what makes a good nucleophile

it decreases from carbon to f

more electronegative elements hold their electrons more tightly, and are less able to donate them to form a new bond.

The vertical periodic trend for nucleophilicity is somewhat more complicated that that for basicity: depending on the solvent that the reaction is taking place in, the nucleophilicity trend can go in either direction. Let’s take the simple example of the SN2 reaction below:

Relative nucleophilicity in a protic solvent increases

Relative nucleophilicity in a polar aprotic solvent decreases down the group

If the electron lone pair on a heteroatom is delocalized by resonance, it is inherently less reactive - meaning less nucleophilic, and also less basic.

Steric hindrance is an important consideration when evaluating nucleophility. For example, tert-butanol is less potent as a nucleophile than methanol. This is because the comparatively bulky methyl groups on the tertiary alcohol effectively block the route of attack by the nucleophilic oxygen, slowing the reaction down considerably