DEVELOPMENT OF THE ATOMIC MODEL From Democritus to Rutherford

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DEVELOPMENT OF THE ATOMIC MODELFrom Democritus
to Rutherford

• Mr. B. McHam
• McKinney H.S. 2006

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c.400 B.C.Ancient Greek Philosopy

• Everything in the universe is made of one or more
  of the basic elements
• Earth, Fire, Water, Air

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What makes up the elements?

• ARISTOTLE
• Matter is infinitely divisible no matter how
  small a piece is, it can always be divided into
  smaller pieces

• DEMOCRITUS
• There exists a smallest piece of matter, which
  cannot be divided any further.
• These pieces are called at?µ?s, or atoms

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Whose Argument Prevails?

• Aristotles viewpoint enjoyed the support of most
  of the world because he was more well-known and
  because Democritus had no evidence to back up his
  claim since these atoms would be too small to
  see.

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1803 John Dalton

• Dalton was an English schoolteacher
• Began teaching mathematics and chemistry at the
  age of 12
• Revived the idea of Democritus smallest piece
  of matter

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Daltons Atomic Model

• All matter is made of tiny particles called
  atoms
• Atoms are indivisible and indestructible
• Atoms of the same element are identical
• Atoms of different elements differ in some
  fundamental way
• Atoms combine in simple whole number ratios to
  form compounds

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Why did people believe Dalton?

• He had EVIDENCE!
• Dalton found that compounds always contained the
  same mass ratio of one element to another
• This is now known as the Law of Definite
  Proportions
• Elements could combine in different ratios, but
  when they did, they would make different
  compounds
• This is now known as the Law of Multiple
  Proportions

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So what?

• These results could only be explained by assuming
  that matter was made of atoms tiny building
  blocks and that these atoms only came in
  certain sizes.

Daltons View of an atom
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Hooray, Dalton!

• Besides giving us his Atomic Theory, Dalton did
  much more in the newly-emerging field of
  chemistry
• He worked with mixtures of gases and determined
  how the pressure was related to their proportions
• He made a list of all the known elements from
  lightest to heaviest

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1869 Dmitri Mendeleev

• Russian chemist
• Arranged elements in tabular form so that
  elements with similar properties were in the same
  column
• When listed in order by mass, elements generally
  repeat properties in groups of 8 (Law of Octaves)

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The First Periodic Table

• Most tables at the time listed elements by mass
• Mendeleev also arranged elements by mass, but
  left several holes in his table and
  occasionally reversed the order of elements to
  fit the properties of others in that column
• The holes were later filled in with newly
  discovered elements that had the properties
  predicted by Mendeleevs table.
• The reason for the reversal of elements was
  explained later by Henry Moseley, who noted that
  the elements were in order by atomic number
  (number of protons) rather than by mass

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1897 J.J. Thomson

• English physicist
• Worked with Cathode-Ray Tubes (CRTs)
• Credited with the discovery of the electron

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CRTs

• Mysterious particles emanated from the cathode
  end
• These particles were deflected by magnetic and
  electric fields
• They were very small and negatively charged

These particles were called electrons and were
assumed to be a part of all matter
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Thomsons Atomic Model
Electrons
Positively charged goo
A.K.A. the Plum-Pudding Model
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1909-11 Robert Millikan

• Set out to discover the charge of a single
  electron
• Famous experiment called the oil-drop
  experiment
• Using his results and the charge-to-mass ratio
  from Thomson, the mass of an electron was found

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The Oil Drop Experiment
To view an animation of this experiment click
below
OIL DROP EXPERIMENT
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1910 Ernest Rutherford

• Expert in radiation
• Famous Gold-Foil Experiment
• Discovered the presence of the nucleus by firing
  alpha particles at a sheet of gold foil

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The Gold Foil Experiment
To view an animation of this experiment click
below
GOLD FOIL EXPERIMENT
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Rutherfords Atomic Model
Electrons
Empty Space
Nucleus Positively charged Made of protons
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1932 James Chadwick

• A fellow researcher with Rutherford, Chadwick
  discovered years later that the nucleus was not
  made of only one particle the proton but of
  two particles.
• This second particle was called the neutron
  because it had no electrical charge