Question

The correct order of electron affinities of S, N, O and Cl is:
A.$O=Cl<S=N$
B.$O<S<Si<N$
C.$N<O<S<Cl$
D.$O<N<Cl<S$

Answer 1

Hint: Electron affinity is also called as electron gain enthalpy, as the name suggests that it is the enthalpy required to add an electron to the gaseous state of an atom to change it into anion. Now this concept depends on the atomic electronic configuration and also on the nuclear charge by which the nucleus attracts the electrons.

Complete step-by-step answer:Electron gain enthalpy or electron affinity is the energy released when an electron is added to the outermost shell of an atom to convert it into ions. So, suppose you have an atom which is equal in number of electrons and protons, now if you add one electron in its outermost shell it will now have more electrons. Thus the ion formed after the electron gain enthalpy process is anion.
The atoms given above are sulphur, nitrogen, oxygen and chlorine. As we know that these are arranged in the periodic table so as that nitrogen is having half stable configuration thus adding electron in nitrogen is very hard thus we need to provide energy for that process. Next we have oxygen and sulphur which are elements are of same group, the difference is that sulphur is having larger size than oxygen thus if we add one electron in outermost of sulphur it feel less repulsion from electrons. This repulsion is higher for oxygen due to its small size and higher electron repulsion. So electron affinity is more in sulphur than oxygen. At last we left with chlorine, an element having highest electronegativity among the given elements so it will have largest electron affinity.
Thus we can write the order of the electron affinity as- $N<O<S<Cl$

Option C is correct.

Note:Nitrogen is having atomic number $7$ and thus its electronic configuration is $1s^{2}2s^{2}2p^3$ there are three electron in p-orbital thus each get one electron. Hence nitrogen gets almost half stable. Therefore it will not have greater electron affinity. This fact of half stability works only in neighborhoods. It means when we have to compare nitrogen, carbon and oxygen these three are neighbors.