Atomic Theory and Structure

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Atomic Theory and Structure

• what everything is made of

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What is an atom?

• The smallest particle into which an element can
  be divided and still be the same substance
• The smallest unit of of a substance
• And when I say small, I mean small
• A penny has 20,000,000,000,000,000,000,000 atoms
  (20 thousand billion billion)

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If its so small, how was it discovered?

• Not long after Greece finally defeated Persia, a
  Greek philosopher named Democritus did some
  thinking
• If you cut something in half, and then cut that
  in half, and keep going, youll end up with
  something you cant cut

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Over 2000 years later

• John Dalton, a teacher, wondered why hydrogen and
  oxygen always combine in the same way to make
  water (a compound)
• It must have something to do with how single
  atoms interact

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So, whats in an atom?

• Atoms have a nucleus
• Made of protons and neutrons
• The nucleus is very small, dense, and
  positively-charged
• Contains most of the atoms mass
• Outside the nucleus, electrons zip around in
  electron clouds

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Protons

• Particles in the atomic nucleus with a positive
  () charge
• We use the number of protons in an atoms nucleus
  to determine what element it is
• Atomic number

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Neutrons

• A particle in the nucleus with NO charge
• Isotopes are atoms that have the same number of
  protons, but different numbers of neutrons
• Mass Number is the sum of protons and neutrons in
  an atom
• Atomic Mass is the average mass of an elements
  isotopes

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Electrons

• A particle outside the nucleus with a negative
  (-) charge
• Found within electron clouds
• Very small
• 1,800 electrons are about the size of one proton
• Their (-) charge cancels out the protons ()
  charge
• An atom with unequal protons and electrons is
  called an ion
• Either () or (-)

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So what?