Chemical formulae and names

A chemical formula tells you which atoms are in a substance and how many of each. For an ionic compound you work it out from the charges on the ions, which must add up to zero. For a molecule of two non-metals, the name tells you the numbers with prefixes such as di- and tri-. Names follow a few simple rules, so you can go from a name to a formula and back again.

Key facts

  • Metals form positive ions: group 1 make 1+, group 2 make 2+, aluminium 3+. Non-metals form negative ions: oxide O²⁻ and sulfide S²⁻, the halides 1− (Cl⁻, Br⁻, I⁻).
  • Some ions are groups of atoms: hydroxide OH⁻, nitrate NO3⁻, sulfate SO4²⁻, carbonate CO3²⁻ and ammonium NH4⁺.
  • In an ionic formula the charges cancel: Mg²⁺ and Cl⁻ make MgCl2. When you need more than one of a group ion, it goes in brackets: Ca(OH)2.
  • A Roman numeral gives the charge on a metal ion: iron(III) oxide contains Fe³⁺, so it is Fe2O3.
  • Names: the metal (or ammonium) comes first. -ide means just two elements (sodium chloride); -ate means oxygen is there too (sodium sulfate).
  • Two non-metals use prefixes for the numbers: mono 1, di 2, tri 3, tetra 4, penta 5, hexa 6. CO is carbon monoxide; CO2 is carbon dioxide.
  • Hydrogen, nitrogen, oxygen and the halogens exist as diatomic molecules: H2, N2, O2, F2, Cl2, Br2, I2.

Worked examples

Example 1

Write the formula of aluminium sulfate.

  1. The ions are Al³⁺ and SO4²⁻.
  2. The smallest number both 3 and 2 go into is 6: two Al³⁺ (+6) and three SO4²⁻ (−6).
  3. Sulfate is a group of atoms, so three of them go in brackets. Al2(SO4)3

Answer: Al2(SO4)3

Example 2

Name these compounds: K2CO3, FeCl3 and N2O4.

  1. K2CO3: potassium with the carbonate ion CO3²⁻ (carbon and oxygen): potassium carbonate.
  2. FeCl3: three Cl⁻ ions balance one iron ion, so the iron is Fe³⁺: iron(III) chloride.
  3. N2O4: two non-metals, so use prefixes: 2 nitrogen (di) and 4 oxygen (tetra): dinitrogen tetroxide.

Answer: potassium carbonate; iron(III) chloride; dinitrogen tetroxide

Example 3

Write the formula of each molecule from its model: ammonia and ethane.

  1. Ammonia: one nitrogen atom joined to three hydrogen atoms: NH3.
  2. Ethane: two carbon atoms and six hydrogen atoms: C2H6.
  3. Carbon compounds are written with C first, then H.

Answer: NH3; C2H6