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The first element of each p-block group (B, C, N, O, F, Ne) differs significantly from the rest due to: (1) very small size, (2) high electronegativity, (3) absence of d-orbitals limiting maximum covalence, and (4) ability to form strong ppi-ppi multiple bonds.

Maximum covalence limits: B(4), C(4), N(4), O(2), F(1). Heavier elements can expand octet using d-orbitals: P(5,6), S(4,6), Cl(7), Xe(2,4,6,8).

Specific anomalies: B never forms $B^{3+}$ ions. C shows maximum catenation and forms ppi-ppi bonds (C=C, C triple bond C). N forms very strong N triple bond N $\frac{946 kJ}{mol}$ but cannot form NCl5. O has max covalence of 2 (OF2 exists, OCl2 does not, OF6 impossible). F-F bond is anomalously weak (lone pair repulsion). HF is unexpectedly weak acid (strong H-F bond).

Diagonal relationships arise as compensation: B behaves like Si, Be like Al, Li like Mg. The diagonal pair has similar charge/size ratio and electronegativity, leading to parallel chemistry despite being in different groups.

These anomalies are among the most frequently tested concepts in JEE p-block chemistry.