MODELS OF THE ATOM
Rutherford
Rutherford’s early work...
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1898 - heard about the discovery of
radioactivity by Marie and Pierre Curie
did his own experiments with radiation
and discovered that there were three
different types
alpha particles - made of matter (bare
helium nuclei) with 4 times the mass of
a proton and two times the charge of a
proton
beta particles - made of matter, the
mass of an electron and the same
negative charge as an electron
gamma rays - made of energy with no
mass or charge
Rutherford’s gold foil experiment
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1909 - Rutherford did further studies on
the atom
used alpha particles shot from a chunk
of polonium
alpha particles were shot at a piece of
gold foil
expected that most of the alpha
particles would pass though the gold
foil and strike the fluorescent screen set
up behind the foil (why? He assumed
that there were fairly large spaces
between the atoms)
he was surprised to find that some of
the alpha particles rebounded from the
foil much like a ball would bounce off
of a wall
Radiation:
Beta Particles (-)
• Fast moving electrons
Gamma Rays
• Energy wave
Alpha Particles (++)
• 4x the mass of a proton
Rutherford used radiation and his gold foil experiment to postulate
that atoms must contain:
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A Nucleus - a very positive, dense and
massive core
An Electron Cloud - a low density, negative
envelope around the
nucleus
Rutherford’s Observations and Conclusions
a)
Most alpha particles travelled straight through the gold foil without
any deflection.
Conclusion: most of the atom is empty space
b)
Some alpha particles were deflected to the side as they travelled
through the gold foil.
Conclusion: since the alpha particles were positively charged they must
have been repelled by a positively charged atom centre.
c)
Some alpha particles bounced straight back towards the polonium
source.
Conclusion: there must be a solid, dense core to the atom which these
alpha particles collided with head on and were thus bounced back.
Rutherford’s Conclusions...
• Atoms are mostly EMPTY SPACE with a
small, dense, positively charged core which
he called the NUCLEUS
• the nucleus was surrounded by a clouds of
electrons that was very large in volume, but
very light in mass compared to the nucleus
• this cloud of electrons was negatively
charged
Rutherford’s Conundrum
• Rutherford found that gold
atoms have 79 protons in their
nuclei, but that this made up
less than half of the mass he
calculated for their nuclei
• thought that the protons could
not be alone in the nucleus and
that there had to be another type
of particle that was uncharged
(“neutral”) in the nucleus
• these neutral particles would
have roughly the same mass as
a proton
• Rutherford’s inference was
confirmed in the 1930’s by
James Chadwick’s discovery of
the NEUTRON
 neutrons are important to the
structure of the atom because
they counteract the repulsive
forces between the protons
within the nucleus
 if there were no neutrons, all of
the protons in the nucleus
would repel each other and the
nucleus would fall apart!