Question

Which has higher electron affinity $Cl$ or $N{a^ + }$?

Answer 1

Electron Affinity is defined as the amount of energy released when an electron is added to a neutral isolated gaseous atom. It has a unit of ev/atom or KJ/mole. This is an exothermic process for all the non-noble gas elements in the periodic table i.e., the value of electron affinity of these elements is negative.

Complete answer:
Suppose we take an element A which gains an electron and converts to its anion along with the release of energy.
$A + {e^ - } \to {A^ - }$ (first electron affinity is always negative)
Now the anion formed gains another electron but in this case we have to provide energy since due to electron-electron repulsion it is resistant to accept another electron.
${A^ - } + {e^ - } \to {A^{2 - }}$(second electron affinity is positive)
Every chemical entity wants to be stable and hence it loses its energy. Thus all those species which attain half-filled or full filled configuration after gaining electrons have highest values of electron affinity.
Some important observations regarding electron affinity are as follows-
1.Electron affinity increases from left to right in a period.
2.Electron affinity decreases from top to bottom in a group.
3.Halogens possess the highest value of electron affinity since their electronic configuration is $n{s^2}n{p^5}$ and hence need only 1 electron to complete its octet. Among the halogens, Chlorine has the highest value of electron affinity and not fluorine because fluorine is of very small size, so the incoming electron faces a lot of repulsion thus decreasing the value. Thus, the order is-
Cl > F > Br > I
4.The more negative the value of electron affinity the higher the affinity of the atom to the electron.
5.The larger the size the smaller the value of electron affinity.
6.Greater the effective nuclear charge the more the value of electron affinity.

Note:
In the given question, we are given $Cl$ and $N{a^ + }$. Now if we look at the electronic configuration of both these chemical entities-
$Cl - [Ne]3{s^2}3{p^5}$
$N{a^ + } - 1{s^2}2{s^2}2{p^6}$
Here as we see chlorine requires only 1 electron to attain stable octet and hence will readily gain an electron however sodium ion has already lost an electron to attain stable noble gas configuration and hence, we have to provide energy to add electrons and overcome electronic repulsions.
Hence we can conclude that $Cl$ has greater value of electron affinity than $N{a^ + }$.