First element of each group (Li, Be, B, C, N, O, F) differs markedly from the rest due to: (1) Very small size and high charge density → predominantly covalent. (2) Absence of d-orbitals → max covalence = 4 (Period 2 limit). Hence N can't form NCl5 but P forms PCl5; O can't expand beyond 2 bonds. (3) Strong ppi-ppi bonding: Only Period 2 elements form effective multiple bonds: C=C, C=O, N=N, N triple bond N, O=O. Heavier elements (Si, P, S) prefer single bonds + higher coordination. This is why CO2 is molecular (O=C=O) but SiO2 is a 3D network (Si-O single bonds). (4) Diagonal relationships: Li-Mg, Be-Al, B-Si — similar charge/radius ratio gives chemical similarities.