Hydrocarbons differ from each other also by the way carbon atoms connect to each other. Straight-chain hydrocarbon, branched hydrocarbon are ca∥ ed structural isomers(结构异构体). Structural isomers have Molecules having the same chemical formula but different structure different physical and chemical properties. The number of possible structural isomers for a chemical formula increases rapidly as the number of carbon atoms increases 3 for csh. 18 for coho. 75 for C10H2,366,319f0rC20H42 HHHH H H CH3H H H CH3 H H-C-C-C-C-C-H H-C-C-C-C-H H—C-C-C—H HHHH H HHH H H CH3 H n-pentane ISo-pentane Neo- pentane• Hydrocarbons differ from each other also by the way carbon atoms connect to each other. Straight-chain hydrocarbon, branched hydrocarbon • Molecules having the same chemical formula but different structures are called structural isomers (结构异构体). Structural isomers have different physical and chemical properties. The number of possible structural isomers for a chemical formula increases rapidly as the number of carbon atoms increases: 3 for C5H12, 18 for C8H18, 75 for C10H22, 366,319 for C20H42. n-pentane iso-pentane Neo- pentane C H H H C C C C H H H H H H H H H C H H H C C C H CH3 H H H H H C H H H C C H CH3 CH3 H H