A line of atoms of the same type in a molecule. In a straight chain the atoms are attached only to single atoms, not to groups. Propane, for instance, is a straight-chain alkane, CH₃CH₂CH₃, with a chain of three carbon atoms. A branched chain is one in which there are side groups attached to the chain. Thus, 3-ethyloctane, CH₃CH₂CH(C₂H₅)C₅H₁₁, is a branched-chain alkane in which there is a side chain (C₂H₅) attached to the third carbon atom. A closed chain is a ring of atoms in a molecule; otherwise the molecule has an open chain.