Question

Highest electron affinity is shown by:
A. ${O^ - }$
B. ${F^ - }$
C. $C{l_2}$
D. ${F_2}$

Answer 1

Hint: Electron affinity is defined as the amount of energy that is released when an electron is added to the outermost shell of a neutral molecule or atom in the isolated gaseous state to form a negative ion. It is also referred to as electron gain enthalpy.

Complete step by step answer:
In order to check the highest electron affinity we have to check the configuration of every option given in the question and the chemical species in which addition of electron takes place will have highest affinity.
In option A,
Electronic configuration of oxygen atom $(O)$ is: $1{s^2}2{s^2}2{p^4}$
When addition of one electron take place to the outermost shell of oxygen atom in the isolated gaseous state to form a negative ion then the electron configuration of ion $({O^ - })$ will be: $1{s^2}2{s^2}2{p^5}$
That means it wants one more electron to complete its octet.
In option B,
Electron configuration of fluorine atom $(F)$ is: $1{s^2}2{s^2}2{p^5}$
When addition of one electron take place to the outermost shell of oxygen atom in the isolated gaseous state to form a negative ion then the electron configuration of ion $({F^ - })$ will be: $1{s^2}2{s^2}2{p^6}$
That means its octet is complete and hence does not need any addition of electrons to complete its octet so it does not have the highest electron affinity.
In option C,
Chlorine molecules already have their complete octet and do not want any electron addition to complete the octet. Hence it does not have the highest electron affinity.
In option D,
Fluorine molecules already have their complete octet and do not want any electron addition to complete the octet. Hence it does not have the highest electron affinity.
From the options only option A needs an electron to complete its octet. So, it will have the highest electron affinity.

Hence, option A is correct.

Note: The decreasing order of electron affinity of halogens are chlorine, fluorine, bromine and iodine. The factors that affect electron affinity are atomic size, nuclear charge and the symmetry of the electronic configuration.