[imsmooth]

I'm going over naming conventions for binary compounds.  I get most of it.

ICl3 iodine TRIichloride (it's covalent, so I use the tri-)
CO2 carbon DIoxide
NaCl sodium chloride (just the elements, since it is ionic)

This one confuses me

Zn2P3 I would think this is Dizinc triphosphide.  Instead, it is zinc phosphide.  How does one know it is the latter and not the former name?

[thetada]

Generally speaking the di- and tri- prefixes are used to distinguish between common variants that would otherwise have the same name. Nitrogen has several oxides, including NO, N2O, NO2 and N2O4, hence the latter is called dinitrogentetroxide, to distinguish it from dinitrogen oxide and nitrogen dioxide. Similarly, carbon dioxide is different from carbon monoxide. Iodine chloride (ICl) exists in equilibrium with iodine trichloride. So my guess would be that there are no other compounds containing only zinc and phosphorous. Rules are useful in chemical naming, but like the rules that dictate plain English, they are subject to various exceptions. In other words, you can't always figure out exactly what something will be called from rules, you need to look it up to make sure. In England, you would probably be given credit in exams for suggesting the name dizinc triphosphide, but I recall in one exam that candidates were penalised for using the term carbon oxide. So maybe the best thing is to go overboard rather than being too basic.

[traceurkid]

That's ionic, isn't it? So it would be named just as you named sodium chloride. If you're saying that it's not ionic just because of the subscripts, that has nothing to do with it. e.g. Lithium oxide Li₂O, an ionic compound