Saturated and unsaturated structures
Hydrocarbons that contain only single bonds are called saturated hydrocarbons because each carbon atom is bonded to as many hydrogen atoms as possible. The figure below shows a molecule of ethane, which is a saturated hydrocarbon.
Definition: Saturated compounds
A saturated compound has no double or triple bonds (i.e. they have single bonds only). All carbon atoms are bonded to four other atoms.
Hydrocarbons that contain double or triple bonds are called unsaturated hydrocarbons because they don’t contain as many hydrogen atoms as possible.
Definition: Unsaturated compounds
An unsaturated compound contains double or triple bonds. A carbon atom may therefore be bonded to only two or three other atoms.
The figure below shows molecules of ethene and ethyne which are unsaturated hydrocarbons. If you compare the number of carbon and hydrogen atoms in a molecule of ethane and a molecule of ethene, you will see that the number of hydrogen atoms in ethene is less than the number of hydrogen atoms in ethane despite the fact that they both contain two carbon atoms. In order for an unsaturated hydrocarbon compound to become saturated, one of the two (or three) bonds in a double (or triple) bond has to be broken, and additional atoms added.