Reading chemical formulas

Ethanol and dimethyl ether both have the formula C₂H₆O, yet one is in drinks and the other is a gas used as a fuel.

A chemical formula is a short sentence written in element symbols. H₂O, CO₂ and NaCl each pack a lot of information into a few characters: which elements are present, in what ratio, and often how the atoms are connected. Reading formulas fluently makes every other part of chemistry easier, from balancing reactions to working out how much of a substance you need.

Symbols and subscripts

Each symbol stands for one atom of that element. A small number written below and to the right, a subscript, tells you how many of the atom just before it. No subscript means one. So H₂O is two hydrogen atoms and one oxygen atom; CO₂ is one carbon and two oxygen; CH₄ is one carbon and four hydrogen. Mind the capital letters: Co is cobalt, but CO is carbon monoxide, a carbon and an oxygen.

Brackets group atoms that come as a set. Calcium hydroxide is Ca(OH)₂: the subscript after the bracket multiplies everything inside, so there is one calcium, two oxygen and two hydrogen. A big number written in front of a formula, a coefficient, counts whole units. 2H₂O means two water molecules, which together hold four hydrogen atoms and two oxygen atoms.

Molecular, ionic and structural formulas

For a covalent molecule the formula gives the real number of each atom in one molecule: glucose is C₆H₁₂O₆, six carbon, twelve hydrogen and six oxygen. For an ionic compound such as NaCl or MgCl₂ it gives only the ratio of ions in the lattice, because there are no separate molecules. A molecular formula does not show how the atoms are connected, and that can matter a great deal. Ethanol and dimethyl ether are both C₂H₆O. In ethanol the oxygen sits at the end with a hydrogen on it, C-C-O-H; in dimethyl ether it sits between the carbons, C-O-C.

That is why chemists also write condensed formulas such as CH₃CH₂OH or C₂H₅OH for ethanol, which hint at the order of the atoms, and draw structural formulas with a line for every bond. Ethanol is a liquid that boils at about 78 °C and mixes with water, partly because its O-H group can hydrogen bond. Dimethyl ether has no O-H group and is a gas at room temperature.

Names follow the formula too. In simple two-element covalent compounds, Greek prefixes count the atoms: mono for 1, di for 2, tri for 3, tetra for 4. CO is carbon monoxide and CO₂ is carbon dioxide, and N₂O₄ is dinitrogen tetroxide. Korean names count the atoms the same way with Sino-Korean numbers. Ionic compounds are named metal first and the nonmetal with the ending -ide, as in sodium chloride and magnesium oxide, without prefixes, because the charges already fix the ratio. Water, ammonia and methane keep their traditional names.

From formula to mass

Add up the atomic weights in a formula and you get its molar mass, the mass in grams of one mole of the substance. With the 2024 IUPAC values, water is 2 × 1.008 + 15.999 = 18.015 g/mol. Carbon dioxide is 12.011 + 2 × 15.999 = 44.009 g/mol. Sodium chloride is 22.990 + 35.45 = 58.44 g/mol. Glucose is 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 180.156 g/mol. This sum is how a recipe written in molecules becomes a recipe written in grams.