[archiematthews]

This is a question I posed to the science department at my school and none of us are yet to find a satisfactory answer. Why would the formula for Mercury(I) Chloride be Hg₂Cl₂ instead of simply HgCl. I understand why Mercury(I) forms the Hg-Hg bond, but could the same thing not be achieved by it existing as a HgCl ionic bond, exchanging electrons?

Furthermore, how in the form Cl-Hg-Hg-Cl would it exist as a lattice, would that not simply become HgCl in a lattice form? Any support with this would be appreciated.

[Hunter2]

You can also ask why is N as N2, O as O2, Cl as Cl2, etc.
Mercury-I compounds exist as bimolecular, so (HgCl)2 as Hg2Cl2 and others. Some disproportuniert  into Hg and Hg2+, already discussed in other thread for Oxid.

[Borek]

Note: "why" is a poor question in physics and chemistry. We can describe the matter, we can sometimes devise from our observations general rules, but now and then the only general rule that survives is "because that's the structure that has the lowest energy". That's the case here - there really is no "why" behind, it just is what it is. You can attempt to explain it with some hand wavy arguments, but even if they will see plausible for this particular case, they won't give any valuable insight into other cases.

We can measure properties of Mercury(I) chloride solutions - its ionic strength, osmotic pressure - they are inconsistent with Hg⁺/Cl^-, but consistent with Hg₂²⁺/2Cl^-.

When you look at the mercury(I) chloride lattice (again determined experimentally, so we are not guessing, we are measuring) mercury atoms are together - that's inconsistent with the structure of other chlorides of monovalent cations, but it is consistent with the presence of Hg₂²⁺.

Compare with Corribus' post from the thread about mercury hydroxide (the one Hunter mentioned) - while it is not about chloride, whole discussion of mercury hydroxide stability is exactly about why "why" is a poor question.

[gavindor]

This is a question I posed to the science department at my school and none of us are yet to find a satisfactory answer. Why would the formula for Mercury(I) Chloride be Hg₂Cl₂ instead of simply HgCl. I understand why Mercury(I) forms the Hg-Hg bond, but could the same thing not be achieved by it existing as a HgCl ionic bond, exchanging electrons?

i'm no expert but i've looked into this kind of subject a bit..
 
Are you asking why the formula is what it is, given the structure.

Or why the structure is the way it is?

The structure is all covalent..  hence described as a linear molecule.  So the Cl--Hg--Hg--Cl

Presumably between each of these Hg2Cl2 structures, is just VDW forces. (I suppose) But certainly just intermolecular forces.

If it were the case that Hg2Cl2 were not a linear molecule, so, if it were ionic. Then, it'd be like Mercury(I) Nitrate  (that's Hg₂(NO3)₂)  (wikipedia has it right , and I understand many other sites have it wrong).  That's the polyatomic ion, Hg--Hg  bonded to the other(s) then we'd have a formula of Hg2Cl2   because the formula for an ionic compound, is simplest ratio of ions, not simplest ratio of atoms.

And still supposing it were an ionic compound, then judging by the formula, it wouldn't be two Hg ions combined with two Cl ions.  'cos Two Hg ions combined with two Cl ions, would be HgCl.  'cos as mentioned, formula of an ionic compound is simplest ratio of ions. The smallest unit would be HgCl. That'd be an ionic compound so forming an ionic lattice, and with no polyatomic ion, the formula of it would be HgCl.

By the way, a covalent compound can be said to form a crystal  lattice too.. solid H2O is a crystal lattice. A solid can be either amorphous , or a lattice.   If a covalent compound forms a lattice it'd be considered to be a molecular lattice. Whereas an ionic compound forming a lattice wouldn't be considered to be a molecular lattice 'cos ionic compounds aren't considered to be molecular.  Some HS level material only uses the word lattice in the context of ionic compounds.  But really it applies also to covalent network and even covalent molecular compounds. The term molecule isn't great and can be used differently by people.

If you ask, Why does something have the structure it has. And no doubt you mean How(incase some misunderstand "why"!). A lot of the time it's not really known why a substance has the structure it has..   Scientists investigated it to find out the structure.  Perhaps there can sometimes be old theories. And some rules of thumb  with  exceptions, that might be used in a school exercise for some reason. But presumably quantum effects play a role in structure, and they are very complicated and not well understood.