Other polyatomic ions may appear on the test, but if they do, the formula and
charges will be provided for you, so you just need to apply the above rules to
determine the formula of any compound they might create.
Ionic Nomenclature
It can sometimes seem like chemistry has its own language. (To an extent, it
does.) The study of naming chemical compounds is called nomenclature.
Nomenclature is based on the type of bonding, and each type of bonding has
different nomenclature rules.
If you are looking at a binary ionic compound (one containing only two
elements), the cation in that compound keeps its name, and the anion changes its
ending to -ide. So, NaF is sodium fluoride, and Li 3 P would be lithium
phosphide. The number of each ion is irrelevant when it comes to naming the
compound. When a polyatomic ion is present as part of the compound, it keeps
its name. Sr(OH) 2 would be strontium hydroxide, BeCO 3 would be beryllium
carbonate, and (NH 4 ) 2 S would be ammonium sulfide.
Things get a little more complicated when we have to name compounds
containing transition metals. Because transition metals can form cations with
multiple charges, the charge of the cation must be specified in the name of the
compound. This is done by using Roman numerals. So, copper (I) sulfate
indicates a copper cation with a charge of positive one, so Cu+. Two of those
copper ions would be needed to balance the negative two charge on the sulfate
ion, making the compound formula Cu 2 SO 4 . However, copper (II) sulfate would
have Cu2+ ions in it, meaning only one copper cation is needed for every sulfate,
creating a compound with a formula of CuSO 4 . Keep in mind that the Roman
numeral represents the charge on the ion, not how many there are. This is a
very common mistake to make!
You should also be able to work backward from the formula of an ionic
compound to its name. MgS would just be magnesium sulfide, and K 3 PO 4 is
potassium phosphate. However, when dealing with a compound containing a
transition metal, this requires you to deduce the charge on the cation. Fe(OH) 2
and Fe(OH) 3 have different names. The hydroxide anion has a charge of
negative one, so in the first compound the charge of the iron cation must be