Least electronegative atom occupies the middle (N), the other two occupy the sides
Count number of valence electrons for each
F= 7
N= 5
O= 6
Total=18
Make one bond to each atom only to start
1. F-N-O
2. Reduce 4 electrons from your total from the 2 bonds to each atom (each bond =2 electrons). 18-4 = 14.
3. Distribute the electrons around the terminal elements first to give them an octet, and then whatever is left is put on the central atom N.
4. There should be 6 on F, 6 on O, and 2 on N.
5. Since the middle atom N has to have a complete octet, remove two lone pair electrons from the O and draw a sing line (single bond) to the N which therefore gives a double bond to the O.
6. F-N=O, with one lone pair on N, and 2 lone pairs on O
