[user529401]

Hello, Why is mercury (I) ion diatomic?

[Mitch]

Why Mercury(I) is diatomic is an excellent question that I'm not sure is well understood. It goes against the trend in that group, Zinc and Cadmium don't behave like this. The obvious answer to give you is that the 4f electrons shield the 4s electrons poorly, so it can ionize to Hg₂⁺ readily.

A more qualitative answer is that this ion is not found in nature as Hg₂⁺, but as salts like Hg₂(H₂O₂)₂²⁺

[Mitch]

Forgot to put the references. I took some of the above information from Advanced Inorganic Chemistry fith edition by Cotton and Wilkinson page 601.

And also from W. R. Mason, Inorg. Chem., 1983, 22, 147

[AWK]

Scientists answer for troublesome question is always "good (or excelent of fascinating) question. Mitch, this is not a cutting remark to your answer, this is real truth.

This is structure of Hg₂(H₂O)₂²⁺ . 2NO₃^- (water hydrogen atoms are not included in ICSD - very old work). Note, there are covalent and ionic bonds between Hg and O atoms (lengths are in Angstrems, drawn in Mercury 1.2)
Oxygen -red
Nitrogen - blue
Mercury - violet

[Mitch]

Do you have too many atoms? I count 12? Can you tell us what the colors are?

[Mitch]

Actually, I think you put up the Hg2N2O8 molecule.  

[AWK]

Mitch, I added some comment in original post - now all is clear, I hope.
AWK

[gregpawin]

Huh

[Julie]

What is meant by diatomic

[AWK]

Diatomic is rather inproper in this case, but user52401 probably meant a metal-metal bond.

AWK

[gregpawin]

So are those atoms in the middle supposed to be nitrogens or mercury atoms?

[Mitch]

Mercury

[gregpawin]

So each atom is attached to a nitrate ion and water molecule?  It doesn't separate from the nitrate?

[Mitch]

I don't think so, because it would change the number of valence electrons on Mercury.

[AWK]

Dative bond between oxygen atom from water and mercury atom is short. Ionic contact between nitrate oxygen atom and mercury atom is longer. I drew only the shortest contacts from crystal structure of mercury(II) nitrate dihydrate, but note this is not one molecule. There are two nitrate anions and one Hg₂(H₂O)₂²⁺ cation.
Concerning primary question - scientist cannot simply explain why mercury(I) form metal-metal bond.
So this is called intristic property of this element. Other elements (but probably not all) also can form chains or rings consisted of  bonds between atoms of the same element (eg. B, C, N, O, Si, S, Se and so on)