Why are the electron affinities of beryllium and magnesium is almost zero ?? Give five reason? ?

Question

Wyatt Atkins · Accepted Answer

Here's my explanation.

Electron affinity

Electron affinity #E_text(ea)# is the energy released when an electron adds to an atom in the gaseous phase.

#"A(g) + e"^"-" →"A"^"-""(g)"#
The higher an atom's tendency to accept an electron, the more positive the electron affinity value will be.

Beryllium and magnesium

The general trend is that electron affinity increases from left to right in the Periodic Table.

#"Be"# and #"Mg"# are the two Group 2 elements closest to the head of the red arrow.

Unlike most elements, they have #E_text(ea) ≤ 0#.

and #"Mg"# have filled #"s"#ubshells. They are much more stable than if they had an extra electron.

An extra electron would have to go into a #"p"# orbital, which is at a much higher energy than an #"s"# orbital. We must add energy to put an electron into a higher energy subshell, so #E_text(ea) ≤ 0#.
I could probably construct three more reasons for the difference, but they would be stating the above comments in different words.

Owen Ambrose · Answer

undefined 1. Beryllium and magnesium belong to Group 2 of the periodic table, characterized by their valence electron configuration of ns². This configuration results in a stable electronic configuration due to the presence of a filled s subshell.

2. Both beryllium and magnesium have relatively small atomic radii, which results in strong electrostatic repulsion between the incoming electron and the already tightly held valence electrons, making it energetically unfavorable to accept an additional electron.

3. The electron affinity trend across a period generally increases from left to right. However, there are exceptions, such as Group 2 elements, where the electron affinity tends to be close to zero or even positive. This is due to the exceptionally stable electronic configuration attained by losing electrons to achieve a noble gas configuration.

4. In the case of beryllium, accepting an additional electron would result in an electron configuration with two electrons in the 2s orbital, which is less stable than the noble gas configuration.

5. Similarly, for magnesium, accepting an additional electron would result in a filled 3s² subshell, which is also less energetically favorable than the preceding configuration.

In summary, the electron affinities of beryllium and magnesium are almost zero due to the stability achieved by their electron configurations and the strong repulsion between incoming electrons and the tightly held valence electrons.