Think about the way that carbon and hydrogen might combine to make alkanes. With one carbon and four hydrogens, only one structure is possible: methane, CH_4. Similarly, there is only one combination of two carbons with hydrogens (ethane, CH_3 CH_3) and only one combination of tree carbons and hydrogens (propane,CH_3 CH_2 CH_3). If larger numbers of carbons and hydrogens combine, however, more than one structure is possible. For example. There are two substances with the formulaC_4 H_10: the four carbons can all be in a row (butane), or they can branch (isobutane). Similarly, there are three C_5 H_12 molecules, and so on for larger alknes. (80)