LINE SPECTRA

Over the nineteenth century, physicists and chemists were increasingly aware of the importance of "line spectra" in the identification of elements. Robert Bunsen was an early spectroscopist and his assistant developed the "Bunsen Burner" to assist in his work. Fraunhofer in the early nineteenth century noticed black lines superimposed on the Sun's spectrum.

To create a "line spectrum", all that is needed is an excited gas, the method of excitement can be

• heat
• electricity ( the passage of electrons )
• light

The lines are the images of the slit of the spectroscope in the various colours of the spectrum.

Two types of line spectra exist

• Emission spectra - these are created when a gas is energised by some means - heat, electricity or light and the gas emits only certain colours which absolutely identify the ion or atom.
• Absorption spectra - these occur when when a gas cloud intercepts the light from a hot body and scatters those same colours as above. As less light now goes straight ahead, these colours are darker than the background full spectrum.

Until the twentieth century, they were a complete mystery.

Some organisation was found in the hydrogen spectrum by Balmer then Rydberg reorganised the equation whereby patterns in the infrared and ultraviolet were predicted.

Rydberg's variation is 1/λ = R ( 1/2² - 1/n² ) from which a generalization of 1/λ = R ( 1/m² - 1/n² ) m,n integers, was made. R = 1.097 x 10⁷ m^-1 is the Rydberg Constant

If m = 1, then for n = 2,3,4,...... we have a series of lines in the UV discovered by Lyman.

m = 2, for n = 3,4,5,.... we have the optical series Balmer originally "explained".

m = 3, for n = 4,5,6,..... we have an infrared series discovered by Paschen

and so on.

The big break through started with Ernest Rutherford's discovery of a "nucleus" (1911) in an atom using Marie Curie's newly discovered radium as a source of the radiation Rutherford called "a" and bombarding very thin gold leaf with them.

Rutherford proposed, in defiance of Maxwell's theory for charges, electrons orbitting a small massive nucleus. This will not work in Maxwell's theory, orbitting charges are accelerating hence will radiate, lose energy and collapse into the centre.This is the beginning of the modern model of the atom.

Niels Bohr (1913) then took up Rutherford's model and mixed into it Einstein's and Planck's E = hf and a radical proposal of his own which restricted the number of or energies of the orbits. What we now call quantum energy levels. The result was a very good modelling of the hydrogen spectrum but increasingly indifferent modelling of higher atomic spectra.

The Hydrogen Atom (Bohr -Rutherford)

Diffraction Gratings