Naming compounds & writing formulas
The idea
Chemical names are a precision language: one name must point to exactly one formula and back again. The first decision is always which naming system applies. A metal with a nonmetal means ionic rules: name the cation, then the anion with an -ide ending (or its polyatomic name like sulfate or nitrate), and never use counting prefixes because the charges already fix the ratio. Two nonmetals mean covalent rules: use prefixes (mono-, di-, tri-, tetra-) because the same pair of elements can form several different molecules, like CO and CO₂.
Ionic formulas come from charge balance, which you already know from ionic bonding: find each ion's charge, then take the smallest ratio that cancels to zero. Transition metals add one wrinkle — many can form more than one cation, so a Roman numeral in the name, as in iron(III), states the charge outright. Polyatomic ions like SO₄²⁻, NO₃⁻, OH⁻, and CO₃²⁻ travel as a unit; when you need more than one, parentheses keep the unit intact.
The classic error is mixing the systems — calling CaCl₂ calcium dichloride. Prefixes belong to covalent compounds only; for ionic compounds the name calcium chloride is already unambiguous, because Ca²⁺ and Cl⁻ can combine in only one neutral ratio.
Worked example
Write the chemical formula for iron(III) sulfate, showing how the charges determine every subscript.
- Decode the name: the Roman numeral (III) says the iron cation is Fe³⁺, and sulfate is the polyatomic anion SO₄²⁻ — a charge you recall from the common-ion list rather than derive.
- Balance the charges with the smallest whole numbers: the least common multiple of 3 and 2 is 6, so use two Fe³⁺ (total 6+) and three SO₄²⁻ (total 6−).
- Assemble the formula, wrapping the polyatomic ion in parentheses because there is more than one of it: Fe₂(SO₄)₃. Without parentheses the subscript would scramble the sulfate unit.
- Verify neutrality: 2 × (+3) + 3 × (−2) = 0, so Fe₂(SO₄)₃ is the correct neutral formula.
Answer. Iron(III) sulfate is Fe₂(SO₄)₃ — two Fe³⁺ ions balancing three SO₄²⁻ ions.
Check your understanding
- Why do covalent compounds need counting prefixes while ionic compounds must not use them?
- Why does iron's name carry a Roman numeral when calcium's never does?
- When exactly are parentheses required in a formula, and what would go wrong without them?
- Starting from a bare formula like Cu(NO₃)₂, what sequence of decisions recovers its correct name?
Build the foundations first
Naming compounds & writing formulas builds on these concepts. If any feel shaky, start there.