Question

Within each pair of elements F and Cl , S and Se and Li and Na respectively, the elements that release more energy upon an electron gain are:
A) F, S and Li
B) Cl, S and Li
C) Cl, Se and Na

Answer 1

Electron gain enthalpy is the energy released due to addition of electrons. If conditions are not in favor of gaining electrons then the electron gain enthalpy will be less negative.

Complete answer:
Electron Gain Enthalpy: Electron gain enthalpy is the energy required to add an electron in a neutral atom. Because, in most of the cases atoms release energy, the electron gain enthalpies are negative.
Factors on which electron gain enthalpy depends are:

Effective Nuclear Charge: Electron gain enthalpy is directly proportional to nuclear charge. With increase in effective nuclear charge, electron gain enthalpy increases.
Atomic size: Electron gain enthalpy is inversely proportional to atomic size, with increase in atomic size, value of electron gain enthalpy decreases.
Electron gain enthalpy across the period:
As we move from left to right in the periodic table the electron gain enthalpy (its negative value), in the same period, increases. As we move across a period from left to right the atomic size decreases and the nuclear charge increases. So both these factors tend to increase the attraction by the nucleus for the incoming electron. Therefore, electron gain enthalpy becomes more and more negative in a period from left to right.
Electron gain enthalpy down the group:
The electron gain enthalpy becomes less negative as we move down a group. As we move down a group, both the atomic size and the nuclear charge increases. But the effect of the increase in atomic size is much more pronounced than the nuclear charge. So with the increase in atomic size, the attraction of the nucleus for the incoming electron decreases. Therefore, the electron gain enthalpy becomes less negative.
In the first pair, that is F and Cl size of Fluorine F is very small due to which after electron gaining results in more electron - electron repulsion which results in less negative electron gain enthalpy than that of Chlorine Cl . So more energy will be released in case of chlorine Cl. Going against the above rule. So it is a case of exception.
Therefore Cl > F
In the second pair, that is S and Se , more energy will be released in the case of S as down the group release of energy during electron gain will decrease. Size of the atom increases which results in a lesser tendency of gaining electrons.
Therefore S > Se

In the third pair, that is Li and Na, more energy will be released in the case of Li as down the group release of energy during electron gain will decrease because the size of the atom increases which results in less tendency for gaining electrons.
Therefore Li > Na

So the correct option is (B).

Note: It is to be noted that if the size is very large or very small then electron gain in that atom will result in less energy release as they won’t be having a tendency to gain electrons.
The electron gain enthalpy for halogens is highly negative because they can acquire the nearest stable noble gas configuration by accepting an extra electron.