How does the rule of zero charge you predict the formula of an ionic compound?

Overall the ionic compound cannot be charged, so all charges of the ions must be balanced. This means if you know the charge of 1 ion (E.G. Ca2+ ion) and you know the charge of the other ion (E.G. Cl1-) then you can work out that to make the overall charge 0, there must be double the amount of chlorine ions than calcium ions present. From this, you can work out the formula must be CaCl2. 

The number of protons in a nucleus

The correct name for n3-ion should be azide.