Download 6.1 Models of the Atom

Models of the
Atom
Greek Philosophers
Greek philosophers were
persons who thought
about things, trying to
understand the world and
humankind’s place in it.
The word, philosopher,
comes from the Greek
words, philos (to love) and
sophia (wisdom).
Philosophers are persons
who try to become wise by
careful thinking about
things.
Greek Philosophers
Greek philosophers thought and discussed ideas but did not do
experiments.
Greek Philosophers
Greek philosophers lived from 624 BC to 320 BC.
Is there a smallest piece of a substance?
Greek philosophers debated whether a substance (like water)
had a smallest piece or if you could keep dividing it up
forever, finding no smallest piece of the substance.
Can you cut a substance in half indefinitely?
Some Greek philosophers thought that you could keep
cutting something in half, never coming to a smallest piece of
the thing.
Other Greek philosophers thought that if you kept cutting
something in half, eventually you would come to the smallest
piece of a thing that could not be cut in half.
The Greek Philosopher, Democritus (460 BC)
Democritus believed that if you cut a thing in half again and
again, eventually you would find a small uncuttable particle
of the thing. He called this thing the uncuttable (atomos in
Greek) which is what the word, atom, means.
The Greek Philosopher, Democritus
Democritus believed that every thing was made up of atoms
and between the atoms was empty space. He thought atoms
were very small which is why they can’t be seen. He argued
that a beach viewed from a distance looks like one thing but
close up it can be seen to be made up of grains of sand.
The Greek Philosopher, Democritus
Democritus thought that all the properties of substances
were due to the shapes and colours of the atoms that they
were made of. He even believed that the soul was made up of
soul atoms. Since he only believed in atoms (He was a
materialist - only matter is real), Democritus was an atheist
and did NOT believe in God. He also proposed the form of
government called a Democracy.
The Greek Philosopher, Aristotle (384 BC)
Aristotle, the teacher of Alexander the Great, taught that you
could cut a thing in half forever, never coming to a smallest
part. He believed that all substances were made up of four
elements: earth, water, air and fire.
The Greek Philosopher, Aristotle
Aristotle argued against the idea of atoms and believed there
was a god who was at the outside of the universe in a realm
of perfection. He thought the earth was in a realm of
imperfection in the centre of the universe.
A New Faith : The Way (Later Called Christianity)
Jesus’ teachings were spread by His disciples after His
death.
Christians Persecuted by Rome from 35 – 313 AD
The early church was persecuted on and off from Jesus’
death to 313 AD.
Constantine Made Christianity Rome’s Favoured Religion
Emperor Constantine with an army of 20,000 fought an
opponent, Maxentius’ army of 100,000 at the battle of the
Milvian bridge in 312 AD.
Constantine Made Christianity Rome’s Official Religion
In a dream, Constantine (whose wife was a Christian) prayed
for help and in his dream felt he would conquer if he put the
first two letters of Christ’s Greek name (chi - X and Rho - P)
on his soldier’s shields. In Christ’s name he would conquer!
Constantine won the battle and made Christianity the favored
religion in Rome by proclamation in 313 AD.
After Constantine, Rome Weakened
As Rome lost influence, the church became stronger and
preserved culture and learning. This time, the Middle Ages
(400 – 1400 AD) is known as the Age of Faith.
Medieval Christians Studied Greek Philosophers
The few Medieval Christians who were educated and who had
access to Ancient manuscripts began reading Greek
philosophers. They rejected Democritus’ atheism and so
they also rejected his ideas of atoms. Since Aristotle
believed in a god, Medieval Christians accepted Aristotle’s
ideas.
Aristotle and
Transmutation
Aristotle taught that one
substance could be
changed into another by a
process he called
transmutation. During the
Middle Ages, persons
called alchemists believed
in transmutation and tried
to convert worthless
substances (like lead) into
valuable substances (like
gold).
The Philosopher’s Stone
Alchemists searched for
the magical substance,
the Philosopher’s stone,
which was thought to be
able to convert lead to
gold and to give a person
eternal life. Because
alchemists used some
occult practices
(witchcraft) and thought
that they could give
themselves eternal life on
their own, they were
often suppressed by the
Church.
Alchemists Develop New Equipment and Techniques
Despite their fanciful and misguided ideas, Alchemists
through experimentation developed many new techniques for
purifying and separating mixtures. They also developed new
equipment for experimenting like ovens, bellows, flasks,
retorts, mortar and pestle, distilling equipment and glassblowing techniques.
Alchemists
Discovered New
Elements
Alchemists also
discovered some new
elements like arsenic,
antimony, bismuth and
phosphorus.
Modern Atomic Theory : Robert Boyle
Robert Boyle (1627-1691)
wrote against Aristotle’s
idea of four elements,
writing that elements are
“certain primitive and
simple, or perfectly
unmingled bodies; which
not being made of any
other bodies, or of one
another, are the
ingredients of which all
these perfectly mixt bodies
are immediately
compounded, and into
which they are ultimately
resolved.”
Modern Atomic Theory : Robert Boyle (1627-1691)
Robert Boyle also appealed to all scientists to base their
science on experiments only. Despite his forward, modern
thinking, Boyle believed in Aristotle’s idea of transmutation.
Joseph Priestley (1733-1804)
•Joseph Priestley discovered the gas oxygen which he called
dephlogisticated air. His experiments paved the way for
Antoine Lavoisier to correctly explain what combustion
(burning) is and its similarity to respiration (the combustion
of food).
Henry Cavendish (1731-1810)
•Henry Cavendish discovered the gas hydrogen and noted
that when hydrogen burns in air it produces water. Thus
Cavendish showed that water was a compound not an
element like Aristotle taught.
Antoine Lavoisier : Brilliant Scientist (1743-1794)
Antoine Lavoisier discovered many laws of chemistry and
helped develop the metric system (SI – System Internationale)
Antoine Lavosier
Antoine Lavosier repeated Priestley’s experiments, naming
the newly found gas, oxygen. He correctly explained that
combustion (burning) involves the combination or addition of
oxygen to a burning substance rather than the loss of
phlogiston, an earlier incorrect theory.
Oxygen is added to a substance that burns.
Combustion is Always a Combination with Oxygen
1.
2.
3.
Wood + oxygen  carbon dioxide (gas) + water + ash
Food + oxygen  carbon dioxide + water
Gasoline + oxygen  carbon dioxide + water
Antoine Lavosier
Antoine Lavosier carefully weighed the substances before a
chemical reaction and then he weighed the products formed
after a chemical reaction. His discovery, called the Law of
Conservation of Mass, was that the total weights of the
reactants before a reaction is equal to the total weights of the
products after a chemical reaction.
Burning Wood : The Law of Conservation of Mass
•If a person weighs just the wood before it burns and the ash
after it burns, it seems like something has been lost.
•But if all reactants and all products are weighed, then the
total weight before equals the total weight after.
Antoine Lavosier
Antoine Lavosier
believed in freedom,
equality and
brotherhood which
were the ideals of the
French Revolution.
However since he
came from a rich
family, the
Revolutionary
government did not
trust him and
unfortunately had him
beheaded.
John Dalton : “Father” of the Modern Atomic Theory
John Dalton, a Quaker school teacher, started to develop his
ideas about atoms in 1802 but it was not until 1827 that he
published a full account of his atomic theory.
Dalton’s Atomic Theory : Birth of Modern Chemistry
•Most of the ideas of Dalton’s atomic theory were not new.
Greeks like Democritus and most scientists after Robert
Boyle philosophically (in thought) agreed on the main points
about atoms.
Dalton’s Atomic Theory : Birth of Modern Chemistry
•Dalton showed how Laws discovered by experiments could
be explained by the atomic theory.
•The Law of Conservation of Mass is explained by the atomic
theory since all chemical reactions are just a rearrangement
of atoms – no atoms are created or destroyed.
Dalton’s Atomic Theory : Birth of Modern Chemistry
•Dalton also used his theory to explain the Law of Constant
Composition, discovered in 1806 by Joseph Proust. The Law
states that for a given compound, the weights of the elements
making up the compound are always in a fixed weight ratio. In
water for example, the weight ratio of hydrogen to oxygen in the
compound water is always 1 : 8 (1 kg of hydrogen is joined to 8 kg
of oxygen, 2.5 g H : 20 g O, 3.5 g H to 28 g O etc.) This is explained
with atoms since every compound molecule is alike and all atoms
of one kind have the same weight.
Dalton’s Atomic Theory : Birth of Modern Chemistry
•Besides showing that his theory fit well with experiments,
Dalton’s 1827 paper suggested a method for finding the
actual weights of different atoms so atoms now were real
enough that they could be weighed! Dalton joined the world
of chemical experiments to the world of philosophical ideas.
•For example, to find the weight of fluorine compared to
hydrogen (the lightest atom), the experimental weight ratio is
1:19 (found in a chem lab expt.). If HF molecules have just 1
H to 1 F, then an F atom must be 19 times as heavy as H.
Main Ideas of Dalton’s Atomic Theory
1.
2.
3.
4.
All matter is made up of small particles called atoms.
Atoms cannot be created or destroyed, or divided into
smaller particles.
All atoms of the same element are identical in mass and
size, but they are different in mass and size from the
atoms of other elements.
Compounds are created when atoms of different
elements link together in definite proportions.
Atoms are real – not just
ideas (They have
unique weights that
can be experimentally
determined)!
Thanks,
J.D.
John Dalton’s Model of the Atom (1827)
• Dalton like the Greeks thought that atoms were hard
indivisible, uncuttable particles, solid like billiard balls.
Electricity and Vacuum Tubes Shed Light on Atoms
• Michael Faraday (1838) found that if a tube with positive
and negative electrodes had the air pumped out, it began
to glow at the negative electrode end. With a better
vacuum pump, Heinrich Geissler (1857) found that a
glowing beam formed between the two electrodes.
The Laws of Electric Charges
• Like charges repel each other away.
• Unlike charges attract each other.
Where is the glowing beam coming from?
• Experiments by William Crookes (1870s) and J.J.
Thompson (1897) showed that the glowing beam was
coming from the negative electrode (cathode) and that the
glowing beam was negatively charged. The glowing beam
was called cathode rays because it came from the cathode.
The Cathode Rays Are Negative Particles
• Since the cathode rays could rotate a paddlewheel inside a
cathode ray tube, experimenters thought they must be
made up of negative particles which came to be called
electrons (from the Greek word for amber, elektron).
Cathode rays found to be in all elements (all atoms)
• The exact same glowing beam formed no matter what
element was used for the cathode. This indicated that
matter (atoms) was made up of negative things and
because matter overall is neutral, it must also have
something positive to neutralize the negative. A new
model of the atom, the raisin bun or plum pudding model
was proposed by J.J. Thompson (1904).
Features of the Raisin Bun or Plum Pudding Atom
• Because an electric force field easily caused negative
particles to be ejected, Thompson thought that the positive
part of an atom must be jelly-like with negative particles
embedded in it which would allow electrons to be easily
pushed/pulled out.
Thompson Measures e/m (charge/mass) Ratio
In 1897, J.J. Thompson devised a cathode ray tube by which
he could measure the charge to mass ratio (e/m) for cathode
rays. He found the e/m for cathode rays (electrons) to be 1.3
x 10 11 C/Kg. The modern value is 1.67 x 1011 C/Kg.
The Usefulness of the e/m Ratio
Once Thompson had determined the e/m ratio, experimenters
needed just to get either the charge or mass of an electron to
compute the other property :
1.76 x 1011 C/Kg = e/m , so
If e (charge on an electron) is known, m = e/1.76 x 1011 C/Kg , or
If m (mass of an electron) is known, e = m(1.76 x 1011 C/Kg)
Millikan’s Oil Drop Experiment
Robert Millikan devised an oil drop experiment in 1909 that
found the charge on a single electron to be 1.59 x 10-19 C
(modern value is 1.60 x 10-19 C). Using this value, Millikan
computed the mass of the electron to be m = 1.59 x 10-19 C /
1.76 x 1011 C/kg = 9.03 x 10-31 kg or 9.03 x 10-28 g.
Using today’s values, the mass of an electron is 9.12 x 10-28 g.
How Millikan Found the Charge on an Electron
Robert Millikan radiated air (knocking out electrons) that had
small oil droplets that picked up some of these electrons. A
hole in an upper plate allowed some oil droplets to fall
through. Charges on the upper and lower plates could be
adjusted to suspend the oil droplets. There was a minimum
charge difference to suspend the droplets and this was the
charge on an electron.
The Gold Foil Experiment and The Nuclear Atom
• Ernest Rutherford designed an experiment in which a beam
of heavy positive alpha particles was shot at a very thin
gold foil sheet that was only a few atoms thick. Rutherford
expected that the alpha particles would plow directly
through the gold atoms (supposedly soft plum pudding
atoms) like an artillery shell would plow through a piece of
tissue paper.
Detecting Alpha Particle Motion
• A circular screen coated with zinc sulphide was placed
around the gold foil sheet. The zinc sulphide glowed
whenever an alpha particle crashed into it.
Gold Foil Experiment : Unexpected Results
• Rutherford found that 99 % of the alpha particles went
straight through the gold foil but a small number of
particles were scattered way off track and some even
bounced back!
Rutherford was Astonished
• The results of the Gold Foil Experiment astonished
Rutherford. He wrote, “… It was almost as incredible as if
you fired a 15 inch shell at a piece of tissue paper and it
came back and hit you.”
A New Model of the Atom: The Nuclear Atom
• Rutherford’s experiment showed him that there was a
small positive centre in the atom that he called the nucleus.
Around this centre, the negative electrons were thought to
rotate much like the planets rotate around the sun.
Rutherford’s atom was mostly empty space with a small
positive centre and moving negative electrons around the
centre.
What is in the Nucleus?
• More experiments convinced Rutherford that the nucleus
must have both positive and neutral particles in it. The
positive particles were called protons. In 1932, an
associate of Rutherford, James Chadwick, discovered the
neutron, a particle that is neutral and weighs the same as a
proton.
Atom Composition: Three Particles
1.
2.
3.
4.
5.
6.
Atoms are composed of
protons, neutrons and
electrons.
The nucleus contains protons
(+) and neutrons (0).
Electrons (-) are moving
rapidly around the nucleus.
The number of electrons
equals the number of protons
in a neutral atom.
The charge on one electron
exactly balances the charge
on one proton. 1+ added to 1gives a charge of 0.
A proton and neutron weigh
1832 X as much as an
electron.
What About the Electrons of an Atom?
• Rutherford’s planetary model of the electrons was shown
to be incorrect since the laws of physics predicted that a
negative charge orbiting a positive nucleus would lose
energy, fall inward and crash into the nucleus, producing a
flash of violet light called the violet catastrophe. This does
not happen in nature so Rutherford’s model was wrong.
The Bohr Model of the Atom : Electron Energy Levels
• In 1913, Niels Bohr proposed that an atom’s
electrons are arranged in energy levels (shells)
around the nucleus. The electrons moved not in
circling orbits but within the shells. The inner
shell holds 2 electrons maximum, the second shell
holds 8 electrons maximum and the third shell 18.
The Bohr Model of the Atom : Electron Energy Levels
• This model helped to explain the spectra of elements.
Light Waves and Wavelength
• White light is a mixture of many colours of light : ROYGBIV (red,
orange, yellow, green, blue, indigo, violet).
• The different colours of light have different wavelengths.
• Violet light has shorter wavelength while red light has longer
wavelength.
Light Waves and Energy
• Light towards the red end of
the spectrum has longer
wavelengths and lower
energies.
• Light towards the violet end
of the spectrum has shorter
wavelengths and higher
energies.
Bright and Dark Line Spectra
• When an element is
energized (by
heating it or passing
electricity through it),
it gives off a
spectrum which is
made up of just
specific energies of
light.
• When white light
(made up of all light
energies) is passed
through a cool gas of
an element, a
spectrum with dark
lines is produced.
Element Spectra : Unique Like Fingerprints
• Each element has its own unique bright line and
absorption spectrum which identify it like different
fingerprints identify different people.
Hydrogen’s Visible Light Spectrum: Balmer Lines
• A Swiss teacher and mathematician, Johann Balmer discovered a
mathematical pattern in bright lines of hydrogen’s visible light
spectrum. The visible light lines in hydrogen’s spectrum are called the
Balmer series.
What Are Stars Made Of ?
• Spectra of stars are
used to determine
what they are made
of and what gases
surround them.
The Bohr Atom and Spectra
• Electrons in the Bohr Atom
are in specific energy levels
or shells. Each shell has a
specific energy value.
• To move from a lower to a
higher energy level also
requires a specific amount of
energy.
• The hydrogen atom has one
electron in the lowest energy
level.
The Bohr Atom and Spectra
• Electrons can move up to higher energy levels if energy is added to
them. These electrons are said to be excited.
• When electrons drop to lower energy levels (due to their attraction to
the nucleus {- and + charges}), they release or give off energy.
Falling Electrons Give off Specific Energies of Light
• Bright line spectra are
produced when
energized electrons
fall back to lower
levels giving off
specific energies, the
specific colours of
light in the bright line
spectrum.
The Bohr Model of the Atom : Main Ideas
1.
2.
3.
4.
5.
The nucleus is just
1/10,000 of the atom’s
volume. Most of the
atom is empty space.
The nucleus contains
99.9% of an atom’s
mass/weight.
The nucleus contains
positive protons and
neutral neutrons which
are 1832 times heavier
than electrons.
The negative electrons
are found in energy
shells around the
nucleus.
The number of
electrons is always
equal to the number
of protons
The Sodium Atom has 11 electrons (and protons)
1.
2.
3.
The first shell (n=1) can hold
only 2 electrons (2*(1)2.
The second shell (n=2) can
hold 8 electrons (2*(2)2.
The third shell (n=3) can hold
18 electrons (2*(3)2 .
The general rule for the number of
electrons that a given energy
level can hold is 2 n2 where n
is the number of the energy
level, a natural number
(1,2,3,4,etc.)
Visualizing an Atom
If an atom were blown up in size to the size of a football field, the nucleus
(protons and neutrons) would be the size of a single pea on the 50 m
(yd) line and the electrons would be like salt grains buzzing around the
field in their shells.
How Small is an Atom?
• In an “average” grain of salt from a salt shaker, there are 1.2 X 1018
atoms or 1,200,000,000,000,000,000 atoms.
How do the Atoms of Different Elements Differ?
• The number of positive protons in the nucleus (the atomic number) of
an atom determines what kind of atom it will be (what kind of element it
will be).
Isotopes: Atoms of the Same Kind That Differ
• Isotopes are atoms of
the same kind (have the
same number of protons
(atomic number) but
different numbers of
neutrons (different mass
numbers/different
weights).
Symbols and Names of Isotopes
When naming an isotope, the
mass number is given with a
hyphen after name.
In the diagram to the
right, from left to right
are lithium-6, lithium-7,
and lithium-8
The Quantum Mechanical Model of the Atom
• A number of persons working from 1925 - 1928 eventually produced
the modern highly mathematical and complex model of the atom called
the Quantum Mechanical Model of the Atom. This is the model still
accepted today. Electrons move rapidly within three-dimensional
shapes called orbitals. Many features of the Bohr atom are kept in the
Quantum Mechanical Model of the atom.
Review of the Models of the Atom from 1800 - 1940
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