A reagent that brings an electron pair to the reactive site is called a nucleophile (Nu:), i.e., nucleus seeking. Nucleophiles attack the electron deficient centre of a substrate. Examples include negatively charged ions with lone pairs like hydroxide ($HO^-$), cyanide ($NC^-$), and carbanions ($R_3C:^-$). Neutral molecules such as $H_2O:$, $R_3N:$, and $R_2NH$ also act as nucleophiles due to lone pairs.

A reagent that takes away an electron pair from the reactive site is called an electrophile ($E^+$), i.e., electron seeking. Electrophiles attack the electron rich centre of a substrate. Examples include carbocations like $CH_3^+$, which have a sextet configuration and are electron deficient. Neutral molecules like alkyl halides ($R_3C-X$) or carbonyl groups ($>C=O$) also act as electrophiles because bond polarity generates a partial positive charge on the carbon atom, creating an electrophilic centre.