Polyatomic Ions

A polyatomic ion is a group of atoms covalently bonded to each other that carries an overall charge and travels through reactions as a single unit. Sulfate goes into the beaker as $\mathrm{SO}{}_4{}^{2-}$ and comes out as $\mathrm{SO}{}_4{}^{2-}$.

Use this sheet while you are writing formulas. The reasoning behind it is in Naming and Formulas.

Charge 1−

Name	Formula
Hydroxide	$\mathrm{OH}{}^{-}$
Nitrate	$\mathrm{NO}{}_3{}^{-}$
Nitrite	$\mathrm{NO}{}_2{}^{-}$
Hydrogen carbonate (bicarbonate)	$\mathrm{HCO}{}_3{}^{-}$
Hydrogen sulfate (bisulfate)	$\mathrm{HSO}{}_4{}^{-}$
Hydrogen sulfite	$\mathrm{HSO}{}_3{}^{-}$
Dihydrogen phosphate	$\mathrm{H}{}_2\mathrm{PO}{}_4{}^{-}$
Acetate (ethanoate)	$\mathrm{CH}{}_3\mathrm{COO}{}^{-}$
Cyanide	$\mathrm{CN}{}^{-}$
Permanganate	$\mathrm{MnO}{}_4{}^{-}$
Hypochlorite	$\mathrm{ClO}{}^{-}$
Chlorite	$\mathrm{ClO}{}_2{}^{-}$
Chlorate	$\mathrm{ClO}{}_3{}^{-}$
Perchlorate	$\mathrm{ClO}{}_4{}^{-}$
Thiocyanate	$\mathrm{SCN}{}^{-}$

Acetate is also written $\mathrm{C}{}_2\mathrm{H}{}_3\mathrm{O}{}_2{}^{-}$. The two formulas describe the same ion; $\mathrm{CH}{}_3\mathrm{COO}{}^{-}$ shows how the atoms are actually arranged and is the better one to learn.

Charge 2−

Name	Formula
Carbonate	$\mathrm{CO}{}_3{}^{2-}$
Sulfate	$\mathrm{SO}{}_4{}^{2-}$
Sulfite	$\mathrm{SO}{}_3{}^{2-}$
Hydrogen phosphate	$\mathrm{HPO}{}_4{}^{2-}$
Chromate	$\mathrm{CrO}{}_4{}^{2-}$
Dichromate	$\mathrm{Cr}{}_2\mathrm{O}{}_7{}^{2-}$
Oxalate	$\mathrm{C}{}_2\mathrm{O}{}_4{}^{2-}$
Peroxide	$\mathrm{O}{}_2{}^{2-}$
Thiosulfate	$\mathrm{S}{}_2\mathrm{O}{}_3{}^{2-}$
Silicate	$\mathrm{SiO}{}_3{}^{2-}$

Charge 3−

Name	Formula
Phosphate	$\mathrm{PO}{}_4{}^{3-}$
Phosphite	$\mathrm{PO}{}_3{}^{3-}$

Charge 1+

Name	Formula
Ammonium	$\mathrm{NH}{}_4{}^{+}$
Hydronium	$\mathrm{H}{}_3\mathrm{O}{}^{+}$

Ammonium is the only common positive polyatomic ion, and it is the only cation in this course that is not a metal. It behaves like a group 1 metal ion in every formula you will write.

Four patterns that cut the memorising in half

The tables above look like forty separate facts. They are closer to twenty facts and four rules.

-ate and -ite differ by one oxygen, and the charge stays the same. Sulfate is $\mathrm{SO}{}_4{}^{2-}$ and sulfite is $\mathrm{SO}{}_3{}^{2-}$. Nitrate is $\mathrm{NO}{}_3{}^{-}$ and nitrite is $\mathrm{NO}{}_2{}^{-}$. Learn the -ate version of each and the -ite follows.

per- and hypo- extend the ladder at both ends. With chlorine and oxygen there are four steps, and the pattern is identical for bromine and iodine:

Name	Formula	Oxygens
Perchlorate	$\mathrm{ClO}{}_4{}^{-}$	one more than chlorate
Chlorate	$\mathrm{ClO}{}_3{}^{-}$	the reference point
Chlorite	$\mathrm{ClO}{}_2{}^{-}$	one fewer than chlorate
Hypochlorite	$\mathrm{ClO}{}^{-}$	one fewer than chlorite

Hypochlorite is the active ingredient in household bleach, which is why this ladder is worth knowing rather than merely learning.

Adding a hydrogen adds an H and cancels one negative charge. A hydrogen ion attaching to a polyatomic ion makes the charge less negative by exactly one:

$\mathrm{PO}{}_4{}^{3-}\to \mathrm{HPO}{}_4{}^{2-}\to \mathrm{H}{}_2\mathrm{PO}{}_4{}^{-}$

Carbonate to hydrogen carbonate works the same way, and it is why baking soda, $\mathrm{NaHCO}{}_3$, needs only one sodium ion while washing soda, $\mathrm{Na}{}_2\mathrm{CO}{}_3$, needs two.

thio- means an oxygen has been swapped for a sulfur. Sulfate, $\mathrm{SO}{}_4{}^{2-}$, becomes thiosulfate, $\mathrm{S}{}_2\mathrm{O}{}_3{}^{2-}$ — same charge, one oxygen replaced.

Brackets are not optional

When a formula needs more than one of a polyatomic ion, the whole ion goes inside brackets with the subscript outside. Calcium nitrate is $\mathrm{Ca}(\mathrm{NO}{}_3){}_2$ — one calcium ion, two nitrate ions, and therefore two nitrogen atoms and six oxygen atoms. Writing $\mathrm{CaNO}{}_{32}$ is a different and non-existent substance, and the mistake propagates silently into every molar mass you calculate afterwards.

The eight to learn first

If you memorise nothing else, memorise these. They account for most of the questions in Naming Practice and nearly every precipitation reaction in Solubility Rules.

$\mathrm{OH}{}^{-}$ · $\mathrm{NO}{}_3{}^{-}$ · $\mathrm{HCO}{}_3{}^{-}$ · $\mathrm{CH}{}_3\mathrm{COO}{}^{-}$ · $\mathrm{CO}{}_3{}^{2-}$ · $\mathrm{SO}{}_4{}^{2-}$ · $\mathrm{PO}{}_4{}^{3-}$ · $\mathrm{NH}{}_4{}^{+}$