Why is aluminum chloride AlCl3 instead of just AlCl?
Aluminum is in group IIIA which means it (usually) makes three covalent bonds and/or forms cations with a +3 charge. In the case of aluminum, it commonly does both. AlCl3 is a molecular compound, not an ionic one like you might expect for a combination of a metal and a nonmetal. Ionic compounds are usually named with the number of each element present, so aluminum chloride would mean one aluminum ion and one chloride ion. But since this is a molecular compound we use a different set of rules. Most elements make a characteristic number of bonds in neutral compounds, which you can predict from their group (column) on the periodic table. For most combinations, there is only one possible (or likely) combination of a given set of elements. Since aluminum makes three covalent bonds and chlorine makes one, and does not make double bonds, the only reasonable covalent combination of Al and Cl atoms is AlCl3. The hard part about naming AlCl3 is that it seems like it should be ionic and it is eas
