A polyatomic ion is two or more atoms that are covalently bonded to one another and possess a charge. (The charge differentiates them from covalent compounds, which are neutral.)
While the definition is nice to have, it doesn’t help much when you’re confronted with a nice little chemical like (NH4)2SO4 and told to give it a name. Many students of chemistry bravely charge right in and try to call it something like “two nitrogen tetra-hydrogen sulfur tetra-oxide” and then proceed to call their teacher all sorts of things that sound even worse. Sometimes they may even be justified. (Not my students though.)
The reason they stumble over a compound like this is that they haven’t learned to recognize a polyatomic ion when they see one. There are clues that can help, and we’ll talk about them, but the first thing I encourage is to memorize a short list of the most commonly seen polyatomic ions. I arbitrarily use 14, because that is small enough to be manageable while giving a wide enough variety to be useful. I’ll list them here, and then discuss them in a little more detail.
formula, charge, name
NH4, +1, ammonium
OH, -1, hydroxide
CN, -1, cyanide
C2H3O2, -1, acetate (an organic polyatomic ion)
ClO, -1, hypochlorite
ClO2, -1, chlorite
ClO3, -1, chlorate
ClO4, -1, perchlorate
NO2, -1, nitrite
NO3, -1, nitrate
CO3, -2, cabonate
SO3, -2 sulfite
SO4, -2, sulfate
PO4, -3, phosphate
The first thing that I would like for you to notice is that EVERY polyatomic ion that ends with oxygen is either an “ite” or an “ate”. That means if you hear or see “ate” or “ite” at the end of a name, you should automatically say, “aha! There’s a polyatomic ion with oxygen.”
Secondly you should notice that there are “families” of ions that share the same first element and only differ in the number of oxygens they have. ALL ions in the same family will always have the same charge, so even if you forget some, you still can figure them out just by knowing one out of each family. For example, if you know that nitrate has a -1 charge, then you know that nitrite also has a -1 charge, whether you’ve memorized nitrite or not.
Within these families, the only thing that varies is the number of oxygens, as we said, so the name has to indicate that difference. In general, the most common form will have the suffix “-ate”. If you make it a point to memorize just the “-ate”s, you can figure out all the rest just by knowing the pattern.
if “X” is the number of oxygens in the “-ate” ion:
per-basename-ate X+1 number of oxygens
basename-ate = X number of oxygens
basename-ite = X-1 number of oxygens
hypo-basename-ite = X-2 number of oxygens
The only sad thing is that the charge of the “-ate” ion is not the same in every family, so you do have to memorize the charge that goes along with each family.
Knowing these fourteen polyatomic ions can also let you make a good guess at the names of other ions you may come across. BrO3 (-1 charge) looks an awful lot like ClO3 (-1 charge), and is named the same way: bromate. And AsO4 (-3) looks like PO4 (-3) and is named arsenate.
When you are writing the names of ionic compounds that contain polyatomic ions, you do it just the same as you do with single element ions, simply say/write the name of the cation (positive ion), then the anion (negative ion). Any extra parentheses or subscripts do not change the name at all, they’re only there to balance the charge.
Parentheses are your friends. if you see parentheses in the formula of an ionic compound, that is a signal to you that the formula inside is a polyatomic ion, so you should name it as a whole instead of saying the individual element names.
With all this at your disposal, let’s look again at (NH4)2SO4.
Parentheses signal an ionic compound, and we know from our list that NH4 is indeed “ammonium”.
The second half has a lot of oxygen paired with another element. Since so many polyatomic ions are just that – something surrounded by oxygens – we should guess that it is also one, and sure enough, it’s there on our list as “sulfate”.
Put the two names together, positive ion first, and we get “ammonium sulfate”.
For a little more practice:
parentheses show a polyatomic, and this is: copper (II) cyanide
recognize NH4 from the list: ammonium bromide
see all those oxygens?: sodium phosphate
more oxygens!: calcium carbonate