How to read and write chemical formulas
A chemical formula is a precise shorthand for the elements in a compound and the number of atoms of each element. In H₂O, for example, every water molecule contains two hydrogen atoms and one oxygen atom. Knowing the formula lets you identify a substance, compare samples and calculate quantities.
Reading and writing compounds becomes easier when every symbol, subscript and bracket has a clear purpose. You need only a few rules: match names to element symbols, identify the relevant ions where applicable, and set the proportions correctly. These habits also support laboratory work, food-label reading and later study of chemical reactions.
What a chemical formula represents
Each element has a one- or two-letter symbol, such as C for carbon, Cl for chlorine and Fe for iron. Capital and lowercase letters matter: Co is cobalt, but CO is carbon monoxide. A single symbol identifies an element; a combination identifies a compound whose atoms are chemically bonded in a fixed overall ratio.
Some compounds exist as molecules, while others form extended ionic lattices. NaCl, for example, is sodium chloride, with a one-to-one ratio of sodium ions to chloride ions rather than a separate molecule shaped like “NaCl”. A formula identifies what is present, but it does not always show the full three-dimensional arrangement.
A formula cannot, by itself, explain every difference between samples. Vitamin C is ascorbic acid, C₆H₈O₆, yet storage, preparation and freshness affect how much remains in food. The explanation of more vitamin C shows why two products containing the same compound can differ in vitamin content.
Reading symbols and small numbers
In a printed formula, a small number low on the line is a subscript counting the atoms of the element before it. H₂O means two hydrogen atoms for every oxygen atom. The subscript 1 is omitted, so NaCl gives a sodium-to-chlorine ratio of 1:1, while CO₂ gives one carbon atom for every two oxygen atoms.
A number before a formula is a coefficient, not a subscript. The formula 2H₂O means two water molecules, whereas the subscript 2 in H₂O describes one molecule containing two hydrogen atoms. In 2H₂ + O₂ → 2H₂O, coefficients show how many units react or form; subscripts remain part of each substance’s identity.
A small number high on the line is an ionic charge. Na⁺ is a sodium ion, and SO₄²⁻ is the sulfate ion; the charge does not change the atom count. These particles also matter in living systems. In biology, the reflex arc describes a rapid response involving nerve signalling, while ions such as Na⁺, K⁺ and Ca²⁺ help cells maintain electrical conditions.
Turning compound names into formulae
Start with the words in the compound’s name. Sodium chloride gives sodium, Na, and chloride, Cl. Calcium chloride needs two chloride ions because calcium forms Ca²⁺ and chloride is Cl⁻, giving CaCl₂. Aluminium chloride is AlCl₃ because one Al³⁺ balances three Cl⁻ ions.
Roman numerals identify the charge used when an element has more than one possible ion. Iron(II) sulfate is FeSO₄, while iron(III) sulfate is Fe₂(SO₄)₃. For named groups, keep each group intact: magnesium hydroxide is Mg(OH)₂, and magnesium nitrate is Mg(NO₃)₂.
Some names describe groups of atoms rather than individual elements. Ammonium chloride is NH₄Cl, and ammonium sulfate is (NH₄)₂SO₄. The brackets around ammonium show that two ammonium ions are needed for each sulfate ion. Recognising these patterns is more useful than treating every formula as unrelated symbols.
Using brackets and crossing over charges
Brackets act like a multiplication sign for everything inside them. In Mg(NO₃)₂, one magnesium ion is paired with two nitrate ions, so the whole NO₃ group appears twice. Aluminium sulfate is Al₂(SO₄)₃ because two Al³⁺ ions balance three SO₄²⁻ ions. The subscripts record the simplest whole-number charge ratio.
This reasoning is often called crossing over. Calcium phosphate contains Ca²⁺ and PO₄³⁻, so place the 2 beside phosphate and the 3 beside calcium: Ca₃(PO₄)₂. Reduce the pair to the smallest whole-number ratio, then check that the charges cancel and the brackets surround the complete phosphate group.
A centred dot introduces water of crystallisation in a hydrate. CuSO₄·5H₂O is copper(II) sulfate pentahydrate, with five water molecules for each copper sulfate formula unit. The 5 after the dot counts the water molecules, while the 4 in SO₄ counts sulfate oxygen atoms; the positions have different meanings.
Checking formulae in everyday Australia
Formula-reading skills meet everyday chemistry around Sydney Harbour, where seawater contains dissolved sodium chloride alongside many other salts. Seawater is a mixture, not one compound, because its ions are not all chemically bonded into a single particle. Evaporation can leave a mixture of salts behind rather than pure NaCl.
Similar care is needed on supermarket shelves. Woolworths and Coles nutrition panels may give values per 100 mL, per serving or per package, using units such as mg, µg and kJ. These describe amounts by mass, volume or energy, not numbers of atoms. The same ingredient can appear in a national-brand product and a cheaper own-brand version, but its chemical identity does not change with the price.
For biology revision, illness information provides context on how nutrients such as iron relate to body processes; chemistry still uses symbols such as Fe to describe composition and quantities. At an Australian garden centre, fertiliser bags show an NPK analysis such as 10-10-10. Those figures express percentages of nitrogen, phosphate and potash rather than an atom ratio, so gardeners compare them alongside price and soil needs.
Practise, verify and apply
Use a consistent checking routine. Identify every element or named ion, write its symbol with the correct capital letter, determine the charge ratio, bracket repeated groups and reduce the ratio. Then read the formula back aloud: Mg(NO₃)₂ means one magnesium and two nitrate groups, not one magnesium, one nitrogen and two oxygens.
Try magnesium oxide, potassium phosphate and calcium nitrate. Magnesium oxide is MgO because Mg²⁺ and O²⁻ combine in a 1:1 ratio. Potassium phosphate is K₃PO₄ because three potassium ions balance one phosphate ion. Calcium nitrate is Ca(NO₃)₂ because two nitrate groups balance one calcium ion.
In your next ten-minute study break at arvo, choose three chemical or ingredient names from a textbook or product at home. Write each formula, label every symbol and number, and check it against the elements in the name. Practise this until the reason for each coefficient, bracket and subscript is clear.