Classifying Ionic Compounds

[skierboy]

1. The problem statement, all variables and given/known data

Which of the following compounds are properly classified as ionic? (select all that apply)
HOCN
SF4
NH4NO3
MgCl2
K2O
Li2
NF3

2. Relevant equations

n/a

3. The attempt at a solution

My initial choices were MgCl2 & K2O ... however, since this was incorrect (it didn't tell me which were correct or incorrect - the entire problem was either correct or incorrect)... I'm guessing that HOCN and NH4NO3 might also classified as ionic.

If this is true, what is the general criteria for recognizing ionic compounds (if the compound has more than 2 types of atoms)?

Thanks!!!
1. The problem statement, all variables and given/known data



2. Relevant equations



3. The attempt at a solution

[Borek]

I'm guessing that HOCN and NH4NO3 might also classified as ionic.

Second for sure, not so sure about the first one.

If this is true, what is the general criteria for recognizing ionic compounds

In the case of binary compounds - large difference in electronegativity. In the case of more complex compounds - almost all salts are ionic.

[epenguin]

Li2?
Am I the only one who has never heard of that?

Is it something that can/does significantly exist in some laboratory in gas phase?

[Borek]

I believe I have seen not only Li2, but also Na2 and K2 reported in gaseous phase. But don't quote me, I can be wrong.

Edit:

K2 at webelements:

http://www.webelements.com/potassium/bond_enthalpies.html

Not exactly what we are talking about, but an obvious sign that there are many exotic diatomic molecules.

[skierboy]

On the same note as NH4NO3, I found that HOCN can be expressed as two different ions as well: H+ and OCN-. Does that make it an ionic compound then?

I'm confused because the individual atoms are covalently bonded...

[Borek]

Not necesarilly. Gaseous HCl is covalent, but it easily dissociates into two ions.