Both these factors favour the addition of an extra electron due to higher force of attraction by the nucleus for the incoming electron. This is due to decreases in size and increase in a period. In general, electron gain enthalpy becomes more and more negative from left to right in a period.Thus, the electron gain enthalpies of third periodic elements, sulphur and chlorine, have negative values than corresponding elements oxygen and fluorine. electron density is high and the addition of electron is not easy. As a result, there is a strong inter-electronic repulsion when extra electrons is added to these atoms, i.e. This is due to small size of the atoms of oxygen and fluorine. 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 factor is predominant in comparison to other factor. This is due to increase in size on moving down the group. The electron gain enthalpies, in general, become less negative in going down from top to bottom 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. VARIATION OF ELECTRON AFFINITY IN A GROUP: Both these factors result into greater attraction for the incoming electron, therefore electron affinity in general increases in a period from left to right. On moving across the period the atomic size decreases and nuclear charge increases. VARIATION OF ELECTRON AFFINITY IN A PERIOD: 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.
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. 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.Įlectron affinity Nuclear charge. FACTORS AFFECTING ELECTRON AFFINITY Nuclear Charge: Units of Electron affinity is Kilo joules per mole (KJ/mol) of atoms or electron volts per atom (eV/atom). (EA-I) is exothermic whereas, (EA-II) is endothermic. 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. The electron affinity of chlorine is 349 KJ/mol. 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.
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