[vhpk]

I want to ask how we write the MO configuration of BN, sorry I have my idea but I can't find the mathematics sign such as sigma or pi so I can't show my attempts

[Bakegaku]

You could use s and p, or spell out sigma and pi.  I think we'd get the idea.

I'd suggest making a picture in photoshop or MS Paint and attaching it to your post, though.

[vhpk]

Yeah, I've seen in some of my books, some authors have written the configuration of BN:
(sigma 2s)² (sigma* 2s)² (pi x,y)³ (sigma z)¹
But the others write:
(sigma z)² (sigma* z)² (pi x,y)²
So I want to ask your idea about this problem and can you give me the general advice to write the configuration of the compounds.
Thanks 

[enahs]

So far you have probably learned about MO with diatomic molecules. I.E. Two of the same atoms, and possibly used the MO diagram to determine the bond order and why certain molecules do not exist/do exist.

The key feature here is that they have all been the same. When you begin to have two different atoms or polyatomic molecules, the MO diagram gets funky. Energy levels begin to cross (say the Pi* is lower then the ?_bondingl levels begin to cross). Also when they differ in energies different "levels" will have more character (I.E. one will tend to go into the bonding orbital while the other might go into the anti bonding orbital).

So I would not worry to much, just follow the rules you have been taught, as that is what your instructor will judge you by.