Bond length and bond shape in molecules can be explained by the hybridisation of orbitals. A bond is formed by the overlap of atomic orbitals. Hybridisation is when atomic orbitals fuse to form newly hybridised orbitals. For example, methane contains for C-H bonds. Carbon has the electronic configuration 1s2 2s2 2p2, so only the two p electrons are unshared. This would mean only two two bonds are possible. However, four bonds are formed. The one s and three p orbitals combine to form hybrid orbitals which have 25% s character and 75% p character. These are sp3 hybridised orbitals. The makes all four bonds in methane equivalent to each other, giving equal bond angles of 109 degrees and methane a tetrahedral shape. In ethane, a pi-bond is required for the double bond. This uses sp2 hybridised orbitals. The 2s orbitals mix with the two of the three available 2p orbitals forming three sp2 orbitals with a remaining p orbital. A single bond has two electrons, with the weakest bond strength and the longest bond length. Double bonds contain 4 electrons, considered to be in the middle for both bond strength and length. Triple bond has 6 electrons, with the strongest bond strength and shortest bond length.
As the number of the electrons shared between two atoms increases, the bond strength increases. The distance between the nuclei decreases and so you get a stronger, shorter bond. So the double C=C bond in ethene is stronger and shorter than the single C-C bond of ethane. The bond strengths and lengths in the benzene ring os an aromatic hydrocarbon are different than those in a straight chain alkene. The formula for benzene is C6H6.
Halogenoalkanes are compounds in which a halogen atom has replaced at least one of the hydrogen atoms in an alkane chain. In the past, halogenoalkanes have been used as refrigerants, aerosol propellents, degreasing agents and dry-cleaning solvents. However, they are no longer used in aerosols because of their damaging effect on the ozone layer. Halogenoalkanes are also important in organic synthesis and can be used to prepare many useful materials. Halogenoalkanes have a the general formula CnH2n+1X, where X represents the halogen atom. Halogenoalkanes contain only single bonds, and their names re based on the alkane homologous series. The appropriate prefix is added to the name of the longest alkane chain. The position of the halogen on the chain is indicated if there is more than one halogen atom in the compound. Fore example, if the halogen was F the prefix would be fluoro-, or if it was CL the prefix would be chloro-. For the halogenoalkane 3-bromo-2-chloropentane shown in Figure 4.