Nature of intermolecular forces

Every halogen (X = F, Cl, Br, I) is more electronegative than carbon, so the carbon atom carrying the halogen develops a partial positive charge (C-delta+) while the halogen carries a partial negative charge (X-delta-): the carbon-halogen bond in an alkyl halide is therefore a polar covalent bond, and alkyl halides as a class are moderately polar compounds. Going down the halogen group from fluorine to iodine, the size of the halogen atom increases, so the C-X bond length increases correspondingly. At the same time the C-X bond strength (bond enthalpy) decreases as the halogen gets larger, because a larger halogen's p-orbital is more diffuse (spread out) and overlaps less effectively with the smaller, more compact orbital on carbon. Table 10.2 tabulates these trends numerically for the four methyl halides: bond length rises from 139 pm (C-F) to 214 pm (C-I) while bond enthalpy falls from 452 kJ/mol (C-F) to 234 kJ/mol (C-I …