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Individual chemical compounds are described by their formulae, such as H₂O for water and CH₄ for methane. A chemical formula describes the number of atoms of each element making up the compound, whereas the structure describes the arrangement of these atoms. The structure of the gas methane, for example, is shown in Figure 1. The molecule is not flat but a three-dimensional tetrahedron, with the carbon atom in the centre and a hydrogen at each of the four corners.

Chemical structure is not just of academic interest. It affects how chemicals react with each other, and with biological systems, including humans and the environment. It is common for chemicals with the same formula to have different structures (called isomers), and therefore have different properties and toxicity. In fact, the seemingly small difference of one molecule being the mirror image of the other (called stereoisomers) can cause major differences in their biological effects.

Table 1 illustrates some of the properties of 1,1,2-trichloroethane and 1,1,1-trichloroethane, which differ because they have different structures even though they have the same formula. One is about 15 times more toxic to rats than the other.

The dictionary of substances and their effects volume 7, The Royal Society of Chemistry, 1994

Although certain rules apply to naming chemicals, most chemicals are known by more than one name, or synonym, and some also have trade names. This means that finding information on specific chemicals can appear complicated but useful information can be gleaned by non-chemists and this becomes easier with familiarity and practice. When searching for information on a chemical, it is always useful to know its CAS registry number. This number is assigned by the US Chemical Abstracts Service, and each chemical has its own unique number. This system is accepted globally. Many books and databases of chemicals are indexed by CAS registry number.

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