Atomic Line Spectra

The spectrum of electromagnetic radiation that we looked at in the last set of notes is called the "continuous spectrum" because it contains 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 prism 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 else pure we do not get a continuous spectrum.  When an electric current discharge passes through the gas the electric current excites, or 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 state.  When a beam of this light is passed through a prism or a spectrometer we do not see a continuous spectrum.  Instead, only a few colours are observed and these are in a series of individual lines.  This series of lines is called the element's atomic spectrum.

Different elements produce different spectra. This different spectra 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 between the frequency of light and its energy.  Atomic spectra show us that 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 do the math you'll see that the frequency is then 4.57 X 1014 Hz.   Using the Planck's constant equation it can be determined that each photon of this light carries 3.03 X 10-19 J of energy.  What is important here is that when hydrogen produces a red line in it atomic spectrum, its frequency is always 4.57 X 1014 Hz and the energy in each photon is always 3.03 X 10-19 J. It is always the same.  This tells us that when an atom is excited and then loses energy, not just any arbitrary amount is lost.  Only certain specific energy changes can occur, which means only certain specific frequencies of light are emitted.

In order to explain this we must use the following model.  In an atom, an electron can have only certain definite amounts of energy 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 electron is quantized, meaning once again that the electron's energy in a 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 the potential energy of a ball on a staircase.  The ball can only come to rest on 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 potential 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 specified by the steps.  So it is with the electrons in an atom.  The electron can only have energies corresponding to the set of electron energy levels in the atom.  When an atom is supplied with energy, as in a gas discharge tube, an electron is raised from a low-energy level to a higher one.  When the electron drops back, energy equal to the difference between the two levels is released and this energy gets emitted as a photon.  Because only certain energy jumps can occur, only certain frequencies can appear in the spectrum.

Go to the Emission Spectra Lab