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(Chem.) The formula of a compound expresses the number of atoms of each element present. The simplest, the empirical formula (EF), simply gives the ratio of atoms. The molecular formula (MF) gives the actual number present. For many compounds, especially organic ones, further information is needed about how the atoms are bonded together and this is given by the structural formula (SF). For butane, the EF is C{TAG(tag=>sub)}2{TAG}H{TAG(tag=>sub)}5{TAG}, the MF is C{TAG(tag=>sub)}4{TAG}H{TAG(tag=>sub)}10{TAG}, and the SF is CH{TAG(tag=>sub)}3{TAG}CH{TAG(tag=>sub)}2{TAG}CH{TAG(tag=>sub)}2{TAG}CH{TAG(tag=>sub)}3{TAG}. Its isomer methylpropane (CH{TAG(tag=>sub)}3{TAG}){TAG(tag=>sub)}3{TAG}CH has the same EF and MF but a different SF. In simple inorganic compounds, round brackets are used if a polyatomic ion such as sulphate or nitrate is present more than once, for example Al{TAG(tag=>sub)}2{TAG}(SO{TAG(tag=>sub)}4{TAG}){TAG(tag=>sub)}3{TAG} or Pb(NO{TAG(tag=>sub)}3{TAG}){TAG(tag=>sub)}2{TAG}. Complex ions are indicated by the use of square brackets as in [Cu(NH{TAG(tag=>sub)}3{TAG}){TAG(tag=>sub)}4{TAG}(H{TAG(tag=>sub)}2{TAG}O){TAG(tag=>sub)}2{TAG}]{TAG(tag=>sup)}2+{TAG}.

(Math.) A formal mathematical expression of some general rule or law. Examples are: A = πr {TAG(tag=>sup)}2{TAG} for the area A of a circle with radius r; the trigonometrical identity sin 2x = 2 sin x cos x; the nth term of the sequence 1, 2, 4, 8,… expressed as 2{TAG(tag=>sup)}n-1{TAG}.



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