A nucleophile donates a pair of electrons — it's a Lewis base
A nucleophile ("nucleus-loving") is electron-rich: it has a lone pair or a π bond it can give away to form a new bond. That is exactly the definition of a Lewis base, so every nucleophile is a Lewis base. A Brønsted base is just the special case where the electrons go to an H⁺ instead of a carbon.
Spot one by looking for a lone pair, a negative charge, or a π system. The more available those electrons, the stronger the nucleophile: charged beats neutral (HO⁻ > H₂O), and down a column polarizability wins (I⁻ > Br⁻ > Cl⁻ in a protic solvent).
Common nucleophiles — a lone pair or negative charge ready to donate. Drawn live.
An electrophile accepts a pair of electrons — it's a Lewis acid
An electrophile ("electron-loving") is electron-poor: a positive charge, an empty orbital, or a δ+ atom created by a polar bond. It accepts the electron pair — the definition of a Lewis acid, so every electrophile is a Lewis acid. A Brønsted acid is the special case where the accepting atom is hydrogen.
The single most common electrophile in organic mechanisms is the δ+ carbon of a carbonyl (C=O). Also watch for carbocations, H⁺ and metal cations, and electron-deficient atoms like the boron of BF₃ or the aluminum of AlCl₃.
Common electrophiles — a positive charge, empty orbital, or δ+ atom ready to accept. Drawn live.
Nucleophilicity and electrophilicity are a spectrum
These words describe how much a species donates or accepts electrons. Down a column of the periodic table, nucleophilicity goes up (the atom is more polarizable, its electrons farther out). Across a row, it tracks basicity. In a polar protic solvent the trend reverses — small charged species get heavily solvated and slowed, so I⁻ outpaces F⁻ as a nucleophile in water even though F⁻ is the stronger base.
Nucleophilicity vs basicity — related, not identical
Basicity measures bonding to H⁺ (a small, naked target); nucleophilicity measures bonding to a carbon electrophile (bigger, hindered). A hindered species like tert-butoxide is a strong base but a poor nucleophile — fine for snatching a small H, too bulky to attack a substituted carbon. That single distinction is what decides E2 vs SN2 in many problems.
Why it matters: >95% of reactions are a nucleophile meeting an electrophile
Once you can label the nucleophile and the electrophile in a problem, the curved arrow almost draws itself — electrons flow from the electron-rich site to the electron-poor site, exactly as in the figure above. Master this one pattern and most of the course becomes variations on it: SN1/SN2, additions to carbonyls, electrophilic aromatic substitution, and more.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.