Physics - Electron energy levels
Electron energy levels
Sometimes the values of the energy levels show how much energy is required
to move an electron from that level to ionisation (free of the atom) and
the energies are shown as negative.
In other cases the levels may show how much energy electrons have compared
with the ground state. In these cases the energy levels are shown as
In either case it is the difference in the energy levels that is
important. For an electron to move from a lower state to a higher state it
must gain the amount of energy that is the difference between the lower
state and the higher state.
Note that an electron can always absorb a photon  that can give it
enough energy to reach ionisation, even a photon with more than enough
energy. This is the reason that electrons are emitted in the photoelectric
effect experiment. For other transitions, however, the photon energy has to
make an exact match.
For example, in the above diagrams, to move from n = 2 to n = 4, an
electron would have to be given exactly 8.8 - 4.9 = 3.9 eV. A photon that
had slightly more or slightly less energy than 3.9 eV would not be absorbed
by an electron in n = 2 of this atom (unless the difference was sufficient
to cause the electron to move to a different energy level or to
If an electron is in an excited state and it 'falls' back to a less
excited state, a photon will be released with exactly the energy that is
the difference in the energy levels.
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