How to predict the formula for binary compounds of main group elements?

Question

How to predict the formula for binary compounds of main group elements?

Answer 1

A "quick and dirty" way to write the formula of a simple binary compound is to "crisscross" the oxidation numbers.  The positive oxidation number becomes the subscript for the second element and the negative oxidation number becomes the subscript for the first element in the formula.  Then you can reduce them, if needed.

Example:

iron (III) oxide

We know that oxide always has an oxidation number of -2 and the (III) tells us that the oxidation number of iron is +3 ...

+3 -2

Fe O  

crisscross, and get

Fe2O3

another example:

lead (IV) sulfide

+4 -2

Pb S

crisscross

Pb2S4

reduce, because 2 and 4 are both divisible by 2

PbS2

This works because the sum of the oxidation numbers for a compound is always zero.