The formal charge of the O -atoms in the ion [:N=O:] will be calculated by this formula:
Formal charge of atom = Total valence electrons − Number of unshared electrons −21 ( Number of bonding electrons)
Where, Total valence electrons =6
Number of unshared electrons =4
Total number of bonding electrons =4
Put the all values in above formula, Then
Formal charge on oxygen atom =6−4−24 =6−4−2=0
Formal charge on oxygen atom will be zero.