What are the Exceptions in electron affinityor electron gain enthalpy and why?

Grade 12th pass

1 Answers

Last Activity: 7 Years ago

Dear Nikita

Notice that the Group 2 elements have much lower electron affinities than the Group 1 elements, with beryllium and magnesium even having positive electron affinities. This is because Group 2 elements already have a full valence s shell and the addition of a new electron would move to the higher energy p shell that is otherwise left empty in the neutral atom. This occupation of a new, higher-energy shell is very energetically unfavorable and so this accounts for the more positive, or lower, electron affinity.
Notice also the same trend in the Group 15 elements as compared to the more negative electron affinities of Group 14.. This is because the addition of an electron to nitrogen's neutral configuration, for example, creates the first paired electrons in the p orbital. Because of electron-electron repulsions, this is energetically unfavorable, making the electron affinity more positive.
Notice that the Period 3 elements actually have the greatest electron affinities, instead of the Period 2 elements as the trend suggests. This is because the atomic radii of the Period 2 elements are considerably smaller, and thus the electrons experience significant electron-electron repulsions that are not completely off-set by the electron-nucleus attractions. For the Period 3 elements, their radii are larger and thus the balance between electron-nucleus attractions and electron-electron repulsions is maximized.

Regards

Arun (askIITians forum expert)