Question

The element with the highest electron affinity will belong to:
(a)- Period 2, group 17
(b)- Period 3, group 17
(c)- Period 2, group 18
(d)- Period 2, group 1

Answer 1

Hint: The element which has the highest affinity belongs to the halogen group. The number of shells in the element is 3. It is a yellowish-green color gas in nature.

Complete step by step answer:
Electron affinity of an element may be defined as the energy released when a neutral isolated gaseous atom accepts an extra electron to form a gaseous negative ion.
Halogens have the most negative electron affinity. This is due to the fact that the valence shell electronic configuration of the halogens is $n{{s}^{2}}n{{p}^{5}}$ and as such they require one more electron to acquire the stable noble gas configuration i.e. $n{{s}^{2}}n{{p}^{6}}$ . As a result, they have a strong tendency to accept an additional electron, and hence their electron affinity is highly negative. However, as we move down from Cl to I, the electron affinity becomes less and less negative due to the corresponding increase in their atomic radii. As the distance of the nucleus from the subshell which receives the additional electron increases, the force with which it is attracted by the nucleus decreases, and hence the electron affinity becomes less negative as we down the group.
The electron affinity of F, is, however, unexpectedly less negative than that of Cl. This is due to its small size. As a result of its small size, the electron-electron repulsion in the relatively compact 2p-subshell is comparatively larger and hence the incoming electron is not accepted with the same ease as in the case of Cl.
Hence, Cl has the highest electron affinity.
So, option (b)- Period 3, group 17 is the correct answer.

Note: You may confuse that option (c) would have the highest electron affinity because they have a fully filled subshell. But due to fully filled subshell, they have the least electron affinity. Don't forget that chlorine atom comes in period 3 and fluorine comes in period 2.