The six common polyatomic ions are ammonium, hydroxide, nitrate, carbonate, sulfate, and phosphate. Each is a covalently bonded group of atoms carrying an overall charge and behaving as one ion in ionic compounds.
| Ion name | Formula and charge | Type |
|---|---|---|
| Ammonium | Cation | |
| Hydroxide |
| Anion |
| Nitrate | Anion |
| Carbonate | Anion |
| Sulfate | Anion |
| Phosphate | Anion |
Names can provide useful clues, but they do not replace knowing the formulas and charges. Most ions ending in “-ate” contain oxygen, although the ending does not determine the charge. Hydroxide commonly appears in bases, while carbonate reacts with acids to form carbon dioxide and water. Nitrate, sulfate, and phosphate commonly occur in salts. When naming an ionic compound, state the cation first and the anion second; do not use molecular prefixes.
In the ionic model (Structure 2.1, SL and HL), oppositely charged ions are held together by electrostatic attraction. The formula of an ionic compound must have zero overall charge, so the numbers of cations and anions are chosen to balance charge.
For example, magnesium ions are and nitrate ions are . Two nitrate ions are needed for each magnesium ion:
Parentheses are used because the formula contains more than one whole nitrate ion. Similarly, ammonium sulfate is : two ions balance one ion.
A common misconception is that the atoms within a polyatomic ion separate when writing a compound. They do not: keep the ion together as a single charged unit. Also distinguish ammonium, , from ammonia, , which is a neutral molecule.
For a deduce or formula-writing question, first write each ion with its charge, find the smallest whole-number ratio giving zero total charge, then add parentheses only when more than one polyatomic ion is required. Do not include charges in the final neutral compound formula, and use the periodic table and supplied data carefully where relevant.
