Question

Which of the following species has the highest electron affinity?
A.${\text{F}}$
B.${\text{O}}$
C.${{\text{O}}^ - }$
D.${\text{N}}{{\text{a}}^ + }$

Answer 1

Hint: To solve this we must know that the electron affinity and electronegativity follow the same trend in the periodic table. The opposite of electron affinity is the ionization energy which also follows the same trend.

Complete step by step solution: The change in energy when an electron is added to an atom or a molecule in its isolated gases state is known as electron affinity. Adding an electron to an atom or a molecule forms a negatively charged ion.
The addition of an electron to an atom or a molecule is an exothermic process and thus, energy is released in the process. As energy is released in the process, the change in energy is negative.
In the periodic table, as we move from left to the right the electron affinity increases. And as we move down the group, the electron affinity decreases.
Thus, the elements with high electron affinity are present at the top right corner of the periodic table.
We are given four species, ${\text{F}}$, ${\text{O}}$, ${{\text{O}}^ - }$ and ${\text{N}}{{\text{a}}^ + }$.
Now ${\text{N}}{{\text{a}}^ + }$ is a s-block element. We know that as we move from left to the right the electron affinity increases and as we move down the group, the electron affinity decreases. ${\text{N}}{{\text{a}}^ + }$ lies in the third period and thus, the electron affinity of ${\text{N}}{{\text{a}}^ + }$ is lowest.
Now ${\text{F}}$ and ${\text{O}}$ lie in the same period. Also they lie to the top right corner of the periodic table. Thus, they have high electron affinities. But fluorine has higher electron affinity than oxygen.
Now we have ${{\text{O}}^ - }$. The size of ${{\text{O}}^ - }$ is small. Thus, ${{\text{O}}^ - }$ experiences greater repulsion. Thus, ${{\text{O}}^ - }$ has the highest electron affinity.
Thus, the species having the highest electron affinity is ${{\text{O}}^ - }$.

Thus, the correct option is (C) ${{\text{O}}^ - }$.

Note: Electron affinity is the negative of electron gain enthalpy. The amount of energy absorbed or released when a free electron is added to an atom in its isolated gaseous state is known as the electron gain enthalpy.