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Theories of the Atom
Early Greek Theories
Democritus (around 400 BC)
No experimental evidence. Proposed that:
matter is made of indivisible particles called _____________
Aristotle (around 350 BC):
supported the idea of _________ ____________
Aristotle's ideas lasted for almost 2000 years
John Dalton (1807)
Dalton's atomic theory
All matter is composed of ____________, ________________ particles called atoms
All atoms of an ____________ have _______________ properties
Atoms are ___________________ to form new substances in chemical reactions,
but are never _______________ or ________________
J.J. Thompson (1897)
Used a cathode ray tube and discovered atoms contain
____________________ charged particles
He called these "_______________ " and his model of the
atom was known as the "raisins in a bun model"
The electrons were considered embedded in a
_______________ "dough"
Ernest Rutherford (1909)
The Gold Foil Experiment
Most of the positive particles went through the foil
but small percentages were deflected at large
Rutherford suggested that:
the center of the atom (called the
____________) has a _______________ charge and contains most of the atom's
the nucleus is surrounded by a cloud of _____________ charged ______________
most of the atom is ____________ ____________
James Chadwick (1932)
discovered undetected ____________ particles in the atom called ____________
neutrons have the ____________ mass as protons
Niels Bohr (1913)
From experimental evidence:
electrons orbit the nucleus
(like planets around the sun)
each orbit holds a certain maximum of
electrons, _____ in the first and _____ in
each of the second and third orbits
This model of the atom is called the
Bohr-Rutherford model
Carbon atom
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