|
Introduction |
|
As the number of electrons becomes greater than one and as the nuclear charge also increases, the electronic energy levels will change form those corresponding to the hydrogen atom to those corresponding to the atom which is being studied. Before outlining how these levels are determined, we shall describe two principles which help in understanding atomic structure. These are the Pauli exclusion principle (which is rigorously valid in all situations) and the aufbau principle (‘aufbau' means ‘building up' in German). Consider two orbital energy level diagrams as shown below |
| |
|
a)
|
 |
| |
| |
|
b) |
 |
| |
|
|
| |
Figure 6.1 Orbital energy levels of: a) hydrogen atom and b) atoms with more than one electron. |
|
|
|
We have seen that in the case of H-atom, many of these levels are degenerate e.g. (2s, 2p x, 2p y and 2pz ) . In addition to the three quantum numbers n, l and ml , there is a fourth one, ms associated with the spin. The electron spin is commonly represented by an arrow. An arrow pointing up represents ms = + ½ and an arrow pointing down represents ms = -½ . The meaning of ms = + ½ is that an electron with this quantum number has a spin angular momentum of ½  in a given direction . (Usually the direction of an applied magnetic field). In atoms with nuclear charge greater than 1and having more than one electron, the degeneracy of s, p, d, f is removed and this is shown in fig 6.1 (b) |
| |
|
|
