Comparative nucleophilic strengths:
The comparative nucleophilic strengths of these atoms are described by looking at the products that would be created if these atoms were to work as nucleophiles. Let us compare the 3 molecules HF, H2O, and NH3 and see what occurs if they were to create a bond to a proton. Because the proton has no electrons, both electrons for the new bond must come from the nucleophilic centers (that is the F, O, and N). the result of it is that, these atoms will gain a positive charge. If hydrogen ?uoride works as a nucleophile, after that the ?uorine atom gains a positive charge. Because the ?uorine atom is strongly electronegative, it does not tolerate a positive charge. Hence, this reaction does not occur. Oxygen is less electronegative and is capable to tolerate the positive charge little better; like that equilibrium is possible among the charged and uncharged species. Nitrogen is the least electronegative of the 3 atoms and tolerates the positive charge very well that the reaction is irreversible and a salt is created.
So, nitrogen is strongly nucleophilic and will generally react like such, while halogens are weakly nucleophilic and will rarely react like such.
Finally, it is worth noting that all these molecules are weaker nucleophiles than their corresponding anions, i.e. HF, H2O, and NH3 are weaker nucleophiles than F-, OH- and NH-2 correspondingly.