Chemical formulae and names · Grade 10
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.