Chemistry / Unit 5: Chemical Names and Formulas
5.2 Polyatomic Ions
Notes
A polyatomic ion is a group of covalently bonded atoms with an overall charge. Treat it as one unit.
| Ion | Formula | Ion | Formula |
|---|---|---|---|
| ammonium | $\text{NH}_4^+$ | hydroxide | $\text{OH}^-$ |
| nitrate | $\text{NO}_3^-$ | nitrite | $\text{NO}_2^-$ |
| sulfate | $\text{SO}_4^{2-}$ | sulfite | $\text{SO}_3^{2-}$ |
| carbonate | $\text{CO}_3^{2-}$ | phosphate | $\text{PO}_4^{3-}$ |
| acetate | $\text{C}_2\text{H}_3\text{O}_2^-$ | hydrogen carbonate | $\text{HCO}_3^-$ |
- "-ate" has one more oxygen than "-ite" with the same charge.
- Use parentheses when you need more than one polyatomic ion: calcium nitrate is $\text{Ca(NO}_3)_2$.
Practice
What is the formula for ammonium sulfate?
Two $\text{NH}_4^+$ ions balance one $\text{SO}_4^{2-}$.
What is the name of $\text{KNO}_2$?
$\text{NO}_2^-$ is nitrite; nitrate is $\text{NO}_3^-$.
What is the formula for calcium phosphate?
$3(+2) + 2(-3) = 0$, and parentheses show two phosphate ions.
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