Introduction
So far you've named simple binary compounds (two elements, like NaCl or CO₂). Today we move to ternary compounds — compounds made of three different elements. Most of the ternary compounds you'll meet in IB Chemistry contain a metal (or ammonium ion) combined with a polyatomic ion, especially an oxyanion (a negatively charged ion containing oxygen). Learning to name these correctly is important because, as IUPAC emphasises, chemical nomenclature exists to make communication between scientists unambiguous and consistent worldwide (Talbot, p.172).
What is a ternary compound?
A ternary compound contains three different elements. The most common type in this course is:
metal cation + polyatomic oxyanion → ionic ternary compound
For example:
- Na⁺ + NO₃⁻ → NaNO₃ (sodium nitrate)
- Ca²⁺ + CO₃²⁻ → CaCO₃ (calcium carbonate)
- K⁺ + SO₄²⁻ → K₂SO₄ (potassium sulfate)
Common oxyanions to memorise
You need to recognise these polyatomic ions and their charges:
- NO₃⁻ — nitrate
- NO₂⁻ — nitrite
- SO₄²⁻ — sulfate
- SO₃²⁻ — sulfite
- CO₃²⁻ — carbonate
- PO₄³⁻ — phosphate
- OH⁻ — hydroxide
- NH₄⁺ — ammonium (this one is a cation, not an anion!)
Naming pattern tip: when an element forms two common oxyanions, the one with more oxygen atoms ends in -ate, and the one with fewer oxygen atoms ends in -ite. Compare NO₃⁻ (nitrate) with NO₂⁻ (nitrite), or SO₄²⁻ (sulfate) with SO₃²⁻ (sulfite).
Naming rules for ternary ionic compounds
- Name the metal cation first (unchanged).
- Name the polyatomic anion second (using its standard name, e.g. nitrate, sulfate).
- Do not use prefixes (di-, tri-) — the formula's subscripts are worked out from balancing charges, not stated in the name.
- If the metal is a transition metal with variable charge, add a Roman numeral in brackets showing its oxidation state.
Worked examples
Example 1: Write the formula for magnesium nitrate.
- Mg²⁺ and NO₃⁻
- Charges must balance: need two NO₃⁻ for every Mg²⁺
- Formula: Mg(NO₃)₂ — note brackets around the polyatomic ion when more than one is needed.
Example 2: Name Fe₂(SO₄)₃.
- Iron combined with sulfate (SO₄²⁻)
- To balance charge: 2 Fe must total +6, so each Fe is +3
- Name: iron(III) sulfate
Example 3: Write the formula for ammonium phosphate.
- NH₄⁺ and PO₄³⁻
- Need three NH₄⁺ for every PO₄³⁻
- Formula: (NH₄)₃PO₄
Example 4: Name Cu(NO₂)₂.
- Copper with nitrite (NO₂⁻), two nitrites means Cu is +2
- Name: copper(II) nitrite
Key takeaways
- Ternary compounds contain three elements, usually a metal cation plus a polyatomic oxyanion.
- Learn the common oxyanions (nitrate, nitrite, sulfate, sulfite, carbonate, phosphate, hydroxide) and remember ammonium (NH₄⁺) is a cation.
- The -ate/-ite ending tells you which oxyanion has more or fewer oxygens.
- Balance charges (not atoms) to find the correct formula, using brackets when more than one polyatomic ion is needed.
- Use Roman numerals to show the oxidation state of transition metals with variable charge.