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Atoms
structures of, properties of, how they
affect each other, history of
Chapter 18
Ancient Greeks
• Philosophers
• Democritus –matter is discrete, not
continuous, could not get a smaller piece
• Leucippus called it atomos,
Late 1700s Dalton Atomic Theory
Behavior of matter
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Tiny indivisible particles called atoms
Atoms of the same element are identical
Can combine to form compounds
Chemical reactions occur when atoms are
separated, joined, or rearranged, but atoms of
one element are not changed into atoms of
another by a chemical reaction
Late 1800sThompson
• 1898 discovery of the Electron
• Plum Pudding model-spherical blob of + and –
charges inside. Raisins in pudding
• Basic unit total plus charges = total – charges
*means atoms are electrically neutral
1896 Becquerel discovered
radioactivity -Uranium
• Put uranium by photographic plates
• The plates were exposed and he had pictures
of the rocks.
• Radioactivity is found in many basic elements
• Alpha Αα
• Beta Ββ
• Gama Γγ
Early 1900s Rutherford (Thomson’s
student) famous experiment
• 1911 mainly studying radioactivity
• Gold foil experiment –when alpha particles
(protons) were aimed at a piece of gold foil
surrounded by a fluorescent screen, most passed
through and a few particles were deflected. The
greatly deflected action was not expected
• Concluded that most passed through because the
mass and positive charges were concentrated in
one spot in the atom. The rest was empty space
and passed through easily (most of it was empty
space)
• Called this the nucleolus . This is the part of
the atom that was deflected instead of passing
through the gold foil and was small compared
to the size of the atom.
• Electrons were charged, randomly arranged
around the nucleolus in random motion. They
moves so that they were not pulled into the
nucleolus.