and
Aluminum nitrate is an ionic compound, meaning that it is made up cations and anions. By convention when writing the formula for an ionic compounds the cation part shall be placed in front the anions. Thus the cation would contain aluminum and the anion shall be a polyatomic nitrate ion consisting of nitrogen and oxygen. The bracket with subscript 3 attached resembles the ratio between the two species, aluminum ions and nitrate ions. For each aluminum ion in the compound there shall be three corresponding nitrate ions.
Aluminum form ions of charge +3 whereas nitrate ions are of charge -1. Write the charge of the ion as the superscript in reverse (i.e., write as in
and
in
, omitting any 1.)