Early Atomic Models

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Early Atomic Models

• Democritus
• World is made of two things empty space and
  indivisible particles, called atoms
• There are different atoms for every material in
  the world

• Aristotle
• Proposed that matter was continuous and not made
  of smaller particles of matter
• He called this continuous flow of matter hyle
• This was accepted until 17th century

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John Dalton (1800s)

• Elements consisted of very tiny particles, called
  atoms, which cannot be broken into smaller
  particles by chemical processes
• All atoms of an element were identical and they
  had the same mass
• Compounds consisted of atoms of different
  elements combined together

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John Daltons Model

• Atoms were tiny, indivisible, indestructible
  particles
• Each one had a certain mass, size and chemical
  behavior

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Dalton used these to prove

• Law of Definite Proportions- specific substances
  always contain elements in the same ratio by mass
• Example NO, NO2, N2O
• Law of Conservation of Matter- matter cannot be
  created or destroyed. A chemical reaction simply
  changes the way atoms are grouped together.

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J.J. Thompson (1890s)

• showed that atoms of any element have negative
  particles with a mass.
• Theorized that positive and negative charges were
  distributed through the atom, called the plum
  pudding model.

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Cathode Ray Tube Parts
Tube- sealed glass tube from which most of the
air is removed. It contains an a metal screen
with material that emits light when struck by
electrons Cathode- negative electrode where
electrons originate Anode- positive electrode
which electrons travel towards For a cathode ray
tube to work you must have power source

• Cathode Ray Tube

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Experiment

• Scientists found that the beam would bend when it
  was in the presence of a magnet. Therefore, he
  concluded that all atoms have particles with a
  mass.
• The beam would bend toward a positively charged
  plate. Therefore he concluded that atoms have
  particles with a charge.

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Experiment continued..

• Finally, Thomson measured the charge to mass
  ratio and found that it was very large. This
  meant the particles were a thousand times smaller
  than an atom.
• He found that charge to mass ratio was the same
  for every element he tried and concluded that all
  elements have this negatively charged particle
  that has mass.
• WE CALL THE PARTICLE THOMSON DISCOVERED THE
  ELECTRON.
• THIS WORK WON JJ THOMSON THE PHYSICS NOBEL PRIZE
  IN 1906.

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The Plum Pudding Model

• JJ Thomson- Protons were found later in a
  modified cathode ray tube
• From this Thomson came up with the plum pudding
  model. This showed that atoms are composed of
  negative charges spread in a positve pudding .

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Other Experiments

• Millikan- Obtained first accurate measurement of
  an electrons charge using the oil drop experiment
• Millikan Experiment
• Roentgen discovered X-rays
• Bequerel discovered radioactivity
• Chadwick- discovered neutrons (1932)

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Lord Rutherford and the Gold Foil Experiment
(1909)

• Used gold foil experiment to find positively
  charged core of the atom which he called the
  nucleus
• Found that the rest of the atom is composed of
  empty space
• Gold Foil Experiment

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Rutherford-Bohr Atom- Planetary Model

• Suggested electrons orbit nucleus the way that
  the planets orbit the sun.
• The more energy an electron has, the farther it
  will exist from the nucleus.
• We use a lot of the same principles in our modern
  day atomic theory, but not that electrons orbit
  the nucleus!