So far we have thought about the radiation emitted by hot solids but we are now going to broaden it by considering gases. Looking at light emitted by a source through a prism or a diffraction grating can tell us a great deal, not only what the substance is but also much about its atomic structure!
This type of spectrum is formed by the passage of light through a cooler
vapour. The vapour then absorbs those regions of the spectrum which it would have emitted had it
been in an excited state. In the cooler gas most of the atoms are in the ground state (the lowest
possible energy state).
Absorption occurs when the electrons in the atoms absorb energy from
the incoming radiation and then reradiate it in all directions.
In 1814 Fraunhofer discovered that the spectrum of the Sun's
radiation was crossed by hundreds of absorption lines which are called Fraunhofer lines. These are
due to the absorption of light from the centre of the Sun by the outer and cooler layers. Study of
these lines gives the astronomers a way of determining the composition of the Sun.
One of
the simplest of spectra to explain is that of hydrogen and it is what we will consider
next.
If we look at the spectrum of atomic hydrogen we can see that it
is made up of series of lines. This arrangement of lines is unique to hydrogen, other monatomic
gases have a line spectrum but no other element shows the same spectrum as hydrogen and it is
sensible to suppose that the spectrum somehow reflects the atomic structure of the atom that
produced it.
Now in 1913 Neils Bohr suggested that the energy of an electron in a hydrogen
atom is quantised - that is the energy can only have certain values. Each level is given a quantum
number.
When energy is given to the atom an electron can jump up from one level to a higher
level. This transition will only take place if the energy provided is equal to the difference in energy
between the two levels.
Energy may be supplied as either:
(a) radiation
(b)
electrical
(c) a particle beam or
(d) heat
When an electron drops from one level
to another a quantum of radiant energy is emitted and this gives a line in the hydrogen spectrum.
The energy of this quantum is given by the formula:
The bigger the energy difference the greater the energy of the emitted quantum and therefore the higher its frequency. Bohr actually derived a formula giving the energy of these levels and using it we can calculate the values for the hydrogen atom shown in the diagram and the following table.
n | Energy (eV) | Energy (J) |
1 | -13.60 | -2.18x10-18 |
2 | -3.39 | -5.42x10-19 |
3 | -1.51 | -2.42x10-19 |
4 | -0.85 | -1.36x10-19 |
5 | -0.54 | -8.71x10-20 |
6 | -0.38 | -6.06x10-20 |
The energy level for n = 1 is called the ground state. Energy
levels with n = 2 and above are called excited states and the level with n = infinity is known as the
ionisation state.
Notice that all the energies are negative compared with the ionisation state
which has zero energy. This means that an electron in any of the other levels has to gain a certain
amount of energy to reach the ionisation level and so escape from the atom.