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Atomic Theory
Democritus to Rutherford
Democritus - Ancient Greece
Philosophical question: Could matter be
divided into smaller and smaller pieces
Democritus thought that everything in the
universe was made of tiny particles called
“atomos” (atoms)
“atomos” means indivisible, not to be cut
Democritus was a philosopher, not a
He never tested his theory
John Dalton (1766-1844)
We might as well
attempt to introduce a
new planet into the
solar system, or to
annihilate one already
in existence, as to
create or destroy a
particle of hydrogen
Dalton’s Atomic Theory
Matter is made up of atoms
Atoms are indestructible They cannot be
created or destroyed
Atoms of an element are identical and have
same mass*
Atoms of different elements have different
Compounds are formed by combining
JJ Thomson’s work
studied electricity
knew that:
• atoms were neutral
• negative charges from his electric stream
charged particles from individual atoms
termed the particle “electron”
model had positive material to balance the
negative electrons
Ernest Rutherford
1910 - devised a method to test Thomson’s
atomic model
Passed alpha particles (positive charged
particles) through a piece of gold foil.
He predicted the alpha particles would pass
straight through.
1 in every 20 000 bounced straight back.
Some deflected at an angle.
Early Theory Summary
Democritus: named “atom”
 Dalton - 1808: atomic theory
 J J Thomson - 1897: “plum pudding”
model, termed “electron”
 Ernest Rutherford - 1910: atom with a
positive nucleus surrounded by
orbiting electrons