Remember, N atom has only 4 bonding AOs, 2s, and three 2p AOs, although it has 5 valence electrons. Thus it forms 3 (as in NH3) or 4 (as in ammonium ion) bonds, never more than 4 bonds. P atom has 3s, 3p, 3d AOs, and can form 3, 4, 5 or 6 bonds as in PF6 negative ion. The diagonal relationship between Li and Mg, Be and Al; B and Si is caused by similarity in size and electronegativity. This is caused by the mixed effects of charge and size. For example, ionic radii of Li+ =0.76 Ang. and 0.72 ang. for Mg2+. Refer to book for other values. S atom has larger size and can form S8 rinds which becomes puckered to get rid of angle strain.