Unsaturated hydrocarbons with triple bonds are called alkynes. In systematic nomenclature, C2H2 would be ethyne, although a more common name for it is acetylene.The simplest alkenes and alkynes are shown in the margin, on the previous page, with the geometry that results from their carbon-carbon bonds.

A new kind of isomerism appears with butene, C4H8. Butene has three different structural isomers, depending on whether the four carbons are in a straight or branched chain, and where the double bond is located.

These structural isomers are 1-butene, 2-butene (with the number describing the position of the double bond), and isobutylene. (The systematic name for isobutylene is 1-methylpropene.) 1-Butene, 2-butene, and isobutylene are genuine structural isomers because their atoms are connected to one another in different ways.

2-Butene has one methyl group on each double-bonded carbon, but these methyl groups can be placed in two different ways relative to the double bond: on the same side of the molecule, or diagonally across the bond. These two possibilities result in two geometrical isomers, cis2-butene and trans-2-butene, which are shown on the next page ( page 11 ). These are geometrical rather than structural isomers because they have the same connections between atoms.