electron density

Meaning: Electron density describes where electrons are most likely to be found in a molecule. It is often shown as partial charges (δ− and δ+) or as shading on an electrostatic map. Density shifts toward more electronegative atoms in polar bonds. It is higher at lone pairs, π bonds, and sites made electron-rich by resonance or by electron-donating groups (alkyl groups, and −OH or −NH₂ through resonance). It is lower next to electron-withdrawing groups (halogens, carbonyls, nitro groups) and at positively charged atoms.

On the MCAT: Passages ask you to locate electron-rich sites (nucleophiles, Lewis bases) and electron-poor sites (electrophiles, such as a carbonyl carbon) and then predict where a reaction occurs. Withdrawing groups pull density away from a conjugate base and stabilize it, so the acid is stronger. In electrophilic aromatic substitution, activating groups raise ring density and speed the reaction, while deactivating groups lower it and slow it. In NMR, lower density around a nucleus means less shielding and a downfield shift.

Common confusion: Electronegativity is a property of an atom, while electron density is the resulting distribution in the molecule. A lone pair is not always available. In an amide, the nitrogen lone pair is delocalized into the carbonyl, so nitrogen is much less basic and less nucleophilic than in an amine.