In Diazonium Ion why does N contain 4 bonds? Nitrogen's valenc number is 3. Sometimes it can form a co-ordinate bond by donating it's lone pair electrons. In diazonium ion N has 4 bonds, then one of those 4 bonds must be a co-ordinate bond. Actually I can't define the co-ordinate bond in diazonium ion.Again In diazonium structure, it is shown the first N with 4 bonds is positive but why??
One way to think about this structure, the benzenediazonium cation, is to consider the structure of benzonitrile - C6H5CN - just replace the positive nitrogen with a neutral carbon atom. This has the same basic electronic structure as the diazonium ion, but differs in that the carbon nucleus has one less positive charge than the nitrogen nucleus.

I don't think the concept of "coordinate bonds" is very helpful here. There are many different species with positively charged nitrogen, such as tetramethylammonium salts, that have four full covalent bonds to nitrogen.