The History of Atomic
Theory
Democritus



Greek philosopher
2400 years ago
The Atom
 Could matter be
divided into smaller
and smaller pieces
forever?
 Or was there a limit to
the number of times a
piece of matter could
be divided?
Atomos



Matter could NOT be
divided into smaller and
smaller pieces forever
This piece would be
indivisible.
“atomos” =“not to be cut.”
Atomos



Atoms= small, hard
particles that were all
made of the same
material
Different shapes and
sizes.
Atoms were infinite in
number, always
moving and capable
of joining together.
Aristotle
Same time period
 Denounced the
idea of atomos
 Earth, Water, Air,
and Fire

Alchemists



Middle Ages
Worked on
turning ordinary
metals into gold
Derived modern
chem
John Dalton
English chemist
 Early 1800’s
 Revisited the
particle theory
 Laid the
groundwork for
Modern theory
 Father of the Atom

Dalton’s Theory
1. All matter  small
particles a.k.a atoms
2. Atoms of the same
element are exactly
alike. Atoms of different
elements are different.
3. Atoms cannot be
divided, created, nor
destroyed
Dalton’s Theory
4. Atoms of different
elements can
combine to form
compounds
5. In chemical reactions,
atoms are combined,
separated, or
rearranged
Known as the “Billiard
Ball Model”
J. J. Thomson
 English
scientist
 1897
 The
electron
 The first to
believe an atom
is made of even
smaller particles.
Thomson Model

Atoms were made
from a positively
charged substance
with negatively
charged electrons
scattered

A.k.a: the “Plum
Pudding Model”
Thomson Model: Cathode Ray

Thomson studied
the passage of an
electric current
through a gas.

As the current
passed through the
gas, it gave off
rays of negatively
charged particles.
Robert A. Millikan



American physicist
1909
Determined the mass
of an electron relative
to the atom
Millikan’s: Oil Drop Experiment



Mist of fine oil
droplets are dropped
into the chamber
Some of the droplets
were negatively
charged
No electricity
electrons fell through:
gravity is only acting
force
Millikan’s Oil Drop Experiment





Voltage adjusted so down
force = up force
Oil drop suspended
between plates
The charge on the droplet
was calculated
Mass of electron =
charge of electron:
charge-to-mass ratio of
electron
Mass = 1/2000 of the
Hydrogen atom
Ernest B. Rutherford
English physicist
 1908
 Gold Foil
Experiment

Gold Foil Experiment: Results


Most of the positively
charged “bullets” passed
right through the gold
atoms in the sheet of gold
foil without changing
course at all.
Some of the positively
charged “bullets,”
however, did bounce
away from the gold sheet
as if they had hit
something solid.
What the Gold Foil Experiment Proves
Niels Bohr
Danish scientist
 1913
 Solar System Model

Bohr Model


According to Bohr’s atomic
model, electrons move in
definite orbits around the
nucleus, much like planets
circle the sun.
These orbits are located at
certain distances from the
nucleus based on relative
energies.
James Chadwick
1932
Neutron: There was unaccounted
mass so there had to be another
subatomic particle
Isotopes: Atoms of the same
element (identical protons) but
different numbers of neutrons and
therefore different masses
Electron Cloud Model
The Electron Cloud Model





Heisenburg Uncertainty Principle: cannot
know speed and location of electron at
same time
Electrons in organized orbitals
Electrons travel so fast that they seem to
form a “cloud” around the nucleus
Location of electron depends on its energy
Light spectrum
Electron Cloud:


Electrons: low NRG are closest to nucleus
Electrons: high NRG are further from nucleus
Historical Models
Indivisible
Greek
X
Dalton
X
Electron
Nucleus
Thomson
X
Rutherford
X
X
Bohr
X
X
Wave
X
X
Orbit
Electron
Cloud
X
X