Download Atomic Theory

The Atomic Theory – p. 82-87
Democritus
 Democritus,
440 B.C.
 Thought matter was made of tiny particles
 Believed these particles could not be cut
into any smaller pieces.
 Called this smallest particle, atom, which
means not able to be divided
Dalton
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British scientist
By the 1700’s scientists knew that elements
combine to form different types of matter
Atoms looked like small, solid marbles
After extensive work, Dalton published his atomic
theory in 1803
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All substances are made from atoms. Atoms
cannot be created, divided, or destroyed
Atoms of the same element are exactly alike, and
atoms of different elements are different
Atoms join with other atoms to make new
substances
J.J. Thomson
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1897 – Made a correction to Dalton’s theory
Discovered that atoms contain negatively
charged particles which he called electrons
Developed the plum pudding model to
describe what he believed an atom to look
like
This model had the electrons located
throughout the atom
Rutherford
 1911-
Rutherford revised atomic theory
 Based on his experiments he proposed
that atoms have a dense small center
which he called a nucleus
 This center contained positively charged
particles which were called protons
 Electrons surrounded the nucleus with
large amounts of space between the
nucleus and the electrons
Bohr
 1913
– Proposed that electrons move
around the nucleus in certain pathways or
energy levels
Modern Atomic Theory
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Electron Cloud Model
Center of an atom very small and very dense.
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Have positively charged particles called protons
Has particles called neutrons that have no
charge
Electrons have a negative charge and move
around the nucleus in orbits. These orbits do not
have set paths so they look like a cloud around
the nucleus