Brief History of Atomic Theory

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Brief History of Atomic Theory
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1st atomic models

• In 400 BC, the model looked like a solid
  indivisible ball

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Atomos

• Democritus theory Matter could not be divided
  into smaller and smaller pieces forever
• He named the smallest piece of matter atomos,
  an indivisible particle

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• This theory was ignored and forgotten for
  more than 2000 years!

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Why?
Aristotle Plato thought earth, fire, air and
water were the elements. Alchemy used this model
for about 2000 years.
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Daltons Model

• In the early 1800s, the English chemist
• John Dalton performed experiments that
  confirmed the idea of atoms.

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Daltons Atomic Theory

• All elements are made of indivisible atoms
• Atoms of the same element are identical, but
  differ from atoms of other elements
• Different elements atoms can combine to form
  compounds.
• Reactions occur when atoms are separated, joined,
  or rearranged.
• WHICH OF THESE ARE NO LONGER CONSIDERED TO BE
  TRUE ?

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Thomsons Plum Pudding Model

• In 1897, J.J. Thomson discovered that atoms
  are made of even smaller particles.

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Thomson Model

• Thomson used cathode ray tube
• The ray consisted of negatively charged
  electrons.

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Thomson Model

• He proposed a model of the atom called the Plum
  Pudding model.
• Atoms were made from a positively charged
  substance with negatively charged electrons
  scattered about, like raisins in a pudding.
• Aka raisin pudding model

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Millikan (1909)

• OIL DROP experiment
• Measures charge on electron

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• Rutherfords gold foil experiment (1911) Involved
  firing a stream of tiny positively charged
  particles at a thin sheet of gold foil (2000
  atoms thick)
• Result most of space occupied by atom is empty
  space! Except for small, dense, positively
  charged NUCLEUS.
• Positive particles named PROTONS
• all protons are in the nucleus.

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Rutherford Nuclear Model

• all of an atoms positively charged particles are
  contained in the nucleus. The negatively charged
  particles were scattered outside the nucleus.

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Bohr Model

• In 1913, the Danish scientist Niels Bohr proposed
  that each electron is in a specific energy level.

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Bohr Model

• According to Bohrs atomic model, electrons move
  in definite orbits around the nucleus, much like
  planets circle the sun.

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Wave Model Louis DeBroglie (1924) es have
wavelike properties
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Wave Model Erwin Schrodinger (1927) Quantum
mechanics model es location probability
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Chadwick (1932)

• Discovers particle with same mass as proton but
  no charge the
• NEUTRON

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Indivisible Electron Nucleus Orbit Electron Cloud
Greek X
Dalton X
Thomson X
Rutherford X X
Bohr X X X
Wave X X X