Nitrogen is anomalous due to: (1) Very strong N≡N bond $\frac{946 kJ}{mol}$ — N2 is chemically inert at RT. (2) No d-orbitals → max covalence = 4 (cannot form NCl5, NF5). PCl5 exists because P has 3d orbitals. (3) Forms strong ppi-ppi multiple bonds (N=N, N=O, N≡N); P forms weaker ppi-dpi bonds instead. (4) Small size and high electronegativity allow strong H-bonding (NH3 has higher b.p. than PH3). (5) Nitrogen shows oxidation states from -3 to +5: NH3(-3), N2H4(-2), NH2OH(-1), N2(0), N2O(+1), NO(+2), N2O3(+3), NO2(+4), N2O5(+5). The wide range of N oxide chemistry is unique.