A History of Atomic Theory
Atomic Models
What is a model ?
MODEL: detailed, 3-D representation of an object,
(typically on smaller scale than original)
model airplane
model car
clothing model
Let’s Take a Trip Through
Time!
It Started with the Greeks
DEMOCRITUS
• Lived in Greece 2500
years ago (460-370 BC)
• “Father of modern
science”
Demacritus said: “All matter is
made of atoms that are tiny,
indestructible and indivisible.”
Democritus
Greek banknote with Democritus and drawing of atom
Where did Democritus get his ideas for
small, indestructible, indivisible atoms?
Atoms are small…
new rings
old ring
EVIDENCE: Old gold rings wear away slowly,
getting thinner and thinner, but you never see
gold atoms on your finger, so gold atoms
must be very small!
Atoms are indestructible…
EVIDENCE:
• Earth has been around long time
• Mountains wash away but rocks continue
to exist
• New plants grow where old plants die
Atoms are indivisible…
EVIDENCE: None
 This was HYPOTHESIS made by
Democritus to explain nature as he saw it.

What did Democritus think
atoms looked like?
Appearance of atoms assumed based on
behavior:
• Liquids pour
• Solids are hard and rigid
Liquids pour. Why?
Liquids require a container
If liquid atoms were like little balls they
would roll out when you tip the container
Greek idea of liquid atom
Solids are rigid. Why?
cocklebur
plant
Velcro® fastener
cockleburs stick on
clothing and each other
Velcro hooks and loops
little hooks on cockleburs
Greek idea of solid atom
Aristotle had other ideas
Greek banknote and coin picturing Aristotle
ARISTOTLE: Famous Greek philosopher, born 384 BC.
• student of Plato (another famous philosopher)
• teacher of Alexander the Great (who later conquered
the world)
Aristotle’s idea of matter
Aristotle did not believe
Democritus’s idea of atoms
was correct
Aristotle believed all matter
made from four elements:
Earth
Air
Fire
Water
Aristotle’s idea of matter
Aristotle was more famous than
Democritus, so people believed
him, even though he was
wrong!
Democritus’s idea of atomos
(atoms) was lost for nearly
2000 years until John Dalton
brought it back in 1803
Atom idea lost for ~2000 years
John Dalton, New Atom,1803
Born in England, 1766
Studied chemistry, physics,
and color blindness
Brought back Democritus’s
idea of an indivisible atom
Color blindness
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Foldable
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Dalton’s Theory of Atoms
Five parts (postulates):
# 1:
Elements are made of extremely
small particles called atoms
# 2:
Atoms of given element are identical
in size, mass, & other properties;
Atoms of different elements are
different in size, mass, & other
properties
(later found not to be exactly correct)
#3:
Atoms cannot be subdivided,
created, or destroyed
(later proved wrong)
So according to Dalton’s theory:
atoms are considered to be like
billiard balls
#4:
Atoms of different elements
combine in simple
whole-number ratios to form
chemical compounds
(H2O
2:1 ratio H:O)
#5:
In chemical reactions, atoms
are combined, separated and/or
rearranged
Ex. of a chemical reaction:

+
+
+
2H2O
+
2Na
 2NaOH + H2
Dalton’s Chemical Symbols
We use different symbols today
Matter is electrical!
Michael Faraday
(English chemist, born 1791)
1834: learned matter
interacts with electricity
Realized electricity had to
be made up of particles that
could be counted, but didn’t
know what they were
(electrons)
J.J. Thomson
English physicist (1856-1940)
won Nobel Prize in 1906
1897:
discovered ELECTRON
- studied cathode rays using
Crooke’s tube
- showed atoms were
divisible
Crooke’s tube
cathode |
| cross-shaped anode
– cross-shaped shadow
Cathode rays (stream of electrons) move from metal
cathode (on left) to cross-shaped anode (on right) &
cast cross-shaped shadow on glass (on right)
So … Electrons are particles with negative charge
Cathode Ray Tube Experiment
thought cathode rays were streams of particles smaller than atoms
electric &
magnetic fields
deflect beam of
charged particles
What do we mean by charge?
ELECTRICAL CHARGE:
• property of matter:
• either more or fewer electrons than protons
batteries have (+) and (-) ends
ELECTRONS are negatively charged (-1)
PROTONS are positively charged (+1)
MATTER IS NEUTRAL (no charge):
• (+) charges equal (-) charges
• charges cancel out each other
More About Charge…
opposite charges attract, like charges repel
JJ Thomson’s discovery of
(-) charged electrons proved
that atoms were divisible!
Thomson’s “Plum pudding” atom
– electron
ENGLISH PLUM PUDDING
raisin
RULE: Matter is NEUTRAL (no charge) so… for
every one (-) charged electron there must be one
some thing of (+) charge
we move from
Dalton’s Billard Ball Model
to
Thomson’s “Plum pudding”
model
Thomson’s Plum Pudding Model:
(+) charge evenly spread
out while…
(-) charge found in bits –
like raisins in plum pudding
(or chocolate chips in cookies)
Ernest Rutherford
1871-1937
• 1920: discovered proton
• 1908: won Nobel Prize (Chemistry)
Discovered that most of
mass of atom is in the center
(nucleus)
Rutherford’s Experiment – 1911
Rutherford’s exp’t: animation
Rutherford tested plum pudding
model of atom:
• sent tiny radioactive alpha (a)
particles, like bullets, towards thin
sheet of gold foil
Rutherford gold foil experiment
expected a particles to pass
straight through, like this…
most a particles went
straight through or didn’t
bump into anything
showing most of atom is
empty space
most a particles went straight through, BUT a few
were deflected, meaning a particles must have hit
something:
 really heavy
OR
 (+) charged
gold foil experiment scattering
results showed:
Plum pudding model of atom was
INCORRECT, so…
Rutherford devised new model that
fit his results…
proposed Nuclear Model
Rutherford concluded:
• all atoms have dense, positive (+) center
(center contains most of mass of atom)
• atom is mostly empty space
(except very tiny (–) electrons; but didn’t say where are)
electrons (–)
/
nucleus
/ [protons (+)]
NUCLEAR
model
Rutherford model
did not speculate how
electrons arranged
around (+) center
(name nucleus came later)
Problem with Rutherford’s Model
To prevent (-) electrons from being attracted to
(+) nucleus, electrons must orbit nucleus like
Earth orbits sun
But…
e- need energy to orbit nucleus;
Rutherford didn’t address this
So how BIG is the nucleus
compared to the entire atom?
 If
atom is as big as football
stadium
 nucleus smaller than flea on
50-yard line!
 If
atom as big as period at end
of sentence in textbook,
 it
would have mass of 70 cars!
.
So how big is an atom?

most atoms are 1-2 angstroms across

1 Å = 1 X 10-10 m

6 Billion Cu atoms in a line = less than 1
meter!

Can we “see” an atom?
James Chadwick
1891-1974



worked with Rutherford
1932: discovered neutron
1935: Nobel Prize (Physics)
Niels Bohr 1885 - 1962
created quantized atom
model, 1915
worked on Manhattan
Project
won Nobel Prize in 1922
Bohr’s model: e- only move from one energy level to
another in atom:
• absorb energy when jump from lower to higher
levels
• emit energy when jump from higher to lower levels
Bohr
addressed issue of atom’s electrons:
 nucleus surrounded by e- orbiting at
different energy levels
 e- have definite orbits
Quantum Mechanical Theory
Electron in a Hydrogen atom
Bohr’s new atomic model had quantized
energy levels
 e- only move by jumping levels
(n = 1, n = 2, etc.)
electrons:
• absorb energy when they jump away from nucleus
• emit energy when they jump towards nucleus
Bohr’s Planetary Model
•
electrons travel only in specific orbits
•
each orbit has definite energy
•
inner most orbit (n=1): least energy
•
outer most orbit (n=7): most energy
•
atoms emit radiation when e- jumps from outer
orbit to inner orbit
•
e – in outermost orbits determine atom’s
chemical properties
Erwin Schrödinger Wave Model
Austrian scientist (1887-1961)
• 1933: won Nobel Prize (Physics
• calculated wave model of
hydrogen atom (1926)
• atomic model called:
• modern model atom model
• quantum mechanical model
• wave model
• cloud model
Schrödinger’s wave model of atom:
• e- behaves as energy wave AND as matter particle
(light also behaves as particle and wave)
Einstein had predicted that energy and matter were
related in his equation E = mc2
If we could see an electron it
might look like this “cloud”
Modern/Wave/Cloud Model
 electron’s
energy is quantized (specific values)
 electrons in probability zones called “orbitals”,
not orbits
- location cannot be pinpointed
 electrons are particles & waves at same time
 electrons move around nucleus at speed of
light (3 x 108m/sec)
Orbitals
A Missing Particle – The Neutron
1932: James Chadwick
• discovered last major piece of
atom: neutron
(“neutral”, no charge)
1935: won Nobel Prize (Physics)
• neutron& proton weigh about
same while electron weighs
almost nothing
(1891-1974)
The Modern Model
an atom is:
• mostly empty space
• nucleus: most of atom’s mass
• nucleus contains protons &
neutrons
• electrons in energy levels
around nucleus
• electrons jump between levels,
emitting & absorbing energy
as jump
Atomic Theory
JJ
Thomson
Democratus
400 BC
1803
John
Dalton
1904
Niels
Bohr
1910
Ernest
Rutherford
1913
1926
Schroedinger
/ Heisenberg
The development of
atomic theory
represents the work
of many scientists
over approximately
100 years
Next Atomic Theory ?
Which one of you will
discover something new to
add to the atomic model of
the atom and win a Nobel
Prize in the future?