TOPIC: Atomic Theory – how the
model of the atom has evolved and
the scientists responsible
Atomic Theory
JJ
Thomson
Democratus
400 BC
1803
John
Dalton
1904
Niels
Bohr
1910
Ernest
Rutherford
1913
1926
Schroedinger
/ Heisenberg
The Complete Modern Atom
An atom is:
• mostly empty space
• nucleus has most of
mass of an atom
• Atom is divisible
• Protons (+1 charge)
• Neutrons (no charge)
• Electrons (-1 charge)
• nucleus contains protons
& neutrons
• electrons are in energy
levels around nucleus
Here’s how we got there…
It Started with the Greeks
DEMOCRITUS
• Lived in Greece 2500
years ago (460-370 BC)
• “Father of modern
science”
• said: “All matter is
made of atoms that
are tiny, indestructible
and indivisible.”
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!
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
Democritus
(Greek)
All matter is made
of atoms that are
tiny,
indestructible,
and indivisible
(can’t be divided)
Atomos
Aristotle’s idea of matter
Aristotle did not believe
Democritus’s idea of atoms
was correct
Aristotle was more famous than Democritus, so
people believed him, even though he was wrong!
Aristotle believed all matter
made from four elements:
Earth
Air (wind)
Water
Fire
Aristotle
Thought
Democritus
was wrong
Matter made of
4 elements –
Earth
Sun
Air
Water
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
Can you see a number in this gray box?
If not, you may be color blind.
(More males are color blind than females)
Dalton’s Theory of Atoms
Five parts to Dalton’s modern atomic theory:
# 1:
Elements are made of extremely
small particles called atoms
# 2:
Atoms of given element are
identical in size, mass, and other
properties;
atoms of different elements are
different in size, mass, and other
properties
(found out not exactly correct)
#3:
Atoms cannot be subdivided,
created, or destroyed
(later proved wrong)
So Dalton’s atoms are kind of like
billiard balls
#4:
Atoms of different elements
combine in simple wholenumber ratios to form chemical
compounds
(H2O
2 Hydrogen :1 Oxygen)
#4:
Atoms of different elements
combine in simple wholenumber ratios to form chemical
compounds
(H2O
2:1 ratio H:O)
#5:
In chemical reactions, atoms
are combined, separated or
rearranged
2H2O + 2Na  2NaOH + H2
#5:
In chemical reactions, atoms are combined,
separated or rearranged
2H2O  2H2 + O2
+
+
+
Dalton’s Chemical Symbols
We use different symbols today
Dalton
1)
2)
3)
4)
5)
Atoms (tiny)
Atoms of same
element are identical *
Indestructible *
Combine in whole #
ratio (H2O)
In chem rxn. are
combined, separated,
rearranged
*turned out to not be
entirely be true
Billard Ball
J.J. Thomson
English physicist (1856-1940)
won Nobel Prize in 1906
Discovered the ELECTRON
in 1897:
-studied cathode rays using
Crooke’s tube
-showed atoms were divisible
Crooke’s tube
JJ Thomson’s
discovery of the
negatively charged
electrons proved
that atoms were
divisible!
Thomson’s “Plum pudding” atom
– electron
ENGLISH PLUM PUDDING
– raisin
JJ Thomson
Discovered the Electron (first
subatomic particle to be
discovered) using cathode
ray tubes
Proved Dalton #3 to be
wrong
the atom is divisible!
Plum Pudding
Model
Ernest Rutherford
1871-1937
• Discovered the proton in 1920
• Won Nobel Prize in
chemistry in 1908
Discovered that most of
mass of atom is in nucleus
And the rest of the atom is empty
space – GOLD FOIL
EXPERIMENT
Rutherford’s Experiment – 1911
Rutherford’s exp’t: animation
Rutherford gold foil experiment
Rutherford expected a particles
to pass straight through, like
this…
What he found is that most
did go straight through (so
atom is mostly empty
space), BUT a few a
particles bounced back
(Concluded there is a
nucleus)
Rutherford shot a particles, which have a positive
charge, since they bounced back he concluded
that the nucleus must have a _____________
Positive
charge
electrons (–)
/
nucleus
/ [protons (+)]
The
NUCLEAR
atom model
Rutherford
-discovered the proton (+
charge)
- Gold Foil Experiment
- Atoms have a positive
nucleus
- Nucleus contain most
of the mass
- Atom is mostly empty
space
Nuclear Model
+
Chadwick
- worked with Rutherford
- Discovered the neutron
Problem with Rutherford’s Model
OPPOSITES ATTRACT: What
prevents (-) electrons from
being attracted to (+) nucleus,
Niels Bohr 1885 - 1962
Bohr addressed the issue of electrons
in the atom
 Nucleus surrounded by electrons
orbiting at different energy levels
 Electrons have definite orbits
Bohr’s new atomic model had quantized
energy levels, meaning the electrons could
only move by jumping between levels
(numbered n = 1, n = 2, n = 3, etc.)
Bohr
- Electrons travel
around the nucleus
in orbits (shells)
- Gain/Emit energy
when moving
from level to level
Planetary
Model
e-
ee-
+
e-
Erwin Schrödinger Wave Model
Austrian scientist (1887-1961)
• won the Nobel Prize in physics
in 1933
• calculated wave model of
hydrogen atom in 1926
• also called cloud model,
quantum mechanical model,
modern model of the atom
• atom model we still use today
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
In Schrödinger’s wave model of atom, electron
behaves as energy wave as well as a 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”
Schrodinger
- Electrons behave
like waves
- Not located in an
exact orbit but
rather a zone
called an orbital
- Orbital =
probable location
of electron
Wave Mechanical
(Cloud) Model =
Modern Model
The Complete Modern Atom
An atom is:
• mostly empty space
• nucleus has most of
mass of an atom
• nucleus contains protons
& neutrons
• electrons are in energy
levels around nucleus
• electrons jump between
levels, emitting and
absorbing energy as jump