Recap and Goal
In the previous lesson you met ternary compounds – compounds built from three elements, usually a metal (or ammonium, NH₄⁺) combined with a polyatomic oxyanion such as sulfate or nitrate. Today we go one step further: naming oxyanions that come in families (with different numbers of oxygen atoms), naming the acids they come from, and combining all of this with transition metal oxidation states.
Naming Oxyanions: the —ate / —ite System
An oxyanion is a polyatomic ion made of a central non-metal atom bonded to oxygen. The basic rule, as introduced before, is: element name + —ate ending (Talbot, p. 130). For example:
- SO₄²⁻ → sulfate
- NO₃⁻ → nitrate
- CO₃²⁻ → carbonate
However, several elements can form two different oxyanions with the same charge but a different number of oxygen atoms. IUPAC distinguishes them using —ate (more oxygen) and —ite (fewer oxygen):
- SO₄²⁻ = sulfate, SO₃²⁻ = sulfite
- NO₃⁻ = nitrate, NO₂⁻ = nitrite
Some elements (especially the halogens) form four different oxyanions, so chemists add prefixes too:
- ClO⁻ = hypochlorite (less oxygen than –ite)
- ClO₂⁻ = chlorite
- ClO₃⁻ = chlorate
- ClO₄⁻ = perchlorate (more oxygen than –ate)
Memory pattern: hypo– < –ite < –ate < per–
From Oxyanion to Oxyacid
If you attach H⁺ ions to an oxyanion, you get the corresponding oxyacid. The naming pattern mirrors the anion:
- —ate → —ic acid (e.g. NO₃⁻ nitrate → HNO₃ nitric acid)
- —ite → —ous acid (e.g. NO₂⁻ nitrite → HNO₂ nitrous acid)
- hypo—ite → hypo—ous acid (HClO = hypochlorous acid)
- per—ate → per—ic acid (HClO₄ = perchloric acid)
Worked Example 1
Name H₂SO₃. This is H⁺ combined with SO₃²⁻ (sulfite). Since the anion ends in –ite, the acid name ends in –ous. Answer: sulfurous acid.
Worked Example 2
Give the formula for calcium chlorate. Chlorate = ClO₃⁻ (charge 1−). Calcium is Ca²⁺, so you need two chlorate ions to balance the charge. Answer: Ca(ClO₃)₂
Worked Example 3
Name Fe₂(SO₄)₃. Sulfate is SO₄²⁻, so three of them give a total charge of 6−. This must be balanced by two Fe cations totaling 6+, meaning each iron is Fe³⁺. Since iron is a transition metal with variable oxidation states, this must be stated (Talbot, p. 130). Answer: iron(III) sulfate.
Why This Matters
Oxyanion nomenclature lets chemists communicate unambiguously about compounds that look almost identical in formula but behave very differently (e.g. sodium sulfate vs sodium sulfite). This systematic approach reflects IUPAC's broader goal of giving chemistry a single, internationally understood language (Talbot, p. 172).
Key Takeaways
- Oxyanions with more oxygen end in –ate; those with fewer end in –ite; extreme cases use hypo– and per– prefixes.
- Oxyacids follow the same pattern: –ate → –ic acid, –ite → –ous acid.
- Transition metals in ternary compounds need a Roman numeral oxidation state (e.g. iron(III) sulfate), but main-group metals with fixed charges (like Ca²⁺) do not.
- Always balance total positive and negative charge when writing formulas from names, and vice versa.