Polyatomic ions are groups of two or more covalently bonded atoms that carry an overall electric charge. They appear throughout chemistry in salts, acids, bases, fertilizers, minerals, and biological molecules. Learning the common polyatomic ions helps you write formulas, name compounds, and predict charges much faster.
A reference chart is useful because many of these ions have fixed names and charges that must be memorized.
Key Facts
- A polyatomic ion acts as one charged unit in a compound, such as NO3^- in NaNO3.
- Use parentheses when more than one polyatomic ion is needed: Ca(NO3)2, Al2(SO4)3, and (NH4)3PO4.
- Common 1- ions include nitrate NO3^-, nitrite NO2^-, hydroxide OH^-, acetate C2H3O2^-, and cyanide CN^-.
- Common 2- ions include sulfate SO4^2-, sulfite SO3^2-, carbonate CO3^2-, chromate CrO4^2-, and dichromate Cr2O7^2-.
- Common 3- ions include phosphate PO4^3-, phosphite PO3^3-, and borate BO3^3-.
- For oxyanions of the same element, -ate usually has more oxygen than -ite: nitrate NO3^- and nitrite NO2^-.
Vocabulary
- Polyatomic ion
- A charged group of covalently bonded atoms that behaves as a single ion in chemical formulas and reactions.
- Oxyanion
- A polyatomic ion that contains oxygen bonded to another element, such as sulfate or nitrate.
- Charge
- The positive or negative electrical value of an ion that determines how it combines with ions of opposite charge.
- Parentheses
- Symbols used in a chemical formula to show that more than one copy of an entire polyatomic ion is present.
- Acid
- A compound that can donate H+ ions, often formed when hydrogen combines with an anion such as nitrate or sulfate.
Common Mistakes to Avoid
- Changing the subscripts inside a polyatomic ion, such as writing NO2 instead of NO3 for nitrate, is wrong because the atoms inside the ion are part of its fixed identity.
- Forgetting parentheses in formulas like Ca(NO3)2 is wrong because CaNO32 does not clearly show two nitrate ions and changes the meaning of the formula.
- Confusing -ate and -ite names is wrong because they represent different numbers of oxygen atoms, such as sulfate SO4^2- and sulfite SO3^2-.
- Ignoring charge balance is wrong because ionic compounds must have a total charge of zero, so Al^3+ with sulfate SO4^2- becomes Al2(SO4)3.
Practice Questions
- 1 Write the correct formula for the ionic compound formed from Mg^2+ and nitrate NO3^-.
- 2 Name the compound (NH4)2CO3 and state the charge on each polyatomic ion.
- 3 Explain why the formula Ca(OH)2 needs parentheses, but NaOH does not.