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Scientist
Democritus—440 B.C
John Dalton—1800
J.J. Thomson—1898
Ernest Rutherford—1911
Niels Bohr—1913
History of Atomic Theory
Greek Philosopher who was the first person to suggest the
idea that atoms caused the changes in matter that could be
observed. He also proposed the name “atom”. “Atom”
comes from a Greek word that means “cannot be
destroyed.”
 Thought that atoms:
1. Were always moving
2. Were infinite in number
3. Could be joined together
Conducted experiments on gases that led to the
development of the first atomic theory.
1. All elements are composed of small, solid balls
called atoms,
2. Said that atoms cannot be created or destroyed.
3. Atoms of the same element are exactly alike, but
atoms of different elements are different from each
other.
4. The atoms of two or more elements can join
together to form types of matter called compounds.
First scientist to suggest that atoms were made up of
smaller particles.
 Discovered negatively charged rays when passing
electric currents through gases.
 He found that all atoms have positive centers and
negatively charged particles evenly scattered
throughout. These negative particles are called
electrons.
Discovered atoms were mostly empty space surrounding a
positive center in his gold foil experiment.
 Concluded that the positively charged particles
were contained in a small central core called
nucleus surrounded by electrons.
Suggested the modern atomic theory
 Electrons in an atom move in specific energy levels
and orbit the nucleus.
 Electrons only exist in certain orbits at certain
energy levels.
 Energy levels are a certain distance from the
nucleus and contain a specific number of electrons
(Level 1—2, Level 2—8, Level 3—18)