When do you use prefixes when naming compounds?

Question

How can I tell by just looking at a periodic table?

Answer 1

You use prefixes when the compound is molecular - not ionic.

A molecular compound contains all nonmetals, while an ionic compound has both a metal and nonmetal.

CO = carbon MONoxide
CO2 = carbon DIoxide
PCl3 = phosphorus TRIchloride
PCl5 = phosphorus PENTAchloride
N2O5 = DInitrogen PENToxide

NOTE: we do not use a prefix of "mono" for the first element, but we do use all the other prefixes for the first element.

Answer 2

Prefixes like alpha, beta, ortho, meta, para etc are used to indicate the presence of functional groups with respect to each other or to indicate its structure or physical properties.