What defines a nucleophile?

A nucleophile is defined as a species that has a lone pair of electrons and is attracted to positive charges. This attraction is due to the nucleophile's electron-rich nature, which allows it to donate its electron pair to form a bond with an electrophile, typically a positively charged or electron-deficient species.

The presence of a lone pair is key, as it enables the nucleophile to participate in chemical reactions, such as nucleophilic substitution or addition reactions. In these reactions, the nucleophile approaches the electrophile, which possesses a partial or full positive charge, facilitating the interaction needed to form new chemical bonds.

The characteristic of being electron-rich highlights the reactivity of nucleophiles; they seek out positive centers in other molecules, making them essential players in many organic reactions. Thus, the defining attributes of a nucleophile are its ability to donate electrons and its affinity for positively charged species, encapsulated correctly by the chosen option.