Understanding Names & Formulas
Chemical nomenclature is the language of chemistry — once you know the rules, you can name or write any compound. This guide covers ionic, molecular, and acid naming with a flowchart approach at each step.
Binary ionic compounds contain exactly two elements: a metal (cation) and a nonmetal (anion). The naming rule is straightforward — metal name first, then the nonmetal stem with "-ide" added. The complication arises with transition metals, which can carry more than one possible charge.
Iron can be Fe²⁺ or Fe³⁺. Copper can be Cu⁺ or Cu²⁺. If you just write "iron chloride," it is ambiguous — it could be FeCl₂ or FeCl₃, two entirely different compounds with different properties. The Roman numeral tells you exactly which ion is present, removing all ambiguity.
For transition metals with multiple charges: determine the metal's charge from the formula, then add a Roman numeral in parentheses immediately after the metal name.
Fixed-charge metals (Na, K, Ca, Mg, Al, Ba): never need Roman numerals.
Fe = iron. Iron is a transition metal — it can be Fe²⁺ or Fe³⁺. A Roman numeral is required.
For the compound to be neutral: Fe must be +2.
Fe = iron (transition metal, needs Roman numeral). O = oxygen (nonmetal, charge = −2).
+6 ÷ 2 atoms = +3 per iron atom → Fe³⁺
b) Name CuCl₂.
c) Name Al₂O₃ — note that Al is always +3 (not a transition metal).
a) Cu is +1 (one Cl at −1 → Cu = +1). Name: copper(I) chloride
b) Cu is +2 (two Cl each −1 → total −2 → Cu = +2). Name: copper(II) chloride
c) Al is always +3. No Roman numeral needed. Al₂O₃ = aluminum oxide
A polyatomic ion is a group of two or more atoms covalently bonded together that carries an overall electric charge. When polyatomic ions form part of an ionic compound, they stay together as a unit. Understanding them is non-negotiable — they appear in hundreds of compounds.
Unlike simple ions (where you know O is −2, Cl is −1), polyatomic ions have names that cannot be derived from first principles. There is no rule that tells you SO₄²⁻ is "sulfate" — it is a name chemists have agreed on and standardized. You must know them to read and write any chemical formula that includes them, which is most of inorganic chemistry.
These seven cover the vast majority of problems you will encounter at the Grade 11 level.
The subscript 2 outside the brackets means there are two complete hydroxide ions (OH⁻). Each OH⁻ carries a −1 charge. Total anion charge: 2 × (−1) = −2, which balances Ca²⁺.
You do not say "calcium dihydroxide" — the polyatomic ion's name is fixed, and Greek prefixes are not used for ionic compounds.
b) Name Mg(NO₃)₂.
c) What is the formula of calcium carbonate?
a) NH₄⁺ = ammonium, Cl⁻ = chloride → ammonium chloride
b) Mg²⁺ = magnesium, NO₃⁻ = nitrate. Subscript 2 means two nitrate groups, but the name doesn't change → magnesium nitrate
c) Calcium = Ca²⁺, carbonate = CO₃²⁻. Criss-cross: 2 and 2 → Ca₂(CO₃)₂ → simplify → CaCO₃
Every ionic compound must have a total charge of zero — positive and negative charges must balance. The criss-cross method is a fast, reliable technique for generating the correct subscripts automatically.
If cation has charge +m and anion has charge −n, the criss-cross gives you m anions and n cations. Total charge = n(+m) + m(−n) = nm − nm = 0. The math always works out to zero, which is why the method is reliable. The only extra step is simplifying if m and n share a common factor.
1. Write the cation (with charge) and anion (with charge) side by side.
2. Swap the magnitudes of the charges and use them as subscripts.
3. Simplify the subscripts to the lowest whole-number ratio.
4. If a polyatomic ion has subscript > 1, enclose it in parentheses.
No brackets needed when subscript is 1.
b) Write the formula for iron(III) oxide.
c) Write the formula for ammonium sulfate.
a) Ca²⁺ and Cl⁻. Criss-cross: Ca₁Cl₂ = CaCl₂. Check: +2 + 2(−1) = 0 ✓
b) Fe³⁺ and O²⁻. Criss-cross: Fe₂O₃. Check: 2(+3) + 3(−2) = 0 ✓ → Fe₂O₃
c) NH₄⁺ (+1) and SO₄²⁻ (−2). Criss-cross: (NH₄)₂(SO₄)₁ = (NH₄)₂SO₄. Check: 2(+1) + (−2) = 0 ✓
Molecular (covalent) compounds form when two nonmetals share electrons rather than transferring them. Unlike ionic compounds, the atoms don't carry predictable fixed charges, so you cannot use the criss-cross method. Instead, the formula itself tells you exactly how many atoms are present, and the name must reflect that using Greek number prefixes.
In ionic compounds, the charge of the metal determines the ratio of ions. In molecular compounds, there is no predictable charge — carbon can bond to oxygen to form CO (one oxygen) or CO₂ (two oxygens), and both are stable, common molecules. The only way to convey which compound you mean is to count the atoms explicitly. Greek prefixes do exactly that.
"mono-" is omitted from the first element when there is only one atom of it (exception: CO is carbon monoxide, not "carbon oxide" — mono stays on oxygen for clarity).
"mono-" is always used on the second element when there is only one.
S = sulfur (nonmetal), O = oxygen (nonmetal) → molecular compound, use Greek prefixes.
Count 1 → "mono-" but omit for first element when count is 1. Just "sulfur".
pent-oxide → 5 oxygen atoms (O)
b) Name N₂O.
c) Write the formula for carbon tetrachloride.
a) C = carbon (1, omit mono), Cl₄ = tetra + chlor + ide = carbon tetrachloride
b) N₂ = di + nitrogen, O = mono + oxide → dinitrogen monoxide
c) "carbon" = 1 C, "tetra-chloride" = 4 Cl → CCl₄
An acid is a compound that releases H⁺ ions when dissolved in water. The naming rule depends on a single question: does the acid contain oxygen? If yes, it is an oxyacid and you look at the polyatomic ion it contains. If no, it is a binary acid and you apply the "hydro-" prefix rule.
Binary acids (like HCl) are simply H bonded directly to a nonmetal. Oxyacids (like H₂SO₄) contain a polyatomic ion with oxygen. The oxyacid naming system is an extension of the polyatomic ion naming system — the "-ate"/"-ite" distinction is preserved and converted to "-ic"/"-ous" to show it is now an acid. The two-path system reflects genuine structural differences between the two types of acids.
Oxyacid: look at the anion:
— anion ends in -ate → acid ends in "-ic acid"
— anion ends in -ite → acid ends in "-ous acid"
HCl has no oxygen → binary acid.
b) Name HNO₂.
c) What is the formula of hydrobromic acid?
a) HNO₃ → anion is NO₃⁻ = nitrate (-ate) → -ic acid → nitric acid
b) HNO₂ → anion is NO₂⁻ = nitrite (-ite) → -ous acid → nitrous acid
c) "hydro" = binary acid, "bromic" → stem "brom" = bromine (Br). Formula: HBr