FS
2 Describe a beta particle. Write its
symbol, including the atomic number
and mass number in the correct
positions. [2 marks]
Critical and creative thinking
O
1 Describe an alpha particle. Write its
symbol, including the atomic number
and mass number in the correct
positions. [2 marks]
11Describe why the Bohr model gives us
much more information than Dalton’s
early model of the atom. [2 marks]
PR
O
Checkpoint
Remember and understand
E
1.3
Radioactivity
EC
TE
D
6 Draw a radioactive decay curve for a
substance that starts with an activity of
1600 counts per minute and has a halflife of 2 hours. [5 marks]
7 Calculate the half-life of a radioactive
substance that decays from 400 counts
per minute to 50 counts per minute in 9
hours. [3 marks]
R
Analyse and evaluate
O
R
8 Alpha radiation consists of particles
that can be easily stopped by our skin.
Explain why they are still considered a
potential risk to our health. [2 marks]
U
N
C
9 Consider the composition of alpha and
beta particles. When a beam of alpha
particles and a beam of beta particles
are fired into an electric field, they
move in opposite directions. Explain
why this is. [2 marks]
a beta particles can pass through
human skin but alpha particles
cannot [2 marks]
b beta particles can be stopped by
a few millimetres of lead foil, but
gamma rays will pass through
several centimetres of lead
[2 marks]
c carbon-14 is a radioactive isotope
but carbon-12 is stable [1 mark]
d alpha decay of an atom always
produces a lighter atomic nuclei,
but beta decay results in a nuclei
that may be of similar mass to the
original atom [2 marks]
e uranium-238 has a half-life that is
1000 times longer than uranium-234
[2 marks]
10Outline how the work of Marie Curie
influenced future work in different
areas of science. [2 marks]
TOTAL MARKS
[ /40]
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1 Fill in the gaps using the words in the Word Bank below:
All ____________ is made up of atoms. Atoms are composed of positively charged
____________ and ____________ neutrons in the nucleus of the atom, surrounded by
shells of orbiting negatively charged ____________.
Our knowledge and understanding of the structure and properties of ____________
has changed as more ____________ has been discovered. The atomic ____________
has been developed by Democritus, Dalton, Thomson, Rutherford, Chadwick and
many others through series of ____________ and research.
1
Model
Atoms
Evidence
Matter
Neutral
Beta
Experiments
Identify that all matter is made up
of atoms
2 Use a periodic table to help you to
complete the table of information about
some elements. [3 marks]
Symbol Relative
atomic mass
Ca
Neon
Atomic
number
Protons
Unstable
Chapter
review
6 Identify one piece of observational
evidence that supports the model of
electrons in stable orbits called shells
around the nucleus. [1 mark]
7 A student draws a picture of an atom
showing a solid nucleus with protons
and neutrons inside it and electrons
orbiting around it. Outline why this is
not an accurate drawing. [3 marks]
EC
TE
D
Element
name
O
Gamma
PR
O
Electrons
E
Alpha
PA
G
Word bank
FS
When the nuclei of an atom is ____________, it can decay and release energy and
particles as radiation to become more stable. ____________ particles are made up of
two protons and two neutrons. ____________ particles are electrons. ____________
radiation is the release of high-frequency electromagnetic energy.
20
C
O
R
R
3 A quick look at a periodic table of
elements shows all atoms have an
atomic number that is a whole number,
but most have a mass number that is
not a whole number. Explain why this
is so. [2 marks]
U
N
Outline the developments to the
atomic theories and models as a
process of refinement and review
of the scientific community
4 Describe the main features of John
Dalton’s idea about atoms. [2 marks]
5 Identify the most important piece of
evidence Rutherford had that supported
his model of the atom. [1 mark]
8 The model of the atom has changed
over time as new information
was discovered and evidence was
established that either extended or
refuted the model. For the Rutherford
model and the Bohr model of the atom,
identify what supporting evidence
led to the establishment of the models.
[2 marks]
Describe the structure of atoms
in terms of protons, neutrons and
electrons
9 Look at the periodic table in Figure
1.19 and use the information to identify
two elements other than oxygen and
carbon that have one predominant
isotope. Explain how you decided
these elements probably had one main
isotope. [3 marks]
2 Radioactivity chapter review 37
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Identify that radioactivity arises
from the decay of nuclei in the
form of particles and energy
11Explain why radiation emitted as an
alpha particle is much more dangerous
than beta particles. [2 marks]
FS
14A medical scientist has a choice of
isotopes to use for a procedure. One
has a half-life of 3 hours and the other
a half-life of 62 minutes. If you were
the patient, which isotope would you
prefer? Explain your answer. [2 marks]
15Evaluate the advantages and
disadvantages of using nuclear power
to generate electricity. [2 marks]
PA
G
U
N
C
O
R
R
EC
TE
D
TOTAL MARKS
[ /30]
12A specific atom of a particular
radioactive isotope has not decayed in
20 half-lives of time. Explain how this is
possible. [2 marks]
13A radioactive isotope used in medicine
has a half-life of 23 minutes. The
isotope is produced in a nuclear reactor
and then couriered to hospitals where
it is used with patients. If it arrives at
the hospital more than 3 hours after
production, it is said to be useless.
Explain why that would be. [3 marks]
O
10An atom of calcium, if placed into water,
will react to form an ion of calcium
(Ca2+) in solution. During that reaction
with water, hydroxide ions (OH–) and
hydrogen gas will be formed from
the breakdown of water molecules.
Predict where the electrons lost from
the calcium are transferred during this
reaction. [2 marks]
Evaluate the benefits and
limitations of the medical and
industrial use of nuclear energy
PR
O
Use models to describe the
arrangement of subatomic
particles in common elements
(additional)
E
1 REVIEW
CHAPTER
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radioactivity. What did he contribute to
this important work? What scientific units
are named after him? Did his children,
like those of Marie and Pierre Curie, also
follow careers in science?
Targeted alpha therapy
CERN
Henri Becquerel shared a Nobel Prize
with Marie Curie for work in discovering
1 What new science laboratory skills
have you learned in this chapter?
6 What are the risks of using radioactive
materials?
3 What were the most difficult aspects
of this topic, and why?
My future
U
N
C
O
R
R
4 Why is it important to know about
atoms and radioactivity?
Key words
alpha particle
atomic number
atomic theory
atoms
background radiation
beta particle
Bohr model
Brownian motion
carbon dating
compounds
decay
electron
electron shell
electron
configuration
elements
emission spectrum
exponential decay
curve
free radicals
gamma ray
half-life
ion
ionisation
isotope
law of constant
composition
law of simple
multiple proportion
mass number
neutron
nucleus
periodic table
proton
radiation
radioactive
radioactive decay
radionuclide
relative atomic mass
Rutherford nuclear
model
shell diagram
Thomson plum
pudding model
valence shell
FS
2 What was the most surprising thing
that you found out about atoms?
My world
1 REVIEW
CHAPTER
O
5 What changes to our lives have
occurred as we have increased our
understanding of atoms?
EC
TE
D
Reflect
Me
PR
O
Henri Becquerel
The European Organization for Nuclear
Research, known as CERN, is based on the
border of France and Switzerland. It has
been responsible for developing scientists’
understanding of atoms. What countries
collaborate in this project? What types
of scientist work at CERN? What current
work is occurring at CERN? What is the
Large Hadron Collider?
E
Targeted alpha therapy (TAT) is a new
therapy for the control of some cancers.
How does this form of therapy work?
What types of cancer are treated by this
method? How widespread is its use?
Identify the risks associated with this form
of radiotherapy and how are they reduced.
PA
G
Research
Choose one of the following topics for a
research project. Some questions have
been included to help begin your research.
Present your report in a format of your
own choosing.
7 Do you think it is important to find out
more about the inside of atoms?
8 What future uses of radioactive
materials do you think will happen in
your lifetime?
2 Radioactivity
1 chapter
chapter review 39
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O
FS
in nuclear reactions, which is why stars give
out so much energy. All elements on the
Earth were formed in stars that were far
bigger than our Sun.
Scientists can only produce new elements
in nuclear reactors and similar devices.
In 2008, the Large Hadron Collider was
turned on for the first time. This gigantic
particle accelerator, a circular tunnel 100 m
underground and almost 27 km long, was set
up to study the fundamental building blocks
from which atoms are made. Physicists hope
Figure 1.42 Matter is sent in all directions when a
star explodes.
U
N
C
O
R
R
EC
TE
D
PA
G
E
MA K ING
C O NNE C TI O NS
From the evidence collected so far, scientists
think the Universe began as an exceptionally
dense cluster of fundamental particles from
which all matter is made. Suddenly this mass
‘exploded’, sending matter in all directions.
This matter collected together in various
places, forming the stars and eventually
planets. This theory about how the Universe
began is known as the Big Bang theory.
Scientists also think the different
elements were formed in the stars as bigger
and bigger nuclei built up from this matter,
in reactions known as nuclear reactions.
This theory is known as the nuclear genesis
theory. Vast amounts of energy are released
PR
O
1
Atoms: past, present and future
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PR
O
O
1 Recall where scientists think the
elements that occur naturally on the
Earth were formed.
R
O
R
C
N
U
Figure 1.45 Carbon
nanotubes are extremely
thin (diameter 10 000
times smaller than a
human hair), hollow
cylinders made of carbon
atoms. They can be used
as ‘packets’ to deliver
substances to specific
sites in the body.
FS
the impact of exposure to radiation.
Much of the recent research into this
area has been aimed at improved
targeting of the damaged
cancer cells that need to be
killed, so that overall doses of
the radiation required can be
reduced. One strategy is the use
of carbon nanotubes to contain the
radioactive material. These nanotubes
could then be delivered (using selective
chemical ‘tags’ attached to the nanotube) to
very specific sites in the body.
2 Describe what this implies about all the
atoms now present on the Earth. Would
they always have been here?
PA
G
E
3 Explain why you think there are only
around 100 naturally occurring
elements. Why aren’t there many
more?
4 Explain why the production
of nuclear wastes with long
half-lives is a potential
environmental problem.
EC
TE
D
to ‘see’ what happened during the Big Bang.
One way to classify the elements we
encounter on the Earth is according to
whether they occur naturally on Earth or
if they have only been produced in nuclear
reactors and particle accelerators.
A further way to classify elements is to
show which ones have radioactive isotopes.
Only atoms with unstable nuclei are
radioactive. Many change from one nucleus
to another, and then to another, until they
eventually form a stable nucleus. Some
nuclei are so unstable that they only last
a fraction of a second, which makes them
very difficult to detect. This is the kind of
challenge facing scientists who are trying
to discover and gather evidence for the
existence of elements with atomic numbers
greater than 118.
Nuclear power and radiotherapies are
areas of science with unknown futures.
Some people see the increase in the use of
nuclear power as a way of reducing climate
change because the amount of carbonbased fuels being burned would be reduced.
Others consider this a dangerous move as
a large amount of radioactive waste would
be produced from the process, with halflives of thousands or even millions of years.
Australia currently does not produce any
of its domestic energy using nuclear power
stations, whereas nuclear power contributes
significantly to electricity generation in
other countries. Recent accidents, such
as damage to the Japanese nuclear power
station in 2011 as a result of an earthquake
and tsunami, have raised further questions
about the safety of nuclear energy. However,
Australia is rich in deposits of uranium and
the sale of these resources is of major benefit
to the Australian economy. Sales of nuclear
fuels remain controversial, because the same
materials can also be made suitable for
nuclear weapons.
In the area of medicine, it is expected
that as our understanding of atoms and
radioactivity increases, more effective
treatments will emerge and will save many
more lives and improve quality of life. But
even in medicine there are concerns about
5 Research which countries
rely on nuclear energy to
produce a large proportion of
their electricity. Why do you think the
governments of these countries have
chosen to do this?
Figure 1.43 The Large
Hadron Collider.
6 Research the countries to which
Australia currently exports
uranium.
7 Why do you think there are
still concerns about the safety
of radiotherapies?
8 Outline what a carbon
nanotube is.
9 Do you find it surprising that,
even after years of research,
scientists are still trying to find out
more about the atom? Discuss you
reasoning.
Figure 1.44 Scientists at
the Fukushima nuclear
power station after the
earthquake and tsunami
in Japan in 2011.
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