chemistry

ELECTRON AFFINITY

The amount of energy released when an electron is added to a neutral gaseous atom is called electron affinity.

Energy is released when the first electron is added to the neutral atom.

The energy is required to add the second electron or e^-s to uninegative ion.

Therefore for most of elements E₁values are negative (energy released) and for all the elements E₂, E₃etc values are positive (energy absorbed).

Even E₁values are positive for some elements due to their stable configurations. E.g.: 'O' group elements, Be, Mg, N.

Numerically I₁of atom 'M' is equal to E₁of M⁺ion. Numerically E₁ of atom 'M' is equal to I₁ of M^–ion. They cannot be determined directly.

They are calculated indirectly using Born-Haber cycle. Electron affinity is measured in kJ mol^–1or eV / atom. Electron affinity depends on

(i)   Size: E.A increases with decrease in size

(ii)Nuclear charge: E.A. increases with increase in nuclear charge

(iii)Screening effect: EA decreases with increase in screening effect.

(iv)Electronic configuration: Atoms with stable electronic configuration have zero or negative electron affinities.

(v) Number of valence electrons in valence shell. E.A increases with increase in the number of valence e^–s.

TREND:

E.A. decreases from top to bottom in a group due to increase in size and increase in nuclear charge. But in most of the groups, the 1^stelement has abnormally low E.A. value than that of the remaining

elements, eg. F < CI, O < S.

EA ↑ from left to right in a period due to ↓ in atomic size and ↑in nuclear charge. In every period halogen has highest E.A value.

E₁trend of few elements: CI > F>Br>I>S>Si.

APPLICATIONS OF E.A.:

(i) We can predict the ability of atom into anion. (ii) Oxidising power of element.

(iii)           We can predict the tightness of binding an electron to the valence shell.