A functional group is an atom or a group of atoms that determine the chemical properties of a compound.
For example the functional group of an alcohol is the -OH group and that of alkenes is the C=C carbon to carbon double bond.
A Homologous series is a group of substances with:
- the same general formula
- similar chemical properties because they have the same functional group
- a trend (graduation) in physical properties
Isomers are molecules with the same molecular formula but with a different structure.