Polyatomic Ions Reference Cheat Sheet

Key Facts

• A polyatomic ion is a bonded group of atoms with an overall charge, such as nitrate, $\mathrm{NO_3^-}$, or ammonium, $\mathrm{NH_4^+}$.
• When writing ionic compounds, the total positive charge must equal the total negative charge, so calcium nitrate is $\mathrm{Ca(NO_3)_2}$ because $\mathrm{Ca^{2+}}$ balances two $\mathrm{NO_3^-}$ ions.
• Use parentheses around a polyatomic ion when more than one copy is needed, as in $\mathrm{Al_2(SO_4)_3}$.
• The ending $\mathrm{-ate}$ usually has more oxygen atoms than the related $\mathrm{-ite}$ ion, such as sulfate $\mathrm{SO_4^{2-}}$ and sulfite $\mathrm{SO_3^{2-}}$.
• The prefixes $\mathrm{per-}$ and $\mathrm{hypo-}$ show oxygen extremes in a series, such as perchlorate $\mathrm{ClO_4^-}$, chlorate $\mathrm{ClO_3^-}$, chlorite $\mathrm{ClO_2^-}$, and hypochlorite $\mathrm{ClO^-}$.
• Most common polyatomic ions are negatively charged, but ammonium, $\mathrm{NH_4^+}$, is a common positive polyatomic ion.
• Hydrogen added to a negative ion usually makes the charge one unit less negative, so carbonate $\mathrm{CO_3^{2-}}$ becomes hydrogen carbonate $\mathrm{HCO_3^-}$.
• Roman numerals in compound names show the charge of a variable-charge metal, as in iron(III) nitrate, $\mathrm{Fe(NO_3)_3}$, where iron is $\mathrm{Fe^{3+}}$.

Vocabulary

Polyatomic ion

A group of covalently bonded atoms that has an overall electric charge.

Ion charge

The positive or negative charge on an ion, shown with a superscript such as $\mathrm{SO_4^{2-}}$.

Subscript

A small number written below and to the right of a symbol that shows how many atoms or ion groups are present, such as the $3$ in $\mathrm{NO_3^-}$.

Parentheses

Symbols used around a polyatomic ion when a formula needs more than one of that entire ion, such as $\mathrm{(OH)_2}$.

Oxyanion

A negatively charged polyatomic ion that contains oxygen, such as nitrate, $\mathrm{NO_3^-}$.

Ionic compound

A neutral compound made from positive and negative ions whose charges add to $0$.