chemical formula

Chemists can represent molecules in a number of ways, all of which can be referred to as a "formula". To avoid confusion it is important to understand the terminology.

Inorganic compounds, those not based on the bonding of carbon into chains or rings, tend to be fairly simple. The most common formula used for these compounds is the molecular formula. This shows exactly how many atoms of each element there are in the molecule or formula unit - NaCl for common salt, CO₂ for carbon dioxide, CuSO₄ for copper(II) sulphate and so on. An alternative to the molecular formula is the empirical formula, which simply shows the ratio of the different elements. For the three compounds above this happens to be identical to the molecular formula, but this is not always the case - hydrazine, for example, has a molecular formula of N₂H₄ and an empirical formula of NH₂.

Organic compounds tend to be more complicated than inorganic compounds and these two types of formulae can be inadequate in providing a useful description of the molecule. The rings and chains of organic chemicals can be represented by three further fundamental types of formula.

The displayed formula of a compound shows all the atoms and all the bonds between them - it is effectively a kind of drawing. A simple example is the displayed formula of methane:

    H
    |
H - C - H
    |
    H

This indicates that one atom of carbon is bonded separately to four hydrogen atoms by single bonds. The displayed formula can also be used for cyclic compounds, and of course it can display functional groups containing any kind of element. For example, the displayed formula of cyclohexanol is:

     H   H
      \ /
  H    C   O - H
   \  / \ /
 H - C   C - H
     |   |
 H - C   C - H
    / \ / \
   H   C   H
      / \
     H   H

Displayed formulae are the most explicit of the basic types. A form of shorthand is the structural formula. This provides the same information but takes certain shortcuts. For straight chain compounds, each section of the molecule can be written, rather than drawn in full. Hexane, for example, has a structural formula of CH₃CH₂CH₂CH₂CH₂CH₃, which can also be shortened to CH₃(CH₂)₄CH₃.

To represent cyclic compounds with a structural formula, a shape is drawn with a point for each carbon atom in the ring. Cyclohexane, for example, is a hexagon:

     _
   /   \
  /     \
  |     |
  |     |
  \     /
   \ _ /
     

The compound consists of carbon and hydrogen, but these are not indicated by letter because they are the basic units of organic chemistry. A structural formula for a cyclic compound assumes that any atom is carbon unless otherwise indicated, and that its tetravalency (need for four bonds) is completed by hydrogen when no other group is indicated. Other types of atoms in or attached to the ring do have to be indicated. Electron delocalisation must also be indicated, by a circle in the ring. Thus the structural formula for pyridine is:

   
      _
    /   \
   /     \ 
   |     |
   |  O  |     
   |     |
   \     /
    \   /
      N

(Maybe representing cyclic structural formulae in ASCII is not my strong point.)

Simple straight chain compounds, such as alkanes and alkenes, can also be represented by a skeletal formula. This consists simply of a zig-zag line, where each point represents a carbon atom and the requisite hydrogen atoms. Butane, for example, has the skeletal formula:

\    /\
 \  /  \
  \/    \

Double bonds and functional groups can also be represented, for example 1-hydroxybut-4-ene:

\\    /\
 \\  /  \
  \\/    \-OH

All of these basic graphical formulae - displayed, structural and skeletal - provide only a limited amount of information about the molecule; they are not a realistic drawing of what the molecule really "looks like", which cannot be achieved simply with letters and lines.

In summary, the molecular formula is the number of each type of atom, the empirical formula is the ratio of each type of atom, the displayed formula is a drawing of all the atoms and bonds in an organic compound, the structural formula is a shorthand for these atoms and bonds, and the skeletal formula is simply a line representing carbon chains in organic compounds.

For the example of butanoic acid, the molecular formula is C₄H₈O₂, the empirical formula is C₂H₄O, the displayed formula is:

    H   H   H    O
    |   |   |   //
H - C - C - C - C
    |   |   |    \
    H   H   H     O - H

the structural formula is CH₃CH₂CH₂CO₂H, and the skeletal formula is:

\    /\
 \  /  \
  \/    \=O
        |
        OH

Not to be confused with "chemical formula" is "chemical name". A chemical, or systematic, name is a description of a molecule that provides a certain amount of information about it. It is not a formula. Chemicals can have many possible "systematic names" (those derived from a logical system of nomenclature), some familiar ("trivial") names, and yet more commercial names. One compound, for example, is known as "ethanoic acid" under the formal IUPAC nomenclature, "acetic acid" by actual working chemists, and "vinegar" when diluted sufficiently and sold in supermarkets.