Question

What is the best way to remember the polyatomic ions? I will give a medal to the answer that is the most helpful... example of a bad answer: "You just have to memorize them."

Answer 1

Nice NO3 -1
Students SO4 -2
Pass PO4 -3
Chemistry CO3 -2
CLass ClO3 -1

Answer 2

that's pretty clever :) but i need to remember the name and charges. and i need to know more than just those... i know a few though... like cyanide is CN- Hydroxide is OH- and Ammonium is NH4-

Answer 3

Br⁻
Bromide

BrO⁻
Hypobromite

BrO₂⁻
Bromite

BrO₃⁻
Bromate

BrO₄⁻
Perbromate

C₂H₃O₂⁻
Acetate

C₂O₄²⁻
Oxalate

Cl⁻
Chloride

ClO⁻
Hypochlorite

ClO₂⁻
Chlorite

ClO₃⁻
Chlorate

ClO₄⁻
Perchlorate

CN⁻
Cyanide

CO₃²⁻
Carbonate

Cr₂O₇²⁻
Dichromate

CrO₄²⁻
Chromate

F⁻
Fluoride

H⁺
Hydrogen

H⁻
Hydride

H₂CO₃
Carbonic Acid

H₂SO₄
Sulfuric Acid

H₃O⁺
Hydronium

H₃PO₄
Phosphoric Acid

HC₂H₃O₂
Acetic Acid

HCL
Hydrochloric Acid

HCO₃⁻
Hydrogen carbonate (bicarbonate)

HNO₃
Nitric Acid

HSO₃⁻
Hydrogen Sulfite (bisulfite)

HSO₄⁻
Hydrogen sulfate (bisulfate)

I⁻
Iodide

IO⁻
Hypoiodite

IO₂⁻
Iodite

IO₃⁻
Iodate

IO₄⁻
Periodate

MnO₄⁻
Permanganate

N³⁻
Nitride

NH₄⁺
Ammonium

NO₂⁻
Nitrite

NO₃⁻
Nitrate

O²⁻
Oxide

O₂²⁻
Peroxide

OH⁻
Hydroxide

P³⁻
Phosphide

PO₃³⁻
Phosphite

PO₄³⁻
Phosphate

S²⁻
Sulfide

S₂O₃²⁻
Thiosulfate

SCN⁻
Thiocyanate

SO₃²⁻
Sulfite

SO₄²⁻
Sulfate

Answer 4

Ah you need to know all of them! The 5 I listed above follow similar naming patterns:

- The parent ion (listed above) has the suffix -ate (e.g. nitrate, sulfate, etc)
- If you add another oxygen, you add per- as a prefix (e.g. ClO4 is called perchlorate)
- If you take off an oxygen, the suffix becomes -ite (e.g. NO2 is called nitrite)
- If you take off 2 oxygens, you also add the prefix hypo- (e.g. SO2 is called hyposulfite)

Note I didn't include the charges in these examples, but the charges STAY THE SAME as the parent ion. For instance, PO5, PO4, PO3, and PO2, all have a -3 charge. Also note that iodate and the related ions have the same charge as chlorate (iodine and chlorine are both halogens and so share similar properties, even as polyatomic ions).

If you start tacking on hydrogens to cancel the negative charge and form oxyacids, the naming is slightly different but follows a similar trend. So if you have SO4(2-) and add 2 hydrogens to make H2SO4, you have sulfuric acid (the -ate suffix becomes -ic). If you start instead with SO3(2-) and add 2 hydrogens to make H2SO3, you have sulfurous acid (the -ite suffix becomes -ous). You can guess then what the molecular formulas of persulfuric acid and hyposulfurous acid are :P

If you add enough hydrogens so that only one negative charge remains, you add the prefix bi- and keep the -ate suffix. So H2CO3 is carbonic acid, but HCO3(-) is bicarbonate. H3PO4 is phosphoric acid, but H2PO4(-) is biphosphate.

A lot of the others are unique and you'll have to start working with them in practice problems to start memorizing their charges/names, but in most cases the charges are -1 or -2 and the names you can probably figure out pretty easily from the formula. A couple things though:

- The thio- prefix means you replaced an oxygen atom with a sulfur atom. So OCN- is cyanate, but SCN- is thiocyanate. Not to be confused with CN-, which is cyanide!
- NH4+ is ammonium and is the only positive polyatomic ion you will need to know
- OH- is the hydroxide ion and is important because it is a BASE (i.e. raises pH)
- H3O+ is the hydronium ion and is important because it is an ACID (i.e. lowers pH). Sometimes people just call it H+ but H3O+ is technically most correct.
- Nitride, iodide, sulfide, etc are all just the sole element, with the charge being however many valence electrons it accepts to complete its octet. So nitrogen has 5 valence electrons and gains 3 more to complete its octet, giving it a charge of -3 and forming the nitride ion.
- Based on the chemical structure, acetic acid you can also remember as CH3COOH (acetate is the anion, CH3COO-), and oxalic acid you can also remember as HOOCCOOH (oxalate is the double anion (-OOCCOO-)

Hope that helps a bit more!