Question

Explain why the electron gain enthalpy of F is less -ve than that of Cl

Answer 1

Electron gain enthalpy is the energy released when an electron is added to an isolated gaseous atom.

With this definition, consider fluorine. It is the smallest element (with respect to atomic radius) in its period. It also has 7 electrons in its outermost shell, the L (2nd) shell. When you add an electron, there is extra repulsion between these electrons.

In the case of chlorine, it also is the smallest in its period, and has 7 electrons in its outermost shell, the M (3rd) shell in this case. Adding another electron to chlorine is relatively easier, as there is more space, and also the empty d-orbital can accomodate this electron.

For fluorine, imagine adding a person to a car with 7 persons.

For chlorine, imagine adding a person to a bus with 7 persons.

This inter-electronic repulsion is what makes the electron gain enthalpy of chlorine the highest, greater than that of fluorine.

Answer 2

Answer:

The negative electron gain enthalpy of fluorine is less than that of chlorine. It is due to small size of fluorine atom. As a result, there are strong interelectronic repulsions in the relatively small 2p orbitals of fluorine and thus, the incoming electron does not experience much attraction.