The correct order of electron gain enthalpy with negative sing F, Cl, Br and I, having atomic number 9, 17, 35 and 53 respectively, is
| A. |
`I > Br > Cl > F` |
| B. |
`F > Cl > Br > I` |
| C. |
`Cl > F > Br > I` |
 |
| D. |
`Br > Cl > I > F` |
As we go down the group in Periodic Table, atomic size increases, force of attraction for the added electron decreases, hence electron gain enthalpy decreases.
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 do 3p-electrons in the larger Cl-atom. Thus, correct answer is (c)