When writeing a formula, how do i know when to add roman numerals?

Answer 1

Iron oxide is Fe2O3.

Since each oxygen atom almost always has two covalent
bonds it means than in this instance the 3 oxygen atoms will
have 6 covalent bonds between them.

These six bonds will be evenly distributed between the two
iron atoms. 6 / 2 = 3, or each Iron atom will have 3 covalent
bonds each.

Therefore each Iron atom is sharing 3 of its outer or valence
electrons making them III.

Iron(III) oxide = Iron oxide = Fe3O2.

The roman numerals are usually used on metals.

Answer 2

Usually, you DON'T use Roman numerals in writing formulas. However, you DO use them to signify the ionic level that an anion (which is usually the case) has in a compound, when the anion can take on different ionic levels. For example Iron(III) oxide is Fe2O3, while Iron(II) oxide is FeO.

Answer 3

It is necessary to use Roman Numerals only if you don't or can't use the common chemical description of the metal or cation that is in the formula. For example, it's either Stannic Chloride or tin(IV) chloride;it's either Ferric Oxide or Iron (III)Oxide; it's either Plumbic Sulfate or Lead (IV) Sulfate; it's either Plumbous Sulfate or Lead (II) Sulfate... etc.