The History of the
Atom
History of the Atom Timeline
Democritus 460 BC
and Dalton 1803 AD
Thomson
Rutherford
Bohr
1897
1912
1913
Modern
Quantum
Cloud Model
post 1930
Marble Model
Plum Pudding
Model
The Nuclear
Model
The Planetary
Model
Black Boxes
Black Boxes are
anything that you
cannot see inside
Like a cell phone…
Or a locked box…
Or an atom…
THE GREEK ATOM
Democritus (around 460-370 BC.),
"a-tomos" and signifies "indivisible".
1.All matter is composed of indivisible atoms, which are
too small to be seen.
2.There is a void, which is empty space between atoms.
3.Atoms are completely solid.
4.Atoms are homogeneous, with no internal structure.
5.Atoms are different in their size, their shape and their
weight.
Early Greek Philosophers
The ancient Greeks (~450 BC) believed there only
four elements:
Earth
Air
Water
Fire
Aristotle
• The idea of the atom was strongly opposed by
Aristotle and others.
• All substances were combinations
of elements and elemental
qualities. Fire, water, earth,
and air.
• The atom receded into the
background.
Democritus’ idea of the atom was largely
ignored until an English schoolteacher did
some experiments over 2000 years later,
he was…
John Dalton
(1766-1804)
Leading to his atomic theory…
Dalton’s Atomic Theory (1803)
1. All matter is made up of small indivisible atoms.
1. All atoms of one element are alike, but they are
different from atoms of other elements
1. In compounds, atoms join in whole number ratios.
(Law Definite Proportions- Proust)
1. Atoms are neither created nor destroyed in any
chemical reaction. (Law of Conservation of MassLavoisier)
1. In chemical reactions, atoms are combined,
separated, or rearranged.
Dalton's Atomic Theory
1. All matter is made up of small indivisible atoms.
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Dalton's Atomic Theory
2. All atoms of one element are alike, but they are
different from atoms of other elements
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Dalton's Atomic Theory
3. In compounds, atoms join in whole number
ratios. (Law Definite Proportions- Proust)
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Dalton's Atomic Theory
4. Atoms are neither created nor
destroyed in any chemical reaction.
(Law of Conservation of MassLavoisier)
5. In chemical reactions, atoms are
combined, separated, or
rearranged.
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John Dalton’s Atomic Theory
Almost right. A good start.
very small
Structure of the atom after Dalton
(ca. 1810)
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Thomson’s CaThode Ray Tube
Experiments and the
Discovery of the Electron
JJ Thomson (1856-1940)
Thomson used the
cathode ray tube to help
establish the identity of
the electron 1897.
Click here to listen
to him talk about it
Gives off electrons
Vacuum
Magnet - redirected the electrons- nobody
knows why the magnetic field bends
J.J. Thomson (1897):
Cathode Rays
Atoms subjected to high voltages
give off cathode rays.
17
•
•
•
•
But, what was that green light?
Was it a light?
Was it a particle?
To test this, he brought a magnet close to the
cathode ray tube to see what would happen.
J.J. Thomson: Cathode Rays
Cathode rays can be deflected by a magnetic field.
Cathode rays are negatively charged particles (electrons).
Electrons are in atoms.
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• So, the magnet caused the cathode ray to
move. What does that tell us?
• Would a magnet affect a light from a
flashlight? (you could try this at home)
• Probably not.
• Therefore,! the cathode ray must be a particle
Thomson also noticed…
That the cathode ray was coming out of the negative
end (cathode) of the tube and going toward the
positive end (anode)
Negative
end
Positive
end
Therefore, because opposites attract, he concluded
that the cathode ray must be negative
He called these particles…
ELECTRONS!
The discovery would alter Dalton’s model of the atom
because now there is something inside it
But, in addition to the negatively charged electrons,
there must be something giving it a positive charge
because the overall charge of the atom is neutral
(not negative)
Let’s look at Thomson’s model of the atom…
Original Model of the Atom
Plum Pudding Model
These newly discovered electrons must have been balanced by some sort of
positive charge. Thomson proposed a 'plum pudding' model, with positive and
negative charge filling a sphere. Even Thomson's student Rutherford, who later
proved model incorrect, believed it at the time.
Thomson’s Plum Pudding
Model of the Atom
He believed the atom was made of
positively charged stuff with negatively
charged particles scattered throughout
Why the “plum
pudding” model?
What is “plum
pudding”?
Plum Pudding is an English dish sort of like bread
pudding with raisins in it.
An American analogy to his atom
would be…
…Is like…
But we’ll still refer to it as the “Plum Pudding” model in class
J.J. Thomson – The Electron
“Plum pudding”
model: Negative
electrons are
embedded in a
positively charged
mass.
Unlike electrical
charges attract, and
that is what holds the
atom together.
Electrons (-)
Positively charged
mass
Structure of the atom after
Thomson (ca. 1900)
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Rutherford and the
Nucleus
Ernest Rutherford
New Zealand physicist
(1871-1937)
In 1909, Rutherford performed the
Gold Foil Experiment.
In it, he shot alpha
particles (very small,
very dense, very fast
particles) at a thin
layer of gold foil.
Radioactivity
• Three types of radiation were discovered by
Ernest Rutherford:
–  particles (positive, charge 2+, mass 7400 times of e-)
–  particles (negative, charge 1-)
–  rays (high energy light)
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• He expected all of the alpha particles to go
straight through
• It would be like if you were shooting bullets at
a cake…all of the bullets (or alpha particles)
would go straight through the cake (or gold
foil atoms)
Ernest Rutherford (1909)
Scattering experiment: firing alpha particles at a gold foil
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Rutherford’s Gold Foil Experiment
Expected- particles to go straight
through the plum pudding with its
diffuse positive charge
Actual- some particles bounced back! 1 in 8000,
Most of the alpha particles went straight
through but a very tiny amount were
deflected at odd angles?
That could only happen if there was
something very tiny in the atom that was
dense enough to deflect the alpha
particles.
Like this…
Rutherford’s Reaction
• The amazed Rutherford commented that it
was "as if you fired a 15-inch naval shell at a
piece of tissue paper and the shell came right
back and hit you."
The Nuclear Atom
Some alpha particles
bounce off the gold
foil. This means the
mass of the atom
must be concentrated
in the center and is
positively charged!
Thomson’s model
could not be correct.
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DISCOVERY OF THE NUCLEUS
Rutherford proved that an atom was
mostly empty space
with a very small, very dense, positively charged
nucleus in it.
Rutherford Video
Ernest Rutherford
The Nucleus and the Proton
The mass is not spread evenly throughout the atom, but is
concentrated in the center, the nucleus.
The positively
charged particles in
the nucleus are
protons.
Electrons (-) are now
outside the nucleus.
Structure of the atom after Rutherford
Planetary Model (published 1911)
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DISCOVERY OF NUCLEONS
Rutherford understood that the nucleus is itself
composed of nucleons. These nucleons are of
two types:
positively charged, it's a proton.
neutrally charged, it's a neutron
The neutron was effectively discovered in 1932
by Chadwick.
James Chadwick – The Neutron
In the nucleus with the protons are particles of similar mass
but no electrical charge called neutrons.
The positively
charged particles in
the nucleus are
protons.
+ nn
Electrons (-) are now
outside the nucleus
in quantized energy
states called
orbitals. (From Niels
Bohr and quantum
mechanics)
Structure of the atom after
Chadwick (1932)
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BOHR ATOM
In order to take account of atomic
stability, Danish physicist, Niels Bohr
created a new model of the atom in
1913..
BOHR ATOM
The orbits of the electrons can't be
just anywhere but are "quantified”;
only certain particular orbits are
permitted for the electron. It's not
until one jumps from one orbit to
another that it can emit (or absorb)
light.
Modern Atomic Model
• Also known as the
quantum atomic model.
• Describes the atom as
mostly empty space with
a tiny massive nucleus with
protons and neutrons at the
center. Surrounding the
nucleus is a cloudlike region of electrons moving too
fast to describe their location in space and time.
Structure of the Atom
proton (+)
neutron
electrons responsible for the
volume and size of
the atom, negatively
charged
10-10 m
10-14
m
nucleus - responsible for the mass
of the atom, positively charged
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What is the relative size of the
atom….?
• An atomic model the size of
Busch Stadium would contain
a pea sized nucleus containing
95.95% of the atoms mass.
• The pea at the pitcher’s
mound would be the nucleus,
and a firefly flying around
outside would be an electron!
• The atom is mostly…..
• Empty space! Nothing!
Subatomic Particles
• Protons and electrons are the only particles that
have a charge.
• Protons and neutrons have essentially the same
mass.
• The mass of an electron is so small we ignore it.
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History of the Atom Timeline
Democritus 460 BC
and Dalton 1803 AD
Thomson
Rutherford
Bohr
1897
1912
1913
Modern
Quantum
Cloud Model
post 1930
Marble Model
Plum Pudding
Model
The Nuclear
Model
The Planetary
Model