Question

how do i find out the valence electron of nitrogen by writing the electronic arrangement

Answer 1

The number of valence electrons is the number of electrons in the outer shell, that the atom uses for bonding. Nitrogen has 5 electrons in its n=2 (outer) shell.

There is a quick way of identifying the number of valence electrons - it is the same as the Group number (not for d-block elements, though). Nitrogen is in Group 5, so it has 5 outer shell electrons.