Types of Formulae

Organic molecules can be shown with several types of formula. Using butane as the example:Molecular formula: the actual number of atoms of each element in one molecule, C₄H₁₀.Empirical formula: the simplest whole-number ratio of atoms of each element, C₂H₅.General formula: an algebraic formula for a whole family of compounds, C_nH_2n+2 for the alkanes.Structural formula: the minimal detail needed to show the arrangement of atoms, carbon by carbon, CH₃CH₂CH₂CH₃. Branches go in brackets, so 2-methylpropane is CH₃CH(CH₃)CH₃.Displayed formula: shows every atom and every bond as a line.Skeletal formula: the carbon chain is drawn as a zigzag of lines. Carbon atoms, and hydrogen atoms bonded to carbon, are not written. Each end of a line and each junction is a carbon atom, and functional groups such as OH or Cl are drawn in full.Worked example: propan-1-ol has the structural formula CH₃CH₂CH₂OH. Counting atoms gives the molecular formula C₃H₈O. The numbers 3, 8 and 1 have no common factor, so the empirical formula is also C₃H₈O.Ethene, C₂H₄, and cyclohexane, C₆H₁₂, both have the empirical formula CH₂, which shows that an empirical formula alone cannot identify a compound.