From Formula to Name: Guided Practice

Introduction

Going from a name to a formula is one skill; going the other way is another. Faced with FeCl₃, Na₂SO₄ or N₂O, you need to recognise the pieces of the formula and turn them back into words. The key is to spot polyatomic ions, notice whether the compound is ionic or covalent, and — for metals with more than one possible charge — work out the correct Roman numeral from the charges in the formula.

Core explanation

Step one: ionic or covalent? If the formula starts with a metal symbol or with NH₄, the compound is ionic. If it contains only non-metals (apart from ammonium compounds), it is covalent and will be named with prefixes.

Naming ionic compounds. Name the positive ion first, unchanged: Na is sodium, Ca is calcium, NH₄ is ammonium. Then name the negative part:

- A single non-metal atom takes an -ide ending: Cl chloride, Br bromide, O oxide, S sulfide, N nitride. - A recognised polyatomic group keeps its own name: SO₄ sulfate, SO₃ sulfite, NO₃ nitrate, CO₃ carbonate, OH hydroxide, PO₄ phosphate, HCO₃ hydrogencarbonate.

Ionic names do not mention the numbers of ions. MgCl₂ is magnesium chloride, not magnesium dichloride, because the charges fix the ratio.

Metals with variable valency. Iron, copper, lead, tin and many other transition metals can form more than one ion. The name must include a Roman numeral showing the charge. To find it, use the known charge of the negative ion and the fact that the compound is neutral.

For FeCl₃: three Cl⁻ ions give −3, so the iron must be +3. The name is iron(III) chloride.

For CuO: one O²⁻ gives −2, so copper is +2. The name is copper(II) oxide.

For Cu₂O: one O²⁻ gives −2, shared between two copper ions, so each is +1. The name is copper(I) oxide.

Naming covalent compounds. Use prefixes to state the number of each atom: mono-, di-, tri-, tetra-, penta-, hexa-. The second element takes an -ide ending. Mono- is left off the first element. So CO is carbon monoxide, SO₂ sulfur dioxide, N₂O dinitrogen monoxide and SiCl₄ silicon tetrachloride. Where a vowel clash occurs, the final "a" or "o" of the prefix is often dropped: tetroxide, monoxide.

Recognising hidden groups. Some formulae disguise polyatomic ions. In Ca(NO₃)₂ the brackets help, but in KNO₃ you must notice that NO₃ is nitrate. Learn the common ion formulae well enough to spot them without brackets.

Step-by-step reasoning

Name Fe₂(SO₄)₃:

1. Fe is a metal, so the compound is ionic. 2. SO₄ is sulfate, charge 2−. 3. Three sulfates give 3 × (−2) = −6. 4. Two iron ions must provide +6, so each is Fe³⁺. 5. Name: iron(III) sulfate.

Visual explanation

Draw a vertical line through the formula between the positive and negative parts: Fe₂ (SO₄)₃. Label the right side with its total charge (−6), then share the opposite charge equally among the metal atoms on the left (+3 each). The Roman numeral appears straight from the diagram.

Real-world analogy

Naming from a formula is like working out a price per item from a receipt. If three identical items cost £6 in total, each must cost £2. Likewise, if two iron ions must balance −6, each carries +3 — the "price per ion".

Real-world example

Iron(II) sulfate, FeSO₄, is used in iron supplement tablets, while iron(III) oxide, Fe₂O₃, is the main component of rust. Although both contain iron, the different charges give different properties and colours, so labels must name them precisely.

Why?

Why do ionic names leave out prefixes while covalent names include them? For a given pair of ions, the charges allow only one neutral ratio, so the name need not repeat it. The one missing piece — the charge of a variable metal — is supplied by the Roman numeral. Non-metal pairs can combine in many ratios, so prefixes are essential.

Common misconception

"The Roman numeral is the number of metal atoms in the formula." It is not. In Fe₂O₃ there are two iron atoms, but the name is iron(III) oxide because each iron ion carries a 3+ charge. The numeral always shows charge, never count.

Worked example

Question: Name (a) PbO₂ and (b) (NH₄)₂CO₃.

Reasoning: (a) Two O²⁻ ions give −4, so lead is +4; lead has variable valency, so the numeral is needed. (b) NH₄ is ammonium and CO₃ is carbonate; ammonium has a fixed charge, so no numeral is used.

Answer: (a) lead(IV) oxide (b) ammonium carbonate.

Quick check

1. Name the compound CuCl₂. Answer: Copper(II) chloride, because two Cl⁻ ions give −2, so the copper ion is +2.

Exam focus

Examiners expect Roman numerals for transition metals and lead or tin, but not for sodium, magnesium, calcium, aluminium or zinc, whose charges are fixed. Show the charge calculation briefly. Do not add prefixes to ionic names: "calcium dichloride" loses marks.

Advanced insight

Older names used endings instead of numerals: ferrous for iron(II) and ferric for iron(III), cuprous for copper(I) and cupric for copper(II). You may still see these on old bottles and in some industries. The modern Stock system, with Roman numerals, is clearer because it states the charge directly.

Summary

To name a compound from its formula, first decide whether it is ionic or covalent. For ionic compounds, name the positive ion, then the negative ion with an -ide ending or its polyatomic name, adding a Roman numeral when the metal has variable valency. For covalent compounds, use prefixes to state the numbers of atoms.

Practice questions

1. Name K₂S. Answer: Potassium sulfide. 2. Name Fe(OH)₂. Answer: Iron(II) hydroxide, because two OH⁻ ions give −2, so iron is +2. 3. Name PCl₃. Answer: Phosphorus trichloride, a covalent compound named with prefixes. 4. Name Mg(NO₃)₂. Answer: Magnesium nitrate; magnesium has a fixed charge, so no numeral or prefix is needed. 5. Name Cu₂O and explain the numeral. Answer: Copper(I) oxide; one O²⁻ gives −2, shared between two copper ions, so each is +1.