The number of structural isomers of butane (C4H10) is

Butane can be written as the straight chain n-butane and as the branched 2-methylpropane (isobutane), so there are 2 structural isomers. Pentane has 3 isomers and hexane 5, which is why 3 or 4 tempt careless students. No other arrangement of four carbons gives a different skeleton.