The History of Atomic Theory

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The History of Atomic Theory

• Mr Nelson

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Democritus
400 BC

• The Greek philosopher Democritus began the search
  for a description of matter more than 2400 years
  ago.
• He asked Could matter be divided into smaller
  and smaller pieces forever, or was there a limit
  to the number of times a piece of matter could be
  divided?

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Atomos

• This piece would be indivisible.
• He named the smallest piece of matter atomos,
  meaning not to be cut.

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Why?

• The eminent philosophers of the time, Aristotle
  and Plato, had a more respected, theory.

Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter.
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Daltons Model

• In the early 1800s, the English Chemist John
  Dalton performed a number of experiments that
  eventually led to the acceptance of the idea of
  atoms.

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Daltons Model

• 1803
• Daltons Model was that atoms are indivisible
  particles.

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Daltons Theory

• He deduced that all elements are composed of
  atoms.
• Atoms of the same element are exactly alike.
• Atoms of different elements are different.
• Compounds are formed by the joining of atoms of
  two or more elements.

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J. J. Thomson

• In 1897, the English scientist J.J. Thomson
  provided the first hint that an atom is made of
  even smaller particles.

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Thomson Model

• Thomson studied the passage of an electric
  current through a gas. Using a CRT.
• As the current passed through the gas, it gave
  off rays of negatively charged particles.

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Thomson Model
Where did they come from?

• the atoms of the gas were uncharged.
• Where had the negative charges come from?

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Thomson

• Thomson concluded that the negative charges came
  from within the atom.
• Thomson called the negatively charged
  corpuscles, today known as electrons.
• Since the gas was known to be neutral, he
  reasoned that there must be positively charged
  particles in the atom.
• But he could never find them.

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Thomson Model

• Plum Pudding model.
• Atoms were made from a positively charged
  substance with negatively charged electrons
  scattered around

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Rutherfords Gold Foil Experiment

• In 1908
• English physicist Ernest Rutherford, began work
  on his gold foil experiment.

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Rutherfords Hypothesis

• Rutherford was trying to verify Thomsons model.
• He expected positively charged alpha particles to
  go straight through a piece of very thin gold.

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What Happened

• Most alpha particles passed straight through the
  gold foil
• A small percentage (1/8000) were deflected at
  large angles or returned to the source
• http//micro.magnet.fsu.edu/electromag/java/ruther
  ford/

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Rutherfords Experiment

• There are 2 reasons alpha particles deflected
• Density of the nucleus
• Repelling charges

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Rutherfords Conclusion

• An atom has
• a small, dense, positively charged center that
  repelled the positively charged alpha particles.
• Named the center of the atom the nucleus
• The nucleus is tiny compared to the atom as a
  whole.
• This could only mean that the gold atoms in the
  sheet were mostly open space.

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Rutherfords Nuclear Model

• Rutherford reasoned that all of an atoms
  positively charged particles were contained in
  the nucleus. The negatively charged particles
  were scattered outside the nucleus around the
  atoms edge.

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Neils Bohr

• Was a Jewish Scientist in Copenhagen during the
  onset of WWII
• Hitler was interested in his research of the
  atom.
• He was moved to the US to protect his knowledge.

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Bohrs Explanation

• Bohr thought that an electron travelled in a
  specific orbit at a certain distance from the
  nucleus called an energy level and had specific
  amounts of energy.
• Worked well for Hydrogen and Helium

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Nuclear symbols

• In this unit we need to be familiar with this
  type of symbol

A
X
Z
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Hyphen Notation

• Includes an element name a - and a number
• Example Sulfur 32
• This sulfur atom has an atomic mass of 32
• Since Sulfur has ______ protons electrons
• It also has ______ neutrons

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