|The spectrum of electromagnetic radiation that we looked
at in the
last set of notes is called the "continuous spectrum" because it
the light of all colours. This spectrum is formed when the light
from the sun, or any object is heated to a very high temperature.
(You have of course heard of metals being heated until they were white
hot). This light can then be spread out by passing it through a
onto a screen. A rainbow is a continuous spectrum of visible light that
has been spread out by tiny water droplets suspended in the air.
|If we look at a pure gas like hydrogen or neon or anything
pure we do not get a continuous spectrum. When an electric
discharge passes through the gas the electric current excites,
energizes the atoms of the gas. The gas then releases this energy
in the form of visible light as the atoms return to a lower energy
When a beam of this light is passed through a prism or a spectrometer
do not see a continuous spectrum. Instead, only a few colours are
observed and these are in a series of individual lines. This
of lines is called the element's atomic spectrum.
|Different elements produce different spectra. This
are called the atomic spectra and are unique enough to be considered as
characteristic as a fingerprint.
|The equation E=hv showed the simple relationship
the frequency of light and its energy. Atomic spectra show us
an atom produces only certain characterisitc frequencies and this means
that there are only certain characteristic energy changes taking place
inside the atom. For example, in the atomic spectrum of hydrogen, there
is a red line. That red line has a wavelength of 656 nm. If you
the math you'll see that the frequency is then 4.57 X 1014
Using the Planck's constant equation it can be determined that each
of this light carries 3.03 X 10-19 J of energy. What
important here is that when hydrogen produces a red line in it atomic
its frequency is always 4.57 X 1014 Hz and the energy in
photon is always 3.03 X 10-19 J. It is always the
This tells us that when an atom is excited and then loses energy, not
any arbitrary amount is lost. Only certain specific energy
can occur, which means only certain specific frequencies of light are
|In order to explain this we must use the following
In an atom, an electron can have only certain definite amounts of
and no others. The electron is restricted to certain energy levels and
must use only these levels. We also say that the energy of the
is quantized, meaning once again that the electron's energy in
particular atom can have only certain values and no others.
|quantized - to have a certain specific quantity.
|The energy of an electron in an atom can be compared to
energy of a ball on a staircase. The ball can only come to rest
a step, and on each step it will have some specific amount of potential
energy. If the ball is raised to a higher step, then its
energy will be increased as well. When the ball drops to a lower
step, its potential energy decreases. But the ball cannot stop between
steps. The ball can only rest at the specific energy levels
by the steps. So it is with the electrons in an atom. The
can only have energies corresponding to the set of electron energy
in the atom. When an atom is supplied with energy, as in a gas
tube, an electron is raised from a low-energy level to a higher
When the electron drops back, energy equal to the difference between
two levels is released and this energy gets emitted as a photon.
Because only certain energy jumps can occur, only certain frequencies
appear in the spectrum.
|Go to the Emission