Electron Affinity (EA)
Periodic Classification of Class 10
ELECTRON AFFINITY (EA):
Electron affinity is defined as the energy released in the process of adding an electron to a neutral atom in the gaseous state to form a negative ion.
X(g) + e- → → X- (g) + Energy (E.A.)
Cl(g) + e- → → Cl-(g) + 349 kJ/mol
The electron affinity of chlorine is 349 KJ/mol. The addition of second electron to an anion is opposed by electrostatic repulsion and hence the energy has to be supplied for the addition of second electron.
O(g) + e- → → O-(g) + Energy (EA-I)
O- (g) + e- → → O2-(g) – Energy (EA-II)
(EA-I) is exothermic whereas, (EA-II) is endothermic. Units of Electron affinity is Kilo joules per mole (KJ/mol) of atoms or electron volts per atom (eV/atom).
FACTORS AFFECTING ELECTRON AFFINITY
Greater the magnitude of nuclear charge greater will be the attraction for the incoming electron and as a result, large will be the value of electron affinity.
Electron affinity α α Nuclear charge.
Atomic size :
Larger the size of an atom is, more will be the distance between the nucleus and the incoming electron and smaller will be the value of electron affinity. (E.A. α α 1/Atomic size 1/Atomic size )
Stable the electronic configuration of an atom lesser will be its tendency to accept the electron and lower will be the value of its electron affinity.
VARIATION OF ELECTRON AFFINITY IN A PERIOD:
On moving across the period the atomic size decreases and nuclear charge increases. Both these factors result into greater attraction for the incoming electron, therefore electron affinity in general increases in a period from left to right.
VARIATION OF ELECTRON AFFINITY IN A GROUP:
On moving down a group, the atomic size as well as nuclear charge increase, but the effect of increase in atomic size is much more pronounced than that of nuclear charge and thus, the incoming electron feels less attraction consequently, electron affinity decreases on going down the group.
- The electron gain enthalpies, in general, become less negative in going down from top to bottom in a group. This is due to increase in size on moving down the group. This factor is predominant in comparison to other factor. i.e. increase in nuclear charge.
- The electron gain enthalpies of oxygen and fluorine, the members of the second period, have less negative values than the elements sulphur and chlorine of the third period. This is due to small size of the atoms of oxygen and fluorine. As a result, there is a strong inter-electronic repulsion when extra electrons is added to these atoms, i.e. electron density is high and the addition of electron is not easy. Thus, the electron gain enthalpies of third periodic elements, sulphur and chlorine, have negative values than corresponding elements oxygen and fluorine.
- In general, electron gain enthalpy becomes more and more negative from left to right in a period. This is due to decreases in size and increase in a period. Both these factors favour the addition of an extra electron due to higher force of attraction by the nucleus for the incoming electron.
- Electron gain enthalpies of some of the members of alkaline earth metals, noble gases and nitrogen are positive.This is because they have stable configurations. Alkaline earth metals have stable configurations due to completely filled ns orbital while nitrogen has extra stability due to half filled p-orbital’s (1s2, 2s2, 2p3) i.e, symmetrical configuration. These atoms resist the addition of extra electron as they do not want to disturb their stability.
- Halogens have highest negative electron gain enthalpies. Following two factors are responsible for this:
- Small atomic size and high nuclear charge of halogen in a period.
- Halogens have the general electronic configuration of ns2np5 configuration. Thus, halogens have very strong tendency to accept an additional electron and their electron gain enthalpies are, therefore, high.