The simplest alkane is methane, CH 4 . Ethane, the next in the series, has the molecular formula C 2 H 6 ,
but this does not convey its structure:
(Keep in mind the convention about drawing the Lewis structures of sp^3 hybridized carbon atoms
discussed above.) The formula for ethane is more informatively (and commonly) written as CH 3 CH 3.
This tells us unambiguously that each carbon atom is attached to three hydrogen atoms. This kind
of notation is known as a condensed structural formula. Similarly, the condensed structural formula
for the next alkane, propane, is CH 3 CH 2 CH 3.
For alkanes with four or more carbons, there are different ways that the carbon atoms can be
connected to each other, which makes the condensed structural formula all the more useful.
Butane, for example, can have either one of the structures shown below.
These two structures have the same molecular formula: C 4 H 10 (and hence the same molecular
weight), but have different physical properties, such as boiling point and melting point. They are
known as isomers (more specifically structural isomers) of each other. One can also say that the
compound butane has two isomers: The top one is known as a straight-chain alkane for obvious
reasons and is given the name n-butane, and the bottom one is a branched alkane and is known as
isobutane.
The number of isomers increases, for each alkane as the number of carbon atoms increases.
Pentane, for example, has three isomers, while hexane has five and decane, with 10 carbon atoms,
has 75.