Why Are Magnesium and Beryllium Not Chemically Inactive?

[ghallya]

Hi all

I was woundering , and I searched for a convincing answer but
I didnt find any , so i hope i will find it here :)

Both Be and Mg (ofcourse othe examples apply ) have closed subshells
, why are they not chemically inactive (inert) ?

 

[Gokul43201]

Having closed subshells is not a condition for chemical inertness. The condition that maximizes stability (minimizes reactivity) is a valence shell octet (or a duet in the case of He).

However, having fully filled (and even half-filled) subshells does contribute to some increased stability. The fact that Be amd Mg are inherently so reactive (being only 2 electrons removed from an octet/duet) makes it a little hard to notice this small stabilization due to the filled subshell. However, careful measurements of ionization energies (and even electron affinities) clearly illustrate this stabilization.

Compare the 1st ionization energies of Be and B, and those of Mg and Al. The periodic trend is for the IE to increase as you go down the period. However, Be has a higher IE (899.5 kJ/mol) than B (800.6 kJ/mol) and Mg has a much higher IE (737.7 kJ/mol) than Al (577.5 kJ/mol). This is telling us that it's harder to pull out a valence electron from Be and Mg than from B and Al. This is a direct result of the extra stabilization from the filled subshells.

Note 1 : A similar disruption of the periodic trend in IEs is observed when you go from the N group (which has a half filled p subshell) to the O group.

Note 2 : Although less convincing (because the periodic trends are not as clear as with IEs), the effect of the stabilization is also seen in the electron affinities.

 

[ghallya]

thank you very much , I really appreciate your help :)