electronegative

Meaning: An electronegative atom pulls shared bonding electrons toward itself. Electronegativity increases across a period (left to right) and up a group, so F is highest, followed by O, N and Cl. The unequal sharing makes a bond dipole: δ− on the more electronegative atom, δ+ on its partner. Through sigma bonds this pull is the inductive effect, which weakens with distance.

On the MCAT: Use it to locate electrophilic sites (a carbonyl carbon is δ+ because O is more electronegative), to rank acidity (Cl near a carboxylic acid stabilizes the conjugate base, so ClCH₂COOH has a lower pKa than CH₃COOH), and to judge polarity and hydrogen bonding (H on N, O or F). C–H bonds are treated as nearly nonpolar.

Common confusion: Electronegativity is not electron affinity. Electron affinity is the energy change when an isolated gaseous atom gains an electron, while electronegativity describes pull within a bond. Also, high electronegativity does not make a better nucleophile or base. Across a row, basicity falls as electronegativity rises because the atom holds its lone pair tightly (NH₂⁻ > OH⁻ > F⁻).