How do you write the formula for ionic compounds? Method to deduce the formulae of ionic compounds:Metals from Groups 1, 2 and 13 combine with non-metals from Groups 15, 16 and 17 to form ionic compounds.Table shows how the metal atoms of Groups 1, 2 and 13 form positive ions with different charges.Group Number of valence electrons Number of electrons donated … [Read more...] about How do you write the formula for ionic compounds?