Atomic theory

1
Atomic theory

• The structure of the atom

2
Early ideas of matter

• Greek philosophy

Believed matter was composed of 4 basic elements.
(included warm, cold, wet and dry)

• Earth
• Air
• Fire
• Water

3
Modern atomic theories

• Democritus (460-370 B.C.)

Suggested all matter is made of small particles
called atoms.
Start of atomic theory. (not well accepted)
4
Modern atomic theory

• Proust (1799)

Proposed the law of definite proportions.
Elements are always in the same proportion by
mass in any sample of the same compound
Carbon dioxide is always the same mass. 27
carbon 73 oxygen
5
Modern atomic theory

• Dalton (1766-1844)

Daltons atomic theory.
All matter is atoms
Atoms cannot be divided.
All atoms of one element are alike, but they are
different from atoms of any other element.
6
Modern atomic theory

• Conservation of mass

Atoms are never created or destroyed in normal
processes, they just combine in different ways.
7
Atomic structure

• The electron

The negatively charged electron was discovered in
a cathode-ray tube.
8
Atomic models

• Thompsons model

Atoms are like scoops of chocolate chip ice cream
Electrons are scattered around the positively
charged mass like chocolate chips in an ice cream
scoop.
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Atomic models

• What is the mass and charge of an electron?
• Robert Millikan used an oil drop experiment to
  show the electron to be
• -1.6 x 10-19 Coulombs
• 9.1 x 10-28 grams

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Atomic structure

• Protons

Anode rays produced positive charged streams that
were deflected the opposite direction of cathode
rays.
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Atomic models

• Rutherford

The gold foil experiment.
Showed the charge had to be concentrated.
12
Atomic models

• What is the mass and charge of a proton?
• Chadwick found the mass and charge were
• 1.6 x 10-19 Coulombs
• 1.67 x 10-24 grams
• 1 AMU

13
Atomic structure

• Neutron

Discovery of isotopes led to knowledge of
neutrons.
HOW?
14
Atomic models

• Nuclear model

The protons and neutrons are found in the
relatively dense nucleus and most of an atom is
empty space.
The electrons are around the nucleus in different
energy levels.
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Atomic models

• What is the electron cloud model?
• This is the current model.
• More to come later

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Atomic models

• How are atoms described?
• Atomic number tells you the protons in an atom.
• Mass number tells you protons neutrons in an
  atom. (isotopes have different mass numbers)
• Charge tells you the protons compared to the
  electron. (ions have charges)

17
Atomic models

• What happens when the nucleus is not stable?
• When the nucleus changes, it is called
  radioactive decay.