Question

Among halogens, the correct order of amount of energy released in electron gain (electron gain enthalpy) is:

A)$F >$ Cl $>$ Br $>$ I

B)$F <$ Cl $>$ Br $>$ I

C)$F >$ Br $>$ Cl $>$ I

D)$I >$ Cl $>$ Br $>$ F

Answer 1

Electron gain enthalpy refers to the amount of energy released when an electron is added to an isolated gaseous atom. Moreover, during the addition of an electron, energy can either be released or absorbed. It becomes less negative as we move down the group due to increase in atomic size.

Complete answer:

Basically, electron gain enthalpy i.e. ${\Delta _{eg}}H$ is the enthalpy change associated with an isolated gaseous atom when it gains an electron to form its corresponding anion. The reaction is as shown:

$X(g) + {e^ - } \to {X^ - }(g)$

So, in general energy is released when an electron is added to an atom. Now, as we move down, the electron gain enthalpy becomes less negative. This is because, as we go down the group, both atomic size and nuclear charge increases but the effect of atomic size is more prominent than that of nuclear charge. Therefore, the force of attraction between the nucleus and the added electron decreases and thus enthalpy becomes less negative.

Now, the electrons that gain enthalpies of F, Cl, Br, and I in $kJmo{l^{ - 1}}$ are – 328, –349, –325 and –295 respectively. Hence, the order will be $F > Br > I$ . The electron gain enthalpy of F is less negative than that of Cl due to electron – electron repulsion in a small-sized F atom.

Hence, option B is correct.

Note: Electron gain enthalpy depends on three factors. These are electronic configuration, atomic radius and nuclear charge. Moreover, there are some exceptions such as in case of Sulphur and oxygen, Sulphur has a higher negative value than oxygen.