Basicity = tendency to donate the lone pair on N to a proton. Three factors compete in solution: (1) +I effect of alkyl groups pushes electron density onto N (increases basicity): 3° > 2° > 1° > NH3. (2) Solvation of conjugate acid (RNH3+, R2NH2+, R3NH+): smaller, less hindered cations are better solvated by water, stabilising them. 1° aminium best solvated > 2° > 3° (fewest N-H for H-bonding). (3) Steric effect: bulky groups around N hinder approach of H+. Net result in water: 2° > 1° > 3° > NH3 $\frac{for standard ethyl}{methyl amines}$. Aromatic amines: much weaker bases. Aniline (pKb 9.4) vs ethylamine (pKb 3.3) — the lone pair on N in aniline is delocalised into the ring (+M), making it less available. EWGs on ring (NO2) further decrease basicity. EDGs (CH3, OCH3) increase it slightly.