A nucleus is any atom that has a partial or fully positive charge associated with it. A nucleophile is an atom or group that is attracted to a source of partial or full positive charge. Alkyl halides act as a nucleus because of the great electronegativity differences between the carbon atom and the halogen atom directly bonded to it. This great electronegativity difference causes the electron density in the overlap region between the carbon and halogen atoms to be pulled toward the halogen atom. This shifting of electron density in the molecule makes the carbon atom partially positive (the nucleus) and the halide ion partially negative (the incipient leaving group).
Figure 1 illustrates the effect of electronegativity differences on bond polarity.
Electrons in the overlap region between the carbon and the halogen atoms are attracted to the more electronegative halogen atom. The carbon atom, which now has less of a share of the bonding electrons, becomes partially positive, and the halogen atom, which has a greater share of these electrons, becomes partially negative.
Remember that a nucleophile is a substance that has a pair of electrons that it can donate to another atom. The weaker the forces of attraction holding the electron pair to the original molecule, the more readily this molecule will share the electrons and the stronger the resulting nucleophile will be. The weakest held electrons on an atom are the nonbonding electron pairs. Electrons in π bonds, although held more strongly than nonbonding electrons, are also loosely held and easily shared, making unsaturated compounds relatively good nucleophiles.
Because they possess a negative charge, anions are always better nucleophiles than their conjugate acids.