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History of Atomic Theory
 Democritus: Atom meaning “unable to divide”. (440 BCE)
 Dalton’s Atomic Theory: (1803)
o All substances are made of atoms.
o Atoms cannot be created, divided, or destroyed.
o Atoms of the same element are exactly alike. Atoms of
different elements are different.
o Atoms join together with other atoms to make new
substances.
 JJ Thomson: “Plum pudding” Model (1897)
o Discovered the electron.
 Rutherford: “Gold foil” experiment (1911)
o Atom has a positive center (nucleus).
o Atoms are made up of mostly empty space.
 Niels Bohr: Electrons are found in energy levels (1913).
o Compared electrons, orbiting the nucleus in energy levels, to
planets orbiting around the sun.
 Modern Model: Electrons are believed to behave more like waves
on a vibrating string than like particles.
o The best scientists can do is calculate the chance of finding
an electron in a certain place within an atom.