History of
Chemistry
Discoveries and Atoms
Early Greeks
Democritus – all matter is made of
small, indivisible particles called
“atomos”
 Aristotle – matter is continuous and
NOT made of smaller particles

Robert
1st
Boyle (1600’s)
true “chemist”
Discovered
a relationship
between pressure and
volume (Boyle’s Law)
Antoine
Lavoisier
Matter
cannot be created
or destroyed
Law of Conservation of
Mass
Joseph
Proust
 Found
that a given compound
always contains exactly the
same proportion of elements
by mass
Law of Definite Proportions
John
Dalton (1800’s)
 The
ratios of the masses of
elements in a compound can
always be reduced to small
whole numbers
Law of Multiple Proportions
Dalton’s Atomic Theory

1) all matter is composed of tiny particles
called atoms
 2) the atoms of an element are always
identical while the atoms of different
elements are different
 3) compounds form when atoms
combine; atoms combine in small whole
number ratios
 4) reactions involve reorganization of
atoms; the atoms themselves do not
change
Dalton
 Proposed
the “Billiard-ball
model” of the atom
Joseph
Gay-Lussac (1809)
 Measured
the volumes of
gases that reacted with one
another to develop the
Law of Combining Volumes of
Gases
Amadeo

Avogadro
at the same temperature and
pressure, equal volumes of
gases contain the same
number of particles
Avogadro’s hypothesis
J.J.
Thomson
 Produced
a “cathode ray”
which was deflected by a
negative electric field
 Thus
the ray must be made of
negative particles (electrons)
J.J.
Thomson
 Since
atoms are neutral, they
must also have a positive area
 Plum
pudding model
J.J.
Thomson
Protons
were found to be
1836 X the mass of an
electron
Charge of proton is +1
Robert
Millikan
 Oil
drop experiment to
determine the magnitude of
the electron’s charge
which is now known as -1
James
Chadwick
Discovered
high energy
particles with no charge
and the same mass as the
proton –
the neutron
Henri
Becquerel
 Accidentally
discovered
radioactivity
 Alpha particles (+2 charge)
 (Also beta particles, gamma
rays)
Ernest
Rutherford (1911)
 Tests
Thomson’s Plum Pudding
Model by shooting alpha
particles through a sheet of gold
foil
Ernest

Rutherford
Nuclear Model
of the Atom
Robert
Bunsen
 Found
that when heated,
different elements produced
different colors in a flame
Niels
Bohr (1912)
Electrons “orbit” the nucleus
somewhat like planets orbit the sun
 Planetary Model

Arnold

Sommerfeld
Expanded the Bohr model
Electrons travel in orbitals, but
the orbitals are not the same shape
-- this leads to the electron cloud
model of the atom
Electron Cloud Model
Wolfgang
Pauli (1924)
 Predicted
that electrons spin
while orbiting the nucleus
Pauli’s Exclusion Principle says
no two electrons do the exact
same thing at the same time
de Broglie and Schrödinger
 Propose
that electrons move
like wave
thus the Wave-Mechanical
Model
Werner
 No
Heisenberg
experiment can measure
the position and momentum
of a quantum particle
simultaneously
Heisenberg’s Uncertainty
Principle
Modern View of the Atom
 Tiny
nucleus surrounded by
electron “cloud”
 Nucleus accounts for all of the
mass
 Arrangement of electrons causes
different chemical properties
Electron Cloud Model

Note: Just as no map can equal a territory,
no concept of an atom can possibly equal
its nature. These models of the atom
simply served as a way of thinking about
them, though they contained limitations
(all models do).