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A Brief History of the
Atomic Theory
460 B.C. : Democritus
Greek Philosopher
Proposed that matter cannot be broken down
At some point you end up with a piece that cannot be
He called the smallest piece an “atom” from the
Greek word atomos, meaning “indivisible”
1807: John Dalton
British chemist
First modern scientist to propose the
existence of atoms
Described it as an invisible,
indestructible, solid sphere
1898: Sir J.J. Thomson
British physicist
Proposed “plum-pudding” model
An atom is a solid mass of
positively charged material with
negative charges (electrons)
scattered throughout
Credited with discovering the
1911: Ernest Rutherford
British physicist
Proved that atoms are
mostly empty space
Discovered the nucleus,
which contains positively
charged particles
First to propose electrons
circle nucleus
1913: Niels Bohr
Danish scientist
electrons move in
different orbits,
or energy levels
1932: James Chadwick
British physicist
Discovered the neutron
Explained why atoms were heavier than the total
mass of protons and electrons
Current Model
Based on Bohr model
Electrons orbit nucleus
in random paths, or
electron clouds