Download Carbon Isotopes

Physics
(14 - 16)
Nuclear Radiation - 1
Introduction
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John Dalton
In 1806, the elements were
defined by Dalton as being
substances composed of only
one type of atom.
He also introduced the idea
that atoms had a ‘solid ball’
structure.
John Dalton 1766 - 1844
We now define an element as a
substance composed of atoms,
each of which has the same
number of protons.
J. J. Thompson
In 1897, J. J. Thompson showed that an
atom contained small negatively charged
particles.
He called these particles electrons.
Thompson knew that the atom was
neutral (had no electrical charge).
J. J. Thompson 1856 - 1940
He proposed that the atom must
be made of a positively charged
sphere which contained the
negatively charged electrons.
This became known as the
‘plum pudding’ model.
Positively charged
sphere
Negatively charged
electron
Earnest Rutherford
In 1910, Earnest Rutherford’s team carried
out experiments in which particles were fired
at a thin sheet of gold.
E. Rutherford 1871 - 1937
Rutherford expected all of the particles to pass
straight through...
Earnest Rutherford
In 1910, Earnest Rutherford’s team carried
out experiments in which particles were fired
at a thin sheet of gold.
E. Rutherford 1871 - 1937
Rutherford expected all of the particles to pass
straight through...
Earnest Rutherford
Press ‘Start’ to take a closer look at Rutherford’s experiment...
Rutherford also predicted the existence of neutrons in the
nucleus, and named the three types of nuclear radiation alpha (),
beta () and gamma ().
The Structure of the Atom
An atom contains a central nucleus around which electrons orbit
in different levels.
The nucleus is composed of neutrons and protons.
The Nucleus
The nucleus is often shown as a static body, whose sub-atomic
particles fill the space of the nucleus.
In fact, the nuclear particles
are in constant motion.
The nucleus of an atom is
tiny compared to the volume
occupied by the electrons
of the atom.
If an atom could be enlarged to
the size of a football stadium, the
nucleus of the atom would be about the
size of a golf ball, somewhere above the centre spot.
The Nucleus
An electron has an incredibly small mass, which is about 1/2000
the mass of a neutron or a proton.
Therefore, electrons are
not considered to contribute
anything to the mass
of an atom.
Hence, the vast majority of the
mass of an atom is found in the nucleus.
The Structure of the Atom
There are many different models used to describe the structure
of an atom.
In this model the
electrons are shown
orbiting around a
central nucleus.
However, electrons also
possess properties typical
of waves.
Electrons
Electrons orbit so fast that they effectively form shells arranged
around the nucleus.
The electrons fill the shells starting from the one nearest to the
nucleus.
Each shell has a maximum number of electrons that it can hold.
Sub-Atomic Particles
Protons, neutrons, and electrons are types of sub-atomic particle.
Protons and electrons are electrically charged, which affects the
structure and stability of the atom.
 Protons are positively charged
 Electrons are negatively charged
 Neutrons have no charge
The charges on the proton and electron are opposite in sign but
equal in magnitude, so they cancel each other out exactly.
Sub-Atomic Particles
All substances on Earth are built from atoms containing these
three sub-atomic particles.
Elements differ from each other in that their atoms contain
different numbers of sub-atomic particles.
The number of protons in an atom is called the atomic number.
This is the factor which decides the identity of an atom - which
element it belongs to, e.g.
 All lithium atoms have three protons
 All carbon atoms have six protons
 All uranium atoms have ninety two protons
All atoms are electrically neutral, so they always have an equal
number of protons and electrons.
Sub-Atomic Particles
The mass of an atom is mainly in the nucleus.
Protons and neutrons have an approximately equal mass of one
atomic mass unit.
Electrons have negligible mass. Therefore, the mass of an atom
is equal to the sum of the number of protons and neutrons.
This number is called the atomic mass number of the atom.
Example:
Carbon atoms that have six
protons and six neutrons,
have an atomic mass of
twelve atomic mass units.
Atomic
Mass Number
12
Atomic
Symbol
Atomic Number
6
Isotopes
All atoms of the same element have the same number of protons.
However, atoms may have different numbers of neutrons.
Atoms of the same element with different numbers of neutrons are
called isotopes of that element.
Within a sample of oxygen, some atoms can have 8, 9 or 10
neutrons - these are the different isotopes of the oxygen atom...
16
17
18
8
8
8
8 neutrons
8 protons
9 neutrons
8 protons
10 neutrons
8 protons
Carbon Isotopes
The most common isotope of carbon (12C) - “carbon-12” can
be represented as follows:
Atomic
Mass Number
12
Atomic
Symbol
Atomic Number
A carbon-12 atom has:
 6 protons
 6 neutrons
 6 electrons
6
Carbon Isotopes
However, another isotope of carbon, 14C can be represented as
follows:
Atomic
Mass Number
14
Atomic
Symbol
Atomic Number
A carbon-14 atom has:
 6 protons
 8 neutrons
 6 electrons
6
Henri Becquerel
In 1896, Henri Becquerel discovered
radioactivity by accident.
H. Becquerel
1852 - 1908
Becquerel was investigating the phosphorescent
properties of uranium. As he was unable to
continue his experiments, he placed the
photographic paper and the uranium salt, with a
key in between them, in a drawer.
Four days later Becquerel looked at his
photographic paper and found it had
an image of the key on it.
Following further experiments, Becquerel
discovered that uranium produced
radioactivity, for which he was awarded
a Nobel Prize.
Image formed by Becquerel
on photographic paper
Marie & Pierre Curie
Marie Curie and her
husband Pierre, spent
many years studying the
radioactive uranium
ore, pitchblende.
Pierre Curie 1859 - 1906
Marie Curie 1867 - 1934
Pitchblende had the unusual property of being more radioactive
than uranium. The Curies deduced that pitchblende must contain
another radioactive element.
Eventually after refining tonnes of pitchblende, Marie and Pierre
identified two new elements, which they called polonium and
radium.
Marie & Pierre Curie
In 1903 Marie Curie,
Pierre Curie and
Henri Becquerel were all
jointly awarded the
Nobel Prize for Physics.
In 1911 Marie Curie was
also awarded the Nobel
Prize for Chemistry and
thus became one of only
two people to be
awarded a Nobel Prize
in two different subjects.
Marie Curie working in her laboratory
Marie & Pierre Curie
Marie died in 1934 of a medical
condition which was almost
certainly caused by working
with pitchblende.
The medical condition was
aplastic anaemia which is
known to be caused by
exposure to radiation.
Marie Curie’s radioactive sources
Radioactivity
 Isotopes such as 14C are not stable and will emit atomic
particles and/or energy - this is radioactivity.
 The emission of a particle and/or energy is called radioactive
decay, and the change that occurs is known as a decay reaction.
 A decay reaction will leave behind a more stable nucleus.
 Heat is always given out.
Radioactivity
 For an individual atom, the decay process is spontaneous and
totally random. However, for a mass of radioactive material,
the rate of decay is predictable.
Types of Radioactivity
Press the buttons to show the three types of radioactivity...
Parent & Daughter Nuclides
An unstable isotope is called a radioisotope.
When a radioactive element decays, it may become...
 A different isotope of the same element
 A new element
The original radioisotope is always referred to as the parent
nuclide (the unstable nucleus), and the resulting isotope as
the daughter nuclide.
Many radioactive elements decay as part of a decay chain, in
which the original parent nuclide forms an unstable daughter
nuclide, which also decays...
Parent & Daughter Nuclides
238
U

234
Th

234
Pa

There are another 11 stages to this chain, ending in...
206
Pb
Click to see the complete decay chain
234
U
Transmutation
A transmutation is where one element changes into another, due
to a nuclear reaction.
Example:
When alpha particles are fired at nitrogen-atom nuclei, the
nitrogen atoms are changed into oxygen and hydrogen atoms.
14
N
7
+
4
He
2

17
O
8
Checking the atomic mass figures...
14 + 4 = 17 + 1
Checking the atomic number figures...
7+2 = 8+1
+
1
H
1
Half-Life
Half-life is the time it takes for half the number of radioactive
nuclei within a sample to decay.
Another way of saying this is...
Half-life is the time it takes for the count rate from a radioactive
sample to fall to half its level.
Half-Life
Half-life is the time it takes for half the number of radioactive
nuclei within a sample to decay.
Another way of saying this is...
Half-life is the time it takes for the count rate from a radioactive
sample to fall to half its level.
What is the half-life of the above radioactive source?
10 years
End of Show
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