Question

Which of the following pairs of elements would have a more negative electron gain enthalpy?

1. O or F
2. F or Cl

Answer 1

(i) $O$ and $F$ are present in the same period of the periodic table. An $F$ atom has one proton and one electron more than $O$ and as an electron is being added to the same shell, the atomic size of $F$ is smaller than that of $O$. As $F$ contains one proton more than $O$, its nucleus can attract the incoming electron more strongly in comparison to the nucleus of $O$ atom. Also, $F$ needs only one more electron to attain the stable noble gas configuration.

Hence, the electron gain enthalpy of $F$ is more negative than that of $O$.

---

(ii) $F$ and $Cl$ belong to the same group of the periodic table. The electron gain enthalpy usually becomes less negative on moving down a group. However, in this case, the value of the electron gain enthalpy of $Cl$ is more negative than that of $F$. This is because the atomic size of $F$ is smaller than that of $Cl$. In $F$, the electron will be added to quantum level $n$ = $2$, but in $Cl$, the electron is added to quantum level $n$ = $3$. Therefore, there are less electron repulsions in $Cl$ and an additional electron can be accommodated easily.

Hence, the electron gain enthalpy of $Cl$ is more negative than that of $F$.