Simply counts the numbers of each sort of atom present in the molecule, but tells you nothing about the way they are joined e.g Propane's molecular formula is C3H8
Shows how the atoms are joined up. The structural formula of propane is CH3CH2CH3.
3 Displayed Formulae
Shows all the bonds in the molecule as individual lines. Each line represents a pair of shared electrons.
3.1 Stick Diagram of Butane
4 Coding the chain length
See p.151 of Edexel IGCSE Chemistry Student Book.
5 Coding for the type of compound
A family of similar hydrocarbons (compounds of carbon and hydrogen only) in which all the carbons are joined to each other with simple covalent bonds. Compounds like this are coded with the ending 'ane' e.g ethane is a two-carbon chain (because of 'eth') with a carbon-carbon single bond, CH3CH3
A family of hydrocarbons which contaiin a carbon-carbon double bond. Their names end in 'ene' e.g ethene is a two carbon chain containing a carbon-carbon double bond, CH2=CH2.
The number in the chain tells you which carbon atom the double bond starts from. e.g but-1-ene, a four-carbon chain with a double bond starting on the first carbon
6 Coding for branched chains
See p.152 of Edexel IGCSE Chemistry Student Book.
7 Structural isomerism
Structural isomers are molecules with the same molecular formula, but with different structural formulae.
WARNING! don't confuse the word isomer with isotope. Isotopes are atoms with the same atomic number but different mass numbers.
Structural Isomerism is the existence of two or more different structures with the same molecular formula. (see p.153 for examples)
To avoid mistakes always draw your isomer so that the longest carbon chain is horizontal.