1) NCl5 does not exist while PCl5 exist. why..?? 2)Oxygen exist in O2 and form multiple bond while S8 forms single bond. Why..?? 3) what is the cause for diagonal relationship.?

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.