Question

Which of the following arrangements for the three halogens Cl, Br, and I when placed in the order of their increasing electron affinity is correct?
(A) Cl > Br > I
(B) I > Br > Cl
(C) Br > Cl > I
(D) I > Cl > Br

Answer 1

Hint: Electron affinity of any element is defined as the amount of energy released or liberated when an electron is added to a neutral atom. If more energy is released when an electron is added to a neutral atom then that atom has great affinity towards the electron as compared to another atom which has less affinity towards the electron and less amount of energy released when an electron is added to that atom.

Complete Step by Step Solution:
Electron affinity is difficult to measure but we can compare the electron affinity of atoms relative to each other. In the given options, chlorine, bromine, and iodine are given. All three atoms belong to the 17^th group of the periodic table. Thus, we need to find an electron affinity trend going down the group for chlorine, bromine, and iodine on the basis of size.

Down the group, size of the atom increases because of adding a new orbital and the number of electrons in the atom increases. Due to the increase of size, an atom does not tend to attract electrons towards itself easily or can say it will have less affinity (love) towards electrons. Thus, less energy will be released when an electron is added to the last atom (large size) of the group. On the other hand, the first atom of the group has great electron affinity because of its small size.

As chlorine’s atomic number is 17, bromine has 35 electrons and iodine’s atomic number is 53. From this, we can conclude that the size of iodine is greater than bromine, and bromine size is larger than chlorine. Thus, chlorine has a greater electron affinity than bromine, and bromine in turn has a greater electron affinity than iodine (Cl > Br > I) because of increases in size of elements down the group.
Thus, the correct option is A.

Note: Along the period, one electron increases, and one proton also increases (nuclear charge) but the electron is added to the same orbital as before. Thus, the effective nuclear charge increases. While going down to the group, the nuclear charge increases with an increase in the number of electrons but this time electrons are added to a new orbital. Thus, the effective nuclear charge will be less, this is also the reason why the last atom of the group has less electron affinity.