Some compounds contain more than two elements.of the names of these compounds either end in the letters -ATE or - ITE but there are some exceptions which end in -IDE. eg sodium sulfite contains sodium, sulfur and oxygen. Copper nitrite contains copper, nitrogen and oxygen. Sodium hydroxide contains sodium, hydrogen and oxygen,
You cannot cross over three valency numbers so a different method is used. In data there is likely to be a list of complex ions as in the table below. Otherwise you will need to learn them.
Sulfate ion has the formula SO42- which means that in each formula unit there is 1 atom of sulfur, 4 atoms of oxygen and because it has added two electrons it has two negative charges. The valency of these ions is the same as the number of charges. So sulfate has a valency of 2.
At this stage, I recommend leaving out the charge when writing the formula and only include it if an ionic formula is asked for,
So to write a formula such as calcium phosphate, find the valency of calcium (Group 2 so VN = 2) and phosphate, PO4, has a VN of 3
Calcium phosphate Ammonium sulphate Calcium hydroxide
Ca PO4 NH4 SO4 Ca OH
2 3 1 2 2 1
Ca3(PO4)2 (NH4)2SO4 Ca(OH)2 (NOT CaOH2 which is wrong)
Notice a bracket is used if ANY of the 'complex' ions has to be multiplied by a number > 1