Lesson 4 of 5

Chemical formulae

Goal: Read and write simple chemical formulae.

Puzzles for this lesson

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A model of a water molecule: one red ball joined to two smaller white balls

A chemical formula uses symbols and numbers to show which elements are in a substance, and how many atoms of each.

  • The symbols show the elements. H2O contains hydrogen and oxygen.
  • A small number counts the atoms of the element just before it. No number means one atom. So each molecule of water, H2O, has two hydrogen atoms and one oxygen atom.
  • The numbers give a ratio that never changes. In carbon dioxide, CO2, there are always two oxygen atoms for every carbon atom.

Naming compounds

When a metal joins a non-metal, the metal is named first. Then look at the ending.

-ideUsually just two elements. Sodium chloride, NaCl. Magnesium oxide, MgO.
-ateThere is oxygen in it as well. Calcium carbonate, CaCO3.
di-Two atoms of that element. Carbon dioxide, CO2.

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Key words in this lesson

Atom The smallest particle of an element that can exist

Element A substance made of only one type of atom

Compound Two or more elements chemically joined together

Molecule A group of two or more atoms joined together

Chemical formula Symbols and numbers that show the elements in a substance and the ratio of their atoms

Oxide A compound of an element with oxygen

Chloride A compound of an element with chlorine

Sodium chloride The compound in table salt

Carbon dioxide A compound with one carbon atom joined to two oxygen atoms

For teachers: AQA 3.5.4 · Pearson Edexcel 14.3, 14.4