Question

Arrange them in descending order of their electron gain enthalpy!!
1. O ,S ,Se ,Te ,Po
2. F ,Cl ,Br ,I ,At

Answer 1

Hint: Electron gain enthalpy depends on three factors, the electronic configuration of the element, the atomic radius of the element and its nuclear charge. Thus electron gain enthalpy should be checked by keeping all the factors in mind.

Complete answer:
The correct answer for the first group is this way, S > Se > Te > Po > O. This represents that sulphur will release the most amount of energy when one electron is added to its atom and oxygen will release the least amount of energy when one electron is added to its atom.
The correct answer for the second group is this way, Cl >F >Br > I> At. This represents that chlorine will release the most amount of energy when one electron is added to its atom and astatine will release the least amount of energy when one electron is added to its atom.
Electron gain enthalpy can also be treated as the amount of energy released when one electron is added to the outermost orbit of an element.
Electron gain enthalpy depends on three factors, electronic configuration, atomic radius and nuclear charge. According to the periodic table, the electron enthalpy should decrease while going down the group as the atomic radius increases.
As atomic radius increases, the distance between outermost orbit and nucleus also increases, thus the attraction of those electrons from nucleus will decrease. And therefore, electron gain enthalpy should decrease while going down the group.
But, in the case of oxygen, it has six electrons in its outermost orbit and thus there is electron repulsive force in the outermost orbit. This force exists in other elements too, but due to the small size of oxygen this force is more. Thus, oxygen has lowest electron gain enthalpy in group 1.
In the case of fluorine, it also has a smaller size compared to chlorine and thus due to the electron repulsive force in outermost orbit, it also has low electron gain enthalpy with respect to chlorine. Thus fluorine is second in group 2.

Note:
It is important to check the factor on which the electron gain enthalpy depends, without checking all the factors one can make a mistake. These exceptions are practically observed and then they are written theoretically, thus it shows how important the dependent factors of electron gain enthalpy are. The electron gain enthalpy of astatine is not practically observed as it is radioactive and has low half-life. Thus, its order is decided on the basis of the factor