polyatomic ion
Summary
A polyatomic ion is a group of two or more covalently bonded atoms that carries a net charge, such as NO₃⁻, SO₄²⁻ or NH₄⁺. It behaves as one unit and generally stays intact when a salt dissolves, so fully dissociated Na₂SO₄ gives 2 Na⁺ and 1 SO₄²⁻ (i = 3).
Detail
Meaning: A polyatomic ion is a group of two or more atoms joined by covalent bonds that carries a net charge, because the group has unequal numbers of protons and electrons. Examples are nitrate NO₃⁻, sulfate SO₄²⁻, phosphate PO₄³⁻, carbonate CO₃²⁻, hydroxide OH⁻, ammonium NH₄⁺ and hydronium H₃O⁺. Bonds within the ion are covalent; the ion is held to its counterion by ionic attraction.
Equation: q = (Σp − Σe) × e, where q is net charge (C), Σp is total protons in the ion, Σe is total electrons, and e = 1.60 × 10⁻¹⁹ C.
On the MCAT: Expect formula writing with charge balance, where parentheses show the whole ion repeated: Ca₃(PO₄)₂ has 3 Ca²⁺ and 2 PO₄³⁻. Colligative property questions count particles: Na₂SO₄ dissociating fully gives 3 particles (i = 3). Buffers (H₂CO₃/HCO₃⁻, H₂PO₄⁻/HPO₄²⁻), solubility and Lewis structures with resonance also use these ions.
Common confusion: Students split the ion into atoms when counting particles, giving Na₂SO₄ seven particles instead of three. Dissolving a salt breaks the ionic attraction, not the covalent bonds inside sulfate, so SO₄²⁻ generally stays one particle.
Sources
- Chemistry: The Central Science
- Chemistry
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.