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First elements of each group (Li, Be, B, C, N, O, F) differ significantly from their group members for four reasons:

1. Very small size → high charge density → predominantly covalent compounds (Li-Mg diagonal, not Li-Na vertical similarity).

2. No d-orbitals in Period 2 → maximum covalence limited to 4. This is why NCl5 doesn't exist (PCl5 does), NF5 is impossible (PF5 exists), and oxygen's max covalence is 2 (sulphur can reach 6 in SF6).

3. Strong ppi-ppi bonding: Only Period 2 atoms form effective pi bonds with each other (C=C, C=O, N=N, N≡N). Heavier elements (Si, P, S) form only weak pi bonds — they prefer single bonds with higher coordination numbers. This explains CO2 (discrete O=C=O) vs SiO2 (3D network of Si-O single bonds).

4. Diagonal relationships: Li-Mg, Be-Al, B-Si show more chemical similarity than vertical pairs. Caused by similar charge/radius ratio and electronegativity.

These anomalies make Period 2 chemistry qualitatively different from the rest of the periodic table.