MYP Chemistry: Writing Chemical Formulae — FAQ

Verdict-first: work out whether the compound is ionic or covalent, then apply the matching method. For ionic compounds, balance the charges of the cation and anion using the criss-cross rule. For covalent compounds, read the prefixes (mono-, di-, tri-) straight from the name. Always simplify the final ratio.

A chemical formula is shorthand for which elements a compound contains and in what ratio — H2O shows two hydrogen atoms bonded to one oxygen atom. In MYP Chemistry you're expected to write a formula from a compound's name, and name a compound from its formula, using valency or ion charges.

Write the cation first, then the anion, then balance total positive and negative charge to zero using the criss-cross method — swap each ion's charge number down as the other's subscript, then simplify. For calcium and chloride (Ca2+ and Cl-), this gives CaCl2, never Ca2Cl.

MYP Chemistry expects you to recognise a core set of polyatomic ions by name and charge: hydroxide (OH-), nitrate (NO3-), sulfate (SO4 2-), carbonate (CO3 2-) and ammonium (NH4+). Treat the whole ion as one unit and bracket it whenever more than one is needed, e.g. Mg(OH)2.

Covalent compound names use Greek prefixes that give you the exact atom count directly — mono- (1), di- (2), tri- (3), tetra- (4) — so carbon dioxide is CO2 and dinitrogen tetroxide is N2O4. Unlike ionic compounds, you don't balance charges here; the prefixes hand you the answer.

Valency is the combining power of an element — the number of bonds it typically forms, shown by its group on the periodic table or given directly in the question. Group 1 metals have valency 1, Group 2 metals have valency 2, and oxygen usually has valency 2. Cross valencies to get subscripts.

Write the two ion charges as plain numbers, ignoring the plus/minus sign, then cross each down as the subscript on the opposite ion. For sodium (Na+) and sulfate (SO4 2-), the 2 becomes sodium's subscript and the 1 stays on sulfate, giving Na2SO4.

The single most common mistake is forgetting to simplify the ratio after crossing charges — writing Mg2O2 instead of MgO. Others include missing brackets around a polyatomic ion needed more than once, like writing CaOH2 instead of Ca(OH)2, and confusing covalent prefixes with ionic charge-balancing rules.

Formula-writing sits mainly under MYP Sciences Criterion A: Knowing and Understanding, which assesses applying scientific knowledge to explanations and problem-solving. According to the IB's MYP Sciences guide, Criterion A is marked out of a maximum of 8, with strands covering applying chemical knowledge to explain phenomena and solve problems.

Individual formula-writing questions on class or unit tests typically carry 1-2 marks each — one for correct symbols and subscripts, one for the correct ratio or brackets. On a full MYP Criterion A task, formula accuracy is usually part of a larger question on bonding, not a standalone strand.

Expect command terms like 'write', 'state' and 'deduce' — 'write' wants the formula itself, 'deduce' wants you to work it out from given information such as valency or an ionic equation, and 'state' wants a quick answer with no working shown.

Yes — MYP formula-writing focuses on naming conventions, valency and simple ionic/covalent rules, while DP Chemistry (first exams 2025) builds on this with oxidation states, wider polyatomic ions, and formulae embedded inside stoichiometric calculations for both SL and HL. The core skill carries straight through unchanged.

Solid formula-writing in MYP is the foundation DP Chemistry assumes is already secure — DP moves straight into mole calculations, equation balancing and stoichiometry, where a shaky grasp of valency costs marks fast. Students who struggle with formulae in MYP 4-5 often struggle with DP Topic 1 stoichiometry a year later.

Practice little and often: pick five compounds a day, write both name-to-formula and formula-to-name, then check against a valency table. Topical Worksheets covering MYP Chemistry bonding and formulae, complete with mark schemes, let you self-check exactly where a ratio or bracket went wrong.

You don't need a chemistry background to help — ask your child to explain their working out loud; if they can't say why MgCl2 has a 2, they haven't understood the valency, not just forgotten a step. Revision Notes summarising MYP Chemistry naming rules make a quick one-page recap for home.