Compounds like the two C_4 H_10 molecules and the three C_5 H_12 molecules, which have the same formula but different structures, are called isomers, from the Greek isos+meros, meaning “made of the same parts.”Isomers are compounds that have the same numbers and kinds of atoms but differ in the way the atoms are arranged. Compounds like butane and isobutane, whose atoms are connected differently, are called constitutional isomers. We’ll sec shortly that other kinds of isomers are also possible, even among compounds whose atoms are connected in the same order. As Table 3.2 shows. The number of possible alkane isomers increases dramatically as the number of carbon atoms increases.