Question: what are the requirements of a good nucleophile?
Answer: Charge. “The conjugate base is always a better nucleophile”. HO- is a better nucleophile than H2O. NH2(-) is a better nucleophile than NH3. HS(-) is a better nucleophile than H2S. ... 
Electronegativity. Nucleophilicity increases as you go to the left along the periodic table.