A nucleophile is a molecule which attracts other molecules that are electron-deficient (electrophiles). Nucleophiles will usually have lone pairs of electrons that are aligned to increase their electrostatic attraction according to the Valence shell electron pair repulsion theory, making them polar (or sometimes amphipathic).

An everyday example of a nucleophile is water. In its normal alignment, water looks like this:

    
 **O**
  / \
 /   \
H     H

With the stars representing lone pairs of electrons. Electrophiles will be attracted to the negative pole of the water molecule and join with an ionic bond.