The chemical formula
The chemical formula is a shorthand way of showing the constituents of a substance. The chemical formula is not required for CLP classification, but is required for Classification and Labelling Inventory notification, and REACH registration. It can also be listed on the Safety Data Sheet for a substance.
The formula comprises the element symbol from the Periodic table, together with a number showing how often it appears in the structure. Where a group of elements is repeated, there may be brackets round the group with a number showing how often it is included in the structure.
The most well-known chemical structure is probably water, H2O, that is two atoms of hydrogen and one atom of oxygen.
The chemical formula sometimes gives an idea of the structure (that is the shape) of the molecule, but usually does not.
An example of this is ethanol, (the alcohol you drink), which can be shown as C2H5OH , which indicates that there is a hydroxyl group in the structure; or it can be given as C2H6O, which does not indicate anything about the structure. In fact, C2H6O can also describe dimethyl ether, CH3OCH3, which is a completely different chemical to ethanol, so you do need to be careful about reading and writing chemical formulae.
A copy of the Periodic Table is published by the Royal Society of Chemistry at: http://www.rsc.org/periodic-table .
Four tables based on atomic number, element name, atomic symbol, and atomic mass are included for reference: