Factors affecting electron affinity
(1) Atomic size
Electron affinity a 1/ Size of atom
Smaller the size of an atom, greater is its electron affinity. As the size of atom increases, the effective nuclear charge decreases or the nuclear attraction for adding electron decreases. Consequently, atom will have less tendency to attract additional electron towards itself. Therefore.
Electron affinity a Effective nuclear charge.
In general, electron affinity decreases in going down the group and increases in going from left to right across the period. On moving down the group atomic size increases and on going from left to right in a period atomic size decreases.
(2) Shielding or Screening Effect
Electron affinity a 1/ Shielding effect
Electronic energy state, lying between nucleus and outermost state hinder the nuclear attraction for incoming electron. Therefore, greater the number of inner lying state, less will be the electron affinity.
(3) Electronic Configuration - The electronic configurations of elements influence their electron affinities to a considerable extent.
Electron affinities of inert gases are zero. This is because their atoms have stable ns2 np6 configuration in their valence shell and there is no possibility for addition of an extra electron.
Electron affinity of beryllium, magnesium and calcium is practically zero. his is attributed to extra stability of the fully completed s-orbitals in them. Thus, if an atom has fully filled or half filled orbitals, its electron affinity will be low.
The electron affinities of Be, Mg and N are almost zero because both Be (Z = 4; 1s2 2s2) and Mg (Z = 12; 1s2 2s2 2p6 3s2) are having s orbital fully filled in their valence shell. Fully filled orbitals are most stable due to symmetry.
Therefore, these elements would be having least tendency to accept electron. Hence, Be and Mg would be having zero electron affinity.
N (Z = 7 ; 1s2 2s2 2px1 2py12pz1) is having half filled 2p-subshell. Half filled sub shells are most stable due to symmetry (Hund's rule). Thus, nitrogen is having least tendency to accept electron. Hence, nitrogen is having almost zero electron affinity.
Electron affinity of fluorine is less than that of chlorine. Although the electron affinity of elements decreases down the group, yet fluorine is having less electron affinity than chlorine because the size of F-atom (0.71 Å) is very small and has only two shells, i.e., n = 1, 2 (9F = 1s2 2s2 2px2 2py2 2pz1).
i) Because of small size of fluorine atom the 2p-subshell becomes compact. There occurs repulsion among electrons of the valence shell and also with electron to be added. This repulsion is responsible for less tendency of F-atom to accept electron.
ii) Because of small size of fluorine there occurs large crowding of electrons
around the nucleus. This crowding is able to screen the nucleus. Because of this, effective nuclear charge gets decreased. Thus, the electron is having less attraction during addition. Hence electron affinity gets decreased.