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Discuss the factors affecting electron gain enthalpy and the trend in its variation in the periodic table.

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Factors affecting electron gain enthalpy: 

(i) Nuclear charge: The electron gain enthalpy become more negative as the nuclear charge increases. This is due to greater attraction for the incoming electron if nuclear charge is high. 

(ii) Size of the atom: With the increase in size of the atom, the distance between the nucleus and the incoming electron increases and this results in lesser attraction. Consequently, the electron gain enthalpy become less negative with increase in size of the atom of the element. 

(iii) Electronic configuration: The elements having stable electronic configurations of half filled and completely filled valence subshells show very small tendency to accept additional electron and thus electron gain enthalpies are less negative.

Variation of electron gain enthalpies in periodic table: 

Electron gain enthalpy, in general, becomes more negative from left to right in a period and becomes less negative as we go from top to bottom in a group. 

(a) Variation down a group: On moving down a group, the size and nuclear charge increases. But the effect of increase in atomic size is much more pronounced than that of nuclear charge and thus the additional electron feels less attraction by the large atom. Consequently, electron gain enthalpy becomes less negative. This is clear from decrease of electron gain enthalpy in going from chlorine to bromine and to iodine. 

(b) Variation along a period: On moving across a period, the size of the atom decreases and nuclear charge increases. Both these factors result in greater attraction for the incoming electron, therefore, electron gain enthalpy, in general, becomes more negative in a period from left to right. However, certain irregularities are observed in the general trend. These are mainly due to the stable electronic configurations of certain atoms. 

Important Trends in Electron Gain Enthalpies: 

There are some important features of electron gain enthalpies of elements. These are:

(i) Halogens have the highest negative electron gain enthalpies. 

(ii) Electron gain enthalpy values of noble gases are positive while those of Be, Mg, N and P are almost zero. 

(iii) Electron gain enthalpy of fluorine is unexpectedly less negative than that of chlorine.

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