Question

Explain electron gain enthalpy and its trends in group and period

Answer 1

Explanation:

Energy is released when an isolated gaseous atom gains an electron and converts into an anion.This energy is called electron gain enthalpy.Larger the amount of energy releasedin this process higher will be the electron affinity. It is the measure of the capacity of an atom to form an anion by gaining electron.

Trends in periodic table

Generally,on moving from left to right in a period,except group 2(completelly filled orbitals) and group 15 (exactly half-filled orbitals), the electron gain enthalpy regularly increases(become more negative) due to decrease in atomic radius. The electron gain enthalpy of halogens in their respective periods is maximum negative because only one electron is required by them to attain stable configuration(s^2p^6).

Generally,the electron gain enthalpies of elements decrease on moving down the group, but some aniomalis are also found in this order, example, in halogens, electron gain enthalpy of F is less than Cl. Due to smaller atomic radius and high electron density of F as compared to Cl it is comparatively difficult to add an electron in F than chlorine. There is high inter electronic repulsion in 2p orbital of F due to its smaller atomic size .As a result the newly entering electron does not experience a greater force of attraction. Consequently, the electron gain enthalpy of F is less negative in comparison to that of Cl.

Exception

1. Electron gain enthalpy of zero group elements is positive because their orbitals are already complete and do not accept new electrons. The new electron in them has to be forced to enter into the higher shell for which additional energy is required.
2. The electron gain enthalpies of 15th group are very low because their valence shell P orbitals being exactly half filled are stable and have no tendency to gain electrons.
3. The electron gain enthalpies of second group elements are positive or almost zero,because their valence shell as orbitals are already complete due to which they cannot accept an additional electrons.
4. the electron gain enthalpy of fluorine is less than than that of chlorine, while on moving down in a group electron affinity decreases regularly. This is due to the smaller size of F. Fluorine has high electron density due to large number of electrons in small space. Hence additional electron experiences some repulsive effect on entering. As a result less energy is produced that is electron gain enthalpy is found to be less than excepted.Similar in the case with 16 and 15th group elements.