Chemical formulas are written with rules according to the type of molecule.
Type 1: Binary Ionic
Between a metal and a non-metal and they’ll end with “-ide” on the second word. The metal’s symbol and charge are written, followed by the non-metals’s symbol and charge. The charges are balanced by using coefficients to indicate the number of atoms.
Type 2: Polyatomic Ionic
They either begin with “ammonium” or end with “-ate” or “-ite” in the second word (except “hydroxide” and “cyanide”—those are polyatomic ions). A polyatomic ion is a group of atoms that together have a charge. The metal’s symbol and charge are written first (or ammonium, NH4+1, the only polyatomic cation). The polyatomic anion’s symbols and charge are written next. The charges are again balanced with subscripts. If a subscript is added to a polyatomic ion, use parenthesis around the ion.
Type 1 or 2 with Multivalent Metals
They will have roman numerals in the name. Multivalent metals are metals that have more than one possibility for the charge. The charge of the metal is indicated with roman numerals following the metal’s name. The formula is then written following the rules for either Type 1 or Type 2.
Type 3: Binary Covalent
Between two non-metals. They will have use prefixes indicated the number of atoms. “Mono-“ is not used on the first element. The element symbols are written, and the prefixes indicate the subscript for each.
Type 4: Acids
The cation for an acid is H+. The anion is based on the format of the name: “hydro___ic acids” end with a single element; “___ic acids” end with the “___ate” polyatomic ion; “___ous acids” end with the “___ite” polyatomic ion. Write the correct anion’s symbol and charge and then balance the charges with subscripts.