Conceptual diagram of a methane molecule consisting of a carbon atom surrounded by four hydrogen atoms. The molecular formula for this is CH₄.

• of the same type – forming an elemental molecule; or
• of different types – forming a compound.

Molecules come in a wide range of sizes – from small hydrogen molecules with just two atoms covalently bound, to the huge macromolecules making up materials such as rubber with hundreds of atoms bound together. Despite their relatively large sizes (compared to atoms), molecules are still too small to be seen with the naked eye.

Molecules can be described by chemical formulae, chemical names and, occasionally colloquial names. For instance, NaCl is sodium chloride which is commonly known as ‘salt’.