Page 258 - Instant notes

Physical chemistry 244

outside a filled core and such repeating patterns in electronic structure form the basis for
periodicity in the elements (see Topic G7).

Single occupancy rule
2
2
The two 2p electrons in the [He]2s 2p ground state electron configuration of carbon
must be distributed amongst three degenerate 2p orbitals. Each electron occupies a
separate p orbital in order to minimize electrostatic repulsion between the electrons. The
p orbitals point in different directions in space (see Topic G5) so electrons in separate
orbitals are, on average, further apart from each other than if they both occupied the same
orbital. The ground state electron configuration of carbon (Z=6) is therefore more
precisely written as and that for nitrogen (Z=7) as ,
and so on. It is a general rule of the Aufbau principle that electrons occupy different
orbitals of a given sub-shell singly before double occupancy.

Hund’s rule

Hund’s rule states that

the ground state electron configuration of an atom maximizes the number of unpaired
electrons.
For example, the Augbau principle shows that the ground state configuration of carbon

is . Since the two 2p electrons occupy different 2p orbitals both
electrons can have a spin quantum number, m s, of +½ (or both of −½) without violating
the Pauli exclusion principle. Hund’s rule dictates that the two 2p electrons in ground
state C have the same (unpaired) spin. Similarly, all three 2p electrons in the ground state
of N have the same spin. The ground state electron configurations of the first row
elements is shown in Fig. 2.

Fig. 2. Ground state electron
configurations of the first row
elements. The representation is
schematic only and does not indicate
relative energy level separation.