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Page 1

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Free Electrons
Outside the atom a free
electron can be given any
Kinetic Energy (limited by
the speed of light).
We can accelerate (or
decelerate) electrons by
applying a potential
difference. This is the
basis of the "electron gun"

Page 2

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Free Electrons
The energy gained by an
electron (of charge e) in a
potential difference V is
exV
So
eV = ½mv2
(N.B. this eV is e x V and
not electronvolts)

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Bound Electrons
So outside the atom an
electron can experience
fractional changes to its
Kinetic Energy.
Inside the atom an
orbiting electron is limited
to a fixed list of allowed
Kinetic Energy values.
So how should we
visualise the atom?

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The Atom




Images like this are badly
out of scale.

Page 5

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The Atom
If you imagine the nucleus
of a typical atom to be the
size of a marble, then the
closest electron will be
orbiting about 2 miles
away (roughly 3km).

Page 6

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The Atom
Obviously the size of
atoms and nuclei vary
greatly from Hydrogen to
Uranium.
Atomic diameter:
60 to 600 pm
(pm = picometres)
Nuclear diameter:
2 to 15 fm
(fm = femtometre)

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Electron Orbits




On this scale, neither
nucleus nor electrons can
be seen.

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Electron Orbits
n=3
n=2



n=4 n=5


Niels Bohr gave each orbit
a number, where n=1
identifies the innermost
orbit.

Page 9

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Electron Orbits
n=3
n=2



n=4 n=5


These orbits correspond to
different amounts of
energy. Electrons can only
occupy these allowed
orbits.

Page 10

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Electron Orbits
n=3
n=2



n=4 n=5


So this orbital diagram can
be redrawn as a more
convenient Energy Level
Diagram.

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