From Name to Formula: Guided Practice

Introduction

Chemical names are not random labels. A systematic name such as "copper(II) nitrate" or "dinitrogen monoxide" contains all the information you need to write the formula, if you know how to decode it. This page gathers together the naming rules you have learned — endings, prefixes, Roman numerals and polyatomic ions — and turns them into one reliable routine for converting any simple name into a formula.

Core explanation

First, decide the type of compound. If the name starts with a metal (or ammonium), the compound is ionic , and you work with ion charges. If the name contains only non-metals and uses prefixes such as di-, tri- or tetra-, it is covalent (molecular) , and the prefixes give the numbers of atoms directly.

Decoding ionic names. The first word names the positive ion; the second names the negative ion.

- A metal name alone means the usual ion: sodium is Na⁺, magnesium Mg²⁺, aluminium Al³⁺. - A Roman numeral gives the charge of a metal with variable valency: iron(III) is Fe³⁺, copper(II) is Cu²⁺. - An -ide ending means a single-element ion: chloride Cl⁻, oxide O²⁻, sulfide S²⁻, nitride N³⁻. (Hydroxide and cyanide are exceptions that are polyatomic.) - An -ate or -ite ending means a polyatomic ion containing oxygen: sulfate SO₄²⁻, sulfite SO₃²⁻, nitrate NO₃⁻, nitrite NO₂⁻, carbonate CO₃²⁻, phosphate PO₄³⁻.

Once you know both ions, balance the charges, use brackets for more than one polyatomic ion, and simplify to the lowest ratio.

Decoding covalent names. Prefixes tell you the number of each atom: mono- 1, di- 2, tri- 3, tetra- 4, penta- 5, hexa- 6. "Mono" is usually left off the first element. So carbon dioxide is CO₂, sulfur trioxide is SO₃, dinitrogen tetroxide is N₂O₄ and phosphorus pentachloride is PCl₅. Do not cross valencies for these — the name already states the atom counts, and the formula is not simplified.

Order of symbols. Write the positive ion (or the less electronegative element) first, matching the order of the name. Sodium chloride is NaCl, not ClNa.

Step-by-step reasoning

Convert "copper(II) nitrate" into a formula:

1. Starts with a metal, so it is ionic. 2. Copper(II) means Cu²⁺. 3. Nitrate is the polyatomic ion NO₃⁻. 4. One Cu²⁺ needs two NO₃⁻ to balance. 5. Bracket the nitrate: Cu(NO₃)₂. 6. Check: +2 + 2 × (−1) = 0.

Visual explanation

Think of the name as a two-part code sheet. Draw two boxes: the left box holds the positive ion with its charge, the right box holds the negative ion with its charge. Draw arrows crossing the charge numbers to become subscripts, then circle any polyatomic ion that needs brackets.

Real-world analogy

Reading a chemical name is like reading a postal address. "Flat 3, 12 High Street" tells a postman exactly where to go, provided he knows the conventions. A systematic name tells a chemist exactly which atoms are present and in what ratio, provided the conventions are understood.

Real-world example

Pharmacists and laboratory technicians read names on labels every day: "magnesium sulfate" (Epsom salts, MgSO₄), "sodium hydrogencarbonate" (baking soda, NaHCO₃) and "calcium carbonate" (antacid tablets and chalk, CaCO₃). Converting names to formulae correctly ensures the right substance is used.

Why?

Why do covalent names use prefixes while ionic names do not? Ionic compounds follow fixed charges, so the ratio can be worked out from the ions alone. Pairs of non-metals can often combine in several ratios — carbon monoxide, CO, and carbon dioxide, CO₂, for example — so the name must state the numbers explicitly.

Common misconception

"Sulfide, sulfite and sulfate are the same ion." They are not. Sulfide is S²⁻, with no oxygen; sulfite is SO₃²⁻; sulfate is SO₄²⁻. Confusing these endings gives the wrong substance, so read the ending carefully before writing anything.

Worked example

Question: Write formulae for (a) aluminium oxide and (b) diphosphorus pentoxide.

Reasoning: (a) Aluminium is a metal, so the compound is ionic: Al³⁺ and O²⁻. Crossing gives Al₂O₃; charges 2 × (+3) + 3 × (−2) = 0. (b) Both elements are non-metals and prefixes are used: di- means two P, penta- means five O.

Answer: (a) Al₂O₃ (b) P₂O₅.

Quick check

1. Write the formula of potassium sulfite. Answer: K₂SO₃, since two K⁺ ions balance one SO₃²⁻ ion.

Exam focus

Examiners often mix ionic and covalent names in one question to check that you choose the right method. Watch the endings -ide, -ite and -ate, and Roman numerals. A common mark-losing error is crossing valencies for a prefixed covalent name.

Advanced insight

The rules here follow the recommendations of IUPAC, the international body that standardises chemical names. Chemists still use some older names: diphosphorus pentoxide actually exists as P₄O₁₀ molecules, but the name reflects the simpler 2 : 5 ratio. Real naming sometimes balances tradition against the true structure.

Summary

To convert a name into a formula, first decide whether the compound is ionic or covalent. For ionic compounds, identify each ion and its charge from the name, balance charges, bracket polyatomic ions if needed and simplify. For covalent compounds, read the atom counts straight from the prefixes. Always write symbols in the order of the name.

Practice questions

1. Write the formula of iron(II) chloride. Answer: FeCl₂, because one Fe²⁺ ion needs two Cl⁻ ions. 2. Write the formula of ammonium carbonate. Answer: (NH₄)₂CO₃, because two NH₄⁺ ions balance one CO₃²⁻ ion. 3. Write the formula of carbon tetrachloride. Answer: CCl₄, reading one carbon and four chlorine atoms from the prefixes. 4. Write the formula of calcium nitride. Answer: Ca₃N₂, since three Ca²⁺ ions (+6) balance two N³⁻ ions (−6). 5. Write the formula of sodium phosphate. Answer: Na₃PO₄, since three Na⁺ ions balance one PO₄³⁻ ion.