[gavindor]

I know that a formula unit is what ionic compounds have. (i.e.  ionic compounds  being not composed of molecules). And I know that the term empirical formula can apply to molecular formulae or to formula units.

Is Formula Unit usually an empirical formula but not always an empirical formula?

e.g. common examples are ionic compounds NaCl and CaCO₃  but from what I understand there are also examples of ionic compounds whose formula unit is not an empirical formula , i.e. the ratio of atoms in the formula are not in their simplest form , for example Hg₂N₂O₆ https://pubchem.ncbi.nlm.nih.gov/compound/129646128   or K₂S₂O₈ https://en.wikipedia.org/wiki/Potassium_persulfate  Their empirical formulae would be HgNO₃ and KSO₄ respectively.

[Hunter2]

Can KSO4 exist? No some bonding missing. HgNO3 theroretical can exist. But mercury-I connect two Hg in there molecules. So it would be Hg2(NO3)2 or Hg2N2O6

[Borek]

Is Formula Unit usually an empirical formula but not always an empirical formula?

Yes.

Also Hg₂Cl₂.

[gavindor]

Can KSO4 exist? No some bonding missing. HgNO3 theroretical can exist. But mercury-I connect two Hg in there molecules. So it would be Hg2(NO3)2 or Hg2N2O6

KSO4 and HgNO3 are empirical formulae. So I think they don't have to exist?

The Formula Units exist Hg2N2O6  and K2S2O8

[Hunter2]

That is what I am saying.