Nucleophilicity with respect to the solvent
Actually nucleophilicity is inversely proportional to electronegativity when comparing compounds across the period, and inversely proportional to size when comparing in the same group. This condition is meant in aprotic solvent.

In protic solvents  the more electronegative ions gets easily solvated to a larger extent (i.e., like a ball with the ion in the centre and the water molecules around it) so the order across the period remains same since electronegativity is not effected but the order about the group changes since the now smaller ions become the larger ones.

For example, the trend for nucleophilicity is  $\ce{F- &gt; Cl- &gt; Br- &gt; I-}$ in aprotic solvent but in protic solvent it is 
$\ce{I- &gt; Br- &gt; Cl- &gt; F-}$    ($\ce{F-}$ is now larger than others in size because it is solvated.)