Nucleophile
In general, within a row across the periodic table, the more basic the ion (the higher the pKa of the conjugate acid) the more reactive it is as a nucleophile. In a given group, polarizability is more important in the determination of the nucleophilicity. In other words, the easier it is to distort the electron cloud around an atom or molecule, the more readily it will react. For example, the iodide ion (I−) is more nucleophilic than the fluoride ion (F−).