[GRUVI MR.]

Hello, friends, I am a freshly minted undergraduate student and have been looking, on-and-off, for the answer to a question about the element mercury. Alas, my introductory chemistry textbook and Google-sleuthing skills have failed me - Summer vacation is nearly over - and so I have come to this forum of wise scholars to beg succor. 

To summarize: What makes it possible for mercury to have the oxidation states Hg₂²⁺ and Hg²⁺, while cadmium, zinc, copernicium, and other transition metals (to my knowledge) are uninclined, or unable, to have both of these oxidation states? If this is too broad a question: What is the physical reason that mercury atoms either have an oxidation state of 2+ by themselves or form a diatomic ion (is this right?) with an overall charge of 2+, while other elements in group 2B don’t?

I thought about this question in chemistry class during the semester and my Professor didn’t know the exact answer, but said that this phenomenon most likely occurs due to mercury’s unique electron configuration of [Xe] 4f¹⁴5d¹⁰6s². From this clue, all I was able to do was conjecture that because mercury has F-block subshells while cadmium doesn’t, Hg somehow gains a tendency to create a metallic bond with itself, rather than invariably opt to lose the two electrons like cadmium and zinc. Perhaps linking two complete F-blocks and ridding the combined electron cloud of 2 electrons offers a stabler charge that is closer, or similar, to + than 2+. Intuitively, I feel this is related to the electron configuration, but I can't figure out how exactly this is the case. Again, though, this is just the ignorant and probably laughably off-the-mark conjecture of a sophomore.

tl;dr version: Why does mercury uniquely possess both Hg₂²⁺ and Hg²⁺ oxidation states?

To conclude, am I on the right track? Are there any pertinent online resources, theories, or articles that I’ve missed on my quest that could enlighten me? Any help on this question would be extremely appreciated, and thank you for reading. 

[Borek]

There are two ways of answering such questions. One is to look for some explanation like you did - but such an explanation is usually hand wavy, especially when it deals with a single case in which given situation arises, as there is no way to generalize the idea and test it if correctly predicts behavior of other ions/elements/molecules/whatever. Second is to simply accept the fact that when you calculate energies of possible systems, the one that we observe in the reality has the lowest energy, period.

In other words - I am not convinced looking for "reasons" behind makes sense. This is similar to "explaining" banana bonds in diborane.

YMMV