I'm trying to figure out how to do polyatomic molecules with the generic ordering of MO's, and I'm having trouble! I'll start with H2O. I'm guessing that since there are no double bonds, no pi-orbitals are used, so I thought the six MO's would be this, with two of each type because there are two O-H bonds:

2 x σ*

2 x n

2 x σ

Is that right? But if it is, I have no idea:

1) How to know when molecules have nonbonding orbitals in the first place (I guessed it here only because I knew I needed six total MO's)

2) How to distribute electrons when you do have them. Do nonbonding orbitals still follow the Pauli exclusion principle?

Thanks!