the correct order of electron gain enthalpy with negative sign of `F,Cl,Br and I` , having atomic number `9,17,35 and 53` respectively is

Options

A. `I gt Br gt Cl gt F`
B. `F gt Cl gt Br gt I`
C. `Cl gt F gt Br gt I`
D. `Br gt Cl gt I gt F`

Answer

C

Explanation

As we go down the group in periodic table, atomic size increases, force of attraction for the added electron decreases, thus, the added electron becomes loosely bound. Hence, electron gain enthalpy decreases down a group.
`X(g)+e^(-)rarr X^(-0)(g)`
Actual order is `Cl gt F gt Br gt I`
The fact that fluorine has a less electron gain enthalpy than chlorine seems to be due to the relatively greater effectiveness of 2p-electron in the small F-atom to repel the additional electron entering the atom than 3p-electrons in the larger `Cl`-atom.