What is the mechanism for the nucleophilic substitution of a halogenoalkane?

A halogenoalkane is a saturated hydrocarbon chain with a halogen atom (F, Cl, Br or I) attached to a carbon instead of a hydrogen. Halogens have a greater electronegativity than carbon. This means they draw the shared electron pair towards them, which polarises the carbon-halogen bond, and gives the carbon a slight delta positive charge. A nucleophile is a species with a lone pair of electrons. It is attracted to the slightly positive carbon, and donates its lone pair of elections to the carbon to form a bond. This pushes the electrons from the C-halogen bond to the halogen (remember carbon can only have 8 electrons in its valence shell), so that the halogen leaves the carbon compound as a negative halide ion.