Models of Atomic
Structure
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John Dalton (1766-1844)
• Dalton was the first to define an
element
• Element = a pure substance
made up of one type of particle
or atom.
• Each element has its own
distinct properties and cannot
be broken down into simpler
substances by means of
chemical change
• Dalton’s vision of the atom is
also called the “Billiard Ball”
model because he thought that
the atom was a solid sphere of
matter
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Dalton’s Atomic Theory:
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all matter is made up of small particles called ATOMS
atoms cannot be created, 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
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Dalton’s Theory continued….
• Dalton thought of atoms as tiny
spherical objects which had hooks on
them
• These “hooks” allowed atoms to
combine with other atoms
• This model of an atom has no protons,
electrons, or neutrons
• Today we know that atoms can be
destroyed via nuclear reactions
• Also, there are different kinds of atoms
(differing by their masses, not chemical
properties) within an element that are
known as "isotopes"
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William Crookes (1832-1919):
• Using a gas discharge tube, he
noticed that a positively charged
pinwheel (“cathode”) would rotate
when an electric charge was
passed through the tube,
• Crookes believed that this electric
charge (called cathode rays) must
have mass as well as motion
• These cathode rays were thought
to be made of fast moving objects
which he called ‘corpuscles’ (tiny
bits of matter)
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Cathode Rays
• A cathode ray tube (CRT) is a hollow vessel with an electrode at
either end.
• The tube is evacuated and then partially filled with a gas.
• When high voltage is applied to the electrodes of a cathode ray
tube, a cathode ray flows from the negative electrode (cathode) to
the positive electrode (anode). This causes the gas to glow.
• Since glowing gas originates from the cathode, it is referred to as a
“Cathode Ray”.
• The cathode ray was assumed to be a beam of negatively charged
particles that we now know as electrons.
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The Cathode Ray
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J.J. Thomson (1856-1940)
• Thomson took Crookes’ work
and created a new experiment
• He designed an experiment to
investigate how these charged
corpuscles would move in an
electric field
• He found that the corpuscles
were attracted towards
positively charged plates therefore the corpuscles must be
negatively charged
• Called them ELECTRONS
Video
Demo
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Protons
• Thomson conducted similar experiments, this time
with anode rays and negatively charged plates
• Anode rays were attracted to the negatively
charged plates - therefore they must contain
positively charged particles
• Thomson called these particles PROTONS
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Thomson’s Model
• Thomson pictured the atom as a
combination of protons and
electrons
• He thought that the electrons
(raisins) were embedded in a sphere
of positively charged protons (bun)
• The electrons were thought to be
mobile particles that leave the atom
when energized by an outside
source such as heat, light, or
electricity
• Called the “Raisin Bun” model
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Thomson’s inferences...
• All atoms contain both protons and electrons.
• All protons are identical, all electrons are
identical. However, electrons differ from protons.
• Electrons have a negative charge, protons have a
positive charge.
• An electron has the same amount of charge as a
proton, even though the charges are opposite in
kind.
• A proton has much more mass than an electron.
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The Electron Microscope
How do Electron Microscopes Work?
• Electron Microscopes (EMs) use a
focused beam of electrons instead of
light to "image" the specimen and
gain information as to its structure &
composition
• A stream of electrons is formed and
accelerated toward the specimen
using a positive electrical potential
• This stream is confined and focused
into a thin, monochromatic (single
colour/wavelength) beam.
• This beam is focused onto the sample
using a magnetic lens
• Interactions occur inside the
irradiated sample, affecting the
electron beam
From www. sciencemuseum.org.uk/on-line/electron/section4/sem.asp
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Rutherford’s early work...
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1898 - heard about the discovery of
radioactivity by Marie and Pierre
Curie
Did his own experiments with
radiation and discovered that there
were three different types
Alpha particles - made of matter
(bare helium nuclei) with 4 times
the mass of a proton and two times
the charge of a proton
Beta particles - made of matter, the
mass of an electron and the same
negative charge as an electron
Gamma rays - made of energy with
no mass or charge
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Rutherford’s gold foil experiment
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1909 - Rutherford did further
studies on the atom
Used alpha particles shot from a
chunk of polonium
Alpha particles were shot at a
piece of gold foil
Expected that most of the alpha
particles would pass though the
gold foil and strike the
fluorescent screen set up behind
the foil (why? He assumed that
there were fairly large spaces
between the atoms)
Found that some of the alpha
particles rebounded from the foil
much like a ball would bounce
off of a wall
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Rutherford’s a-particle experiment:
Video
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Radiation
Beta Particles (-)
• Fast moving electrons
Gamma Rays
• Energy wave
Alpha Particles (++)
• 4x the mass of a proton
Rutherford used radiation and his gold foil experiment to postulate
that atoms must contain:
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A Nucleus - a very positive, dense and
massive core
An Electron Cloud - a low density, negative
envelope around the
nucleus
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Rutherford’s Inferences
• There must be a small dense, positively charged
particle within the gold foil (to account for the
rebounding of a small percentage of the particles)
• the atoms must be mostly empty space (to account
for the high percentage of particles that simply
pass straight through the foil)
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Rutherford’s Conclusions...
• Atoms are mostly EMPTY SPACE with a small,
dense, positively charged core which he called the
NUCLEUS
• The nucleus was surrounded by a clouds of
electrons that was very large in volume, but very
light in mass compared to the nucleus
• This cloud of electrons was negatively charged
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Rutherford’s Conundrum
• Rutherford found that gold
atoms have 79 protons in their
nuclei, but that this made up
less than half of the mass he
calculated for their nuclei
• Thought that the protons could
not be alone in the nucleus and
that there had to be another type
of particle that was uncharged
(“neutral”) in the nucleus
• These neutral particles would
have roughly the same mass as
a proton
• Rutherford’s inference was
confirmed in the 1930’s by
James Chadwick’s discovery of
the NEUTRON
• Neutrons are important to the
structure of the atom because
they counteract the repulsive
forces between the protons
within the nucleus
• If there were no neutrons, all of
the protons in the nucleus
would repel each other and the
nucleus would fall apart!
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Niels Bohr
• So far we have seen that
Rutherford’s experiment gave
rise to the nuclear model of the
atom
• In 1912, Niels Bohr began
working with Ernest Rutherford
• He believed that Rutherford’s
model was incomplete
• Why weren’t the negatively
charged electrons attracted to
the positively charged nucleus?
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Bohr’s Experiments…
• Bohr looked at light
emitted from gas
discharge tubes,
specifically hydrogen
• He noticed that the light
spectrum for hydrogen
showed only four lines,
rather than a continuous
spectrum
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Bohr’s Conclusions…
• Bohr decided that the only way the spectrum could show
four lines would be if the electrons were forced to stay in
fixed orbits, much like the planets around the sun
• The only way the atom could give off light would be if the
electrons jumped from one orbit to another
• Bohr decided these regions were three dimensional and
sphere like
• These regions are known as electron shells
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The Bohr-Rutherford Model
• This model combines the
ideas of both Rutherford
and Bohr
• It suggests that the atom
contains a dense positive
core (the nucleus) and that
the electrons exist in orbits
or electron shells
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Introducing Isotopes
• We have seen that atoms of the same element are all
similar, each having the same number of electrons,
protons, and neutrons
• This would also tell us that the atomic mass of each of
these elements would be the same
• However, scientists studying radioactivity found that some
substances had different atomic masses, while maintaining
the same chemical properties
• They concluded that these must be different atomic forms
of the same element
• These atoms that differ in atomic mass are called Isotopes
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Isotopes
• Atoms with the same number of electrons in their
orbits, and protons in their nuclei, but different
numbers of neutrons
• Since these atoms have the same number of
protons, they have the same atomic number and
are atoms of the same chemical element, but
because of the different number of neutrons they
differ in their mass number
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Isotopes of Hydrogen and Carbon
6 Protons 6 Neutrons
6 Protons 7 Neutrons
6 Protons 8 Neutrons
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Half-life
• The half life of an isotope is the amount of time it
takes for half of the atoms to decay into a more
stable form
• There are many naturally occurring isotopes that
exist around us because their half-lives are longer
than the age of the earth!
• E.g Uranium – 238 has a half life of 4.5 billion
years
• Most isotopes have short half lives and must
therefore be made in the lab to study or use
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Radioactive decay
• Alpha decay
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Many nuclei are radioactive. This
means they are unstable, and will
eventually try to decay to more
stable isotopes
They do this by emitting a particle
which allows them to enter a lower
energy state
This process occurs until a stable
nucleus is reached
There are 3 common types of decay
– alpha, beta, and gamma
The difference between them is the
particle that is emitted by the
nucleus
– Nucleus emits an alpha particle
(helium nucleus)
– Alpha particles are very useful
because the don’t travel far in
air before being absorbed
• Beta decay
– Most often emits an electron
– Neutron is changed into proton
– More penetrating than alpha
particles, but much less than
gamma particles
• Gamma decay
– Nucleus changes from high to
low energy level
– Energy is released as light
– Very penetrating
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Carbon Dating
"Radiocarbon (C-14) Dating," Microsoft® Encarta® Encyclopedia 2000.
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