Why does methane have no isomer

Ethane consists of two carbons, so the molecule simply cannot form branches and only exists in an unbranched configuration:

H3C-CH3

Propane consists of three carbons, but just like ethane is too short to form branches. Therefore, it also exists in an unbranched configuration only:

H3C-CH2-CH3

Keep in mind, however, that an unbranched configuration doesn't mean that the molecule is a straight line. Take a look at a propane molecule which is, in fact, kinky:

http://www.lloydminsterheavyoil.com/propane.jpg

Butane has four carbons and is the smallest alkane that can form isomers. There are two options. The first one is an unbranched configuration, called n-butane, which looks like this:

http://www.it.swin.edu.au/personal/fwang/Mom/n_but...

The second one is a branched configuration where a methyl group is positioned on the second carbon (called isobutane, or i-butane for short). It looks like this:

http://www.lloydminsterheavyoil.com/Isobutane.JPG

I hope this helps! Good luck!

Source(s): BA Chemistry