Name this ion
PO43-
Worked solution
Answer: phosphate
- Identify the ionsPO43- is phosphate.
- Where it sits on the ladderphosphite 3 Ophosphate 4 O Within one family, -ate is the reference. -ite has one oxygen fewer, hypo...ite one fewer again, and per...ate one more than -ate. The charge stays the same all the way up.
- Write the namephosphate, PO43-.
How to actually memorise the polyatomic ions
Most students meet the polyatomic ions as a list of about forty rows and try to learn them one at a time. That takes weeks and it does not survive an exam. The list is not forty independent facts. It is roughly a dozen facts plus a pattern that generates the rest, and once you can see the pattern the drill above stops being memorisation and starts being recall.
Learn the families, not the rows
Start with the four chlorine oxyanions. They all contain chlorine and oxygen, they all carry a 1- charge, and the only thing that changes across the four is the number of oxygen atoms:
- Perchlorate ClO4–, four oxygens
- Chlorate ClO3–, three oxygens
- Chlorite ClO2–, two oxygens
- Hypochlorite ClO–, one oxygen
The -ate form is the reference. -ite is one oxygen fewer. hypo-…-ite is one fewer again. per-…-ate is one more than -ate. The charge never changes as you move up and down that ladder, which is the part people get wrong under time pressure.
That single pattern covers chlorine, bromine and iodine. It also explains sulfate against sulfite, nitrate against nitrite, and phosphate against phosphite. You have just learned about fifteen ions by learning one rule and four reference points.
The ones with no pattern
A short list has to be learned outright, because nothing generates them:
- Ammonium NH4+, the only common positive one in the table
- Hydroxide OH–, cyanide CN– and peroxide O22-, the three that end in -ide without being single elements
- Carbonate CO32-, which has no -ite partner in ordinary use
- Acetate C2H3O2–, permanganate MnO4– and dichromate Cr2O72-
Seven or eight facts. That is a reasonable evening. Everything else on the list falls out of the ladder or out of the hydrogen rule below.
The hydrogen rule
Bicarbonate, bisulfate and bisulfite look like three more things to learn. They are one thing. Add a hydrogen to a 2- ion and the charge moves up by one, to 1-:
- Carbonate CO32- becomes hydrogen carbonate HCO3–
- Sulfate SO42- becomes hydrogen sulfate HSO4–
- Phosphate PO43- becomes hydrogen phosphate HPO42-, then dihydrogen phosphate H2PO4–
The older names use bi- for the same thing, so bicarbonate and hydrogen carbonate are the same ion. Both are accepted in the drill above, because both are what courses teach.
The two that share a formula
Permanganate and manganate are both MnO4. Nothing in the formula separates them. Permanganate is 1- and manganate is 2-, and that is the entire difference. This is why every question in the drill shows the charge: an ion quiz that asks you to name MnO4 has two right answers.
Why spacing beats repetition
Running through the whole list five times in one sitting feels productive and does almost nothing for recall a week later. Twenty ions a day for four days beats eighty in one evening, and the gap between sessions is doing the work rather than the total time.
The drill above tracks which ions you get wrong in this browser and shows those more often. Ten minutes on the twenty you keep missing is worth an hour on the full table.
What to do when you get one wrong
A red cross tells you nothing you did not already know. Read what the drill says instead. There is a real difference between confusing sulfate with sulfite, which means you have the ladder but not the reference point, and writing the wrong charge, which means you have the name but not the arithmetic. Those two mistakes need different fixes, and they will keep recurring until you fix the right one.
Where to go next
Once the ions are solid, the compounds follow quickly, because naming an ionic compound is mostly just naming its two ions. The ionic compound naming practice works from a formula to a name, and the formula writer goes the other way.
For the full reference table, including the common salt each ion appears in, see the complete list of polyatomic ions. If you want something to work on away from a screen, the printable flashcards and worksheets come with answer keys.
Common questions
How many polyatomic ions do I need to know?
Most introductory courses examine about twenty five. The set marked “the 20 you need first” in the drill is the intersection of what several common syllabuses require.
Do I have to learn the charges as well as the names?
Yes, and the charges are the part that matters. You cannot write a formula without them, and permanganate against manganate cannot be told apart any other way.
Why does peroxide end in -ide when it contains oxygen?
Because it is not an oxyanion of another element. Hydroxide, cyanide and peroxide are the three common exceptions to the rule that -ide means a single element. Learn them as exceptions and the rule holds everywhere else.