The ions cr3+ and o2- combine to form the ionic chromium oxide. in what proportions do these ions combine to produce a neutral compound?

When ions combine to form neutral compounds, their charges are usually crisscrossed to cancel out any net charge. For this example, Cr3+ and O2- have different charges. Applying the crisscross method, the neutral compound is Cr2O3. This proportion balances out a net charge because two Cr3+ gives a +6 charge, and three O2- gives -6 charge. 

The correct answer is "atomic weight".

Seems cool i guess ttyl