Page 490 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
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478
QUESTIONS
15 Figure 30.18 shows another energy level diagram. In this case, energies are given in electronvolts (eV). From the list below, state which photon energies could be absorbed by such an atom: 6.0eV 9.0eV 11eV 20eV 25eV 34eV 45eV
Energy / eV
0 –2
–4 –13
–22
–47
Figure 30.18 An energy level diagram – see Question 15.
16 The line spectrum for a particular type of atom is found to include the following wavelengths: 83nm 50nm 25nm
a Calculate the corresponding photon energies
in eV.
b Sketch the energy levels which could give rise to these photons. On the diagram indicate the corresponding electron transitions responsible for these three spectral lines.
Electron energies in solids
So far, we have only discussed the spectra of light from hot gases. In a gas, the atoms are relatively far apart, so they do not interact with one another very much. Gas atoms that exert negligible electrical forces on each other are known as isolated atoms. As a consequence, they give relatively simple line spectra. Similar spectra can be obtained from some gemstones and coloured glass. In these, the basic material is clear and colourless, but it gains its colour
from impurity atoms, which are well separated from one another within the material.
In a solid or liquid, however, the atoms are close together. The electrons from one atom interact with those of neighbouring atoms. This has the effect of altering
the energy level diagram, which becomes much more complicated, with a large number of closely spaced energy levels.
electron energy
many energy levels
energy bands
forbidden gaps
ab
Figure 30.19 a In a solid, the electron energy levels are very close together. b The energy levels form bands with forbidden gaps between them.
Figure 30.19a shows the result. Instead of individual energy levels we have bands of many levels, all close together. In between the bands are gaps with no allowed energy levels.
Figure 30.19b shows a more conventional representation of these energy bands in a solid. An electron can have an energy at any level in one of the bands. However, it cannot have an energy value which lies in the forbidden gap between bands (just as an electron in an isolated atom cannot have an energy which lies between two energy levels).
Band theory and electrical conduction
We can use this band theory of solids to explain why some materials are better conductors than others. In Figure 30.20, the energy bands are shown with green shading where they are occupied and with grey shading where they are unoccupied.
■■ In a metal, one band, known as the conduction band, is only partially filled. The electrons in the conduction band are the conduction or free electrons which give the metal its conductivity, as discussed in Chapter 11.
■■ In an insulator, the conduction band is unoccupied. The band below this, known as the valence band, is fully occupied. An electron whose energy lies in the valence band is bound to an individual atom.