Periodicity of electron affinity

Electron affinity is the amount of energy released when an electron is added to an isolated gaseous atom.

Electron affinity is the ability of an atom to hold an additional electron. If the atom has more tendency to accept an electron then the energy released will be large and consequently the electron affinity will be high. Electron affinities can be positive or negative. It is taken as positive when an electron is added to an atom. It is expressed as electron volts per atom (eV per atom) or kilo joules per mole.

Factors affecting electron affinity

• When the nuclear charge is high there is greater attraction for the incoming electron. Therefore electron affinity increases as the nuclear charge increases.

• With the increase in the size of the atom the electron affinity decreases because the distance between the nucleus and the incoming electron increases.

• Electron affinities are low or almost zero in elements having stable electronic configurations (half filled and completely filled valence subshells) because of the small tendency to accept additional electron.

Variation along a period

The size of an atom decreases and the nuclear charge increases on moving across a period. This results in greater attraction for the incoming electron. Hence the electron affinity increases in a period from left to right.

Variation down a group

As we move down a group the atomic size and nuclear size increases. As the effect of increase in atomic size is more pronounced the additional electron feels less attracted by the large atom. Consequently the electron affinity decreases.

However there are some irregularities observed in the above general trend. They are:

The halogens have highest electron affinity because they have only one electron less than the stable noble gas configuration.They have a strong tendency to accept an additional electron. This makes their electron affinity values high.

The electron affinity value of noble gases are zero because of the stable electronic configuration of ns²np⁶ which has no tendency to take in additional electron. No energy is released and their electron affinities are zero.

The electron affinity values for Be, Mg, N and P are almost zero because of the extra stability of completely filled orbitals in Be and Mg and half filled orbitals in N and P.

Electron affinity of fluorine is unexpectedly less than that of chlorine. The low electron affinity value of F is due to the very small size of F atom. This small size results in strong inter electronic repulsions in the relatively compact 2p subshell of fluorine: thus the incoming electron does not feel much attraction. Electron affinity for some third period elements (e.g., P, S, Cl) are greater than corresponding second period elements (e.g., N, O, F) because of the smaller atom size of second period elements, which produces larger electronic repulsions for the additional electron.

Succesive electron affinities

The second electron is added to the negatively charged ion and the addition is opposed by coulombic repulsions. The energy has to be supplied to force the second electron into the anion.

First electron affinity

Second electron affinity

The second electron affinities in which energy is absorbed have negative values while the first electron affinity have positive values as energy is released.

Problem

9. Arrange the following in the decreasing order of electron affinity: B, C, N, O.

Solution

All these elements belong to the same period. The size of an atom decreases and the nuclear charge increases on moving across a period. This results in greater attraction for the incoming electron. Hence the electron affinity increases in a period from left to right O, C, B, N.