What is the formula of aluminum carbonate?
Aluminium generally has a charge of +3
The electron configuration of Aluminium is
The 3p electron and the 2 3s electrons are outer valance electrons and are lost to form the +3 charge.
The Carbonate ion has a charge of -2
so the formula is
Because the charge of aluminium is 3 and the charge of carbonate is -2, you need to multiply each to balance out at 6. So, "swap and drop":