Answer to Question #48940 in Inorganic Chemistry for krishan kumar

Question

First electron affinity of sulphur is - 200 kJ mol -1 but second electron affinity is +565 kJ mol -1. Explain.

Answer 1

Energy is released as an atom becomes more stable. Atoms are most stable, when the outer shell of electrons is full. For atoms of elements beyond Helium, this means 2 electrons in the s and 6 in the p sub-shells. The p sub-shell has 3 orbitals, px, py, pz. Each orbital can hold 2 electrons.
The most stable arrangement of electrons is 2 in the s and 2 in each of the p’s. Sulfur is in row 3, so the most electron configuration of the 3rd shell = 3s2, 3px2, 3py2, 3pz2.
Sulfur’s electron configuration is 1s2, 2s2, 2p6, 3s2 3p4. The outer shell configuration = 3s2, 3px2, 3py1, 3pz1.
The 1st electron affinity is the energy released as 1 electron is added to the outer electron shell of the atom. When 1 electron is added to Sulfur, the outer shell configuration = 3s2, 3px2, 3py2, 3pz1.
S + e- → S-1 + 200 kJ/mol
The 2nd electron affinity is the energy released as 1 more electron is added to the outer electron shell of the atom. When 1 more electron is added to Sulfur, the outer shell configuration = 3s2, 3px2, 3py2, 3pz2.
S-1 + e- → S-2 + 649 kJ/mol
Now, Sulfur has a full outer shell. Now Sulfur is the most stable that is can be. The magnitude of the energy released as electrons are added to an atom is proportional the increase of stability.
S-1 is more stable than S; but S-2 is much more stable than S-2. So, the energy released as the 2nd electron is added an S-1 ion is more than 3 times the energy released as 1 electron is added to a Sulfur atom.