Dipole moment arrows confusion

[Sho Kano]

My teacher posted on his website, Lewis structures of different compounds. One of them were N3-

Why isn't there a dipole moment arrow pointing from the central atom to the other Nitrogen? Does the arrow only point from neutral atom to negatively charged atom?

 

[Borek]

I am not sure I understand your question.

First of all - is it obligatory to mark a dipole in the Lewis structure?

 

[Sho Kano]

I am not sure I understand your question.

First of all - is it obligatory to mark a dipole in the Lewis structure?

Don't know but He wants it done