For a molecule to act as a nucleus or substrate in a nucleophilic substitution reaction, it must have both a polar bond and a good leaving group. For an atom or a group to be a good leaving group, it must be able to exist independently as a relatively stable, weakly basic ion or molecule. Groups that act as leaving groups are always capable of accommodating the negative charge through a high electronegativity or by delocalization. Because halogen atoms have high electronegativities and form relatively stable ions, they act as good leaving groups.