The History of Atomic Theory
• The discovery of the atom as we know it today
was a progression.
• Many scientists contributed to the
development of modern day atomic theory.
Democritus (~460 BC ~370 BC)
• Greece
• Believed all matter was
composed indivisible
particles
• “atomos”
• Ideas were rejected by
society (but correct)
Aristotle (~322 BC)
• Greece
• Believed everything
in universe was
composed of four
elements: earth, fire,
water, air
• Ideas accepted by
society (but
incorrect)
John Dalton (1766-1844)
• England
• Chemist
Dalton’s Atomic Theory
1. All matter is composed of atoms. Atoms
are indivisible (didn’t know about
protons, neutron, and electrons).
2. All atoms of an element are identical
(didn’t know about isotopes).
3. Compounds are formed by two or more
types of atoms (still true).
4. A chemical reaction is rearrangement of
atoms (still true).
Dalton’s Billiard Ball Model of Atom
• Solid sphere
J. J. Thomson (1856-1940)
• England
• Physicist
• In 1897, Thomson discovered the electron
while studying cathode rays using cathode ray
tubes
Thomson’s
Plum Pudding Model of Atom
Robert Milliken (1868-1953)
• USA
• Physicist
Milliken’s Oil Drop Experiment
• 1909
• Discovered the charge
on an electron
Ernest Rutherford (1871-1937)
• New Zealand
• Physicist
Rutherford’s Gold Foil Experiment
• 1911
• Fired alpha particles at
gold foil and measured
deflection
• Surprised most particles
went straight through
while a few bounced
back
• Discovered nucleus
• Results not consistent
with plum pudding
model
Rutherford’s Gold Foil Experiment
Rutherford’s
Nuclear Model of Atom
• electrons circling in
fixed orbits
Niels Bohr (1885-1962)
• Denmark
• Physicist
Bohr’s Planetary Model of Atom
• Electrons move in fixed
orbits at different
distances from nucleus
Erwin Schrodinger (1887-1961)
• Austria
• Physicist
Schrodinger’s
Electron Cloud Model of Atom
• What we use today
• Protons and neutrons
located in small dense
nucleus
• Electrons move about
nucleus in cloud
James Chadwick (1891-1974)
• England
• Physicist
• In 1932, discovered neutrons