I suppose you mean Hg, not Mg.
....The fact of the matter is ionic and covalent bonds really shouldn't be viewed a completely different things. Rather, it is better visualize them along a continuous spectrum of potential interactions between adjacent nuclei, differentiated mostly by the degree to which electrons are shared. The ionic case is simply the extreme case where electrons are almost completely localized around one of the two nuclei (in a diatomic pair).
Thanks, yes I mean Hg not Mg, I can't see an edit button for earlier posts in a thread.
And I understand that covalent and ionic is a spectrum, with purely covalent on one end (with something like Cl2). And going to non polar covalent, to polar covalent, to predominently ionic, and with nothing being 100% ionic / purely ionic.
When I say ionic I mean the bond between the Mercurous cation, and each of the Chlorine anions, being predomimently ionic.
Maybe it could be modelled covalently without writing ionic charges and treating it as a cation and two anions?
The way I see things is.. If we were to take NaCl, and boil it very hot, so it split into ionic monomers and ionic dimers.. So clusters some are just an NaCl pair, a molecule. Some are NaClNaCl(ionic dimer) Na2Cl2. But let's say we get it to all just discrete NaCl pairs. Then if we let it cool so it forms crystals, we'd then say it's not molecular, presumably because the bond between the Na and Cl, is ionic and the bonds between one NaCl and another, is ionic. And so it's not seen to be distinguishable entities.. discrete molecules. In contrast, if we take O2 gas and freeze it, then we have a molecular solid. There's VDW forces between the O2 molecules (and within them). But the O2 molecules while they have covalent bonds within the molecule. Between the molecules they don't. So the O2 molecules are all considered very much separate.
Now, if I look at this picture on the wikipedia page for Hg₂Cl₂
What I am seeing is a structure. Cl---Hg₂---Cl
(and that has a covalent bond between the Hg atoms, and an ionic bond between the Hg2 and each Cl)
So that three ion entity is ionic - ionic meaning composed of ions. And Hg2 like any polyatomic ion is internally covalent.
And those three ion entities(as in, those entities each consisting of three ions), are not connected to each other with any covalent or ionic bond.
Just like with the O2 molecules.
So those "three ion entities", those ionic entities (ionic as in composed of ions), are technically molecules. 'cos they are all separated/distinct.
And I guess like in the O2 case, what is between one molecule and another, is just VDW forces.
So it seems to me what we have is a crystal that is molecular, where each molecule is ionic(composed of ions).
(and one of the ions has a covalent bond - which isn't unusual).
Is that right?