[Kalibasa]

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!

[cth]

I'll start with H2O. I'm guessing that since there are no double bonds, no pi-orbitals are used

It doesn't work that way.

What you should look at is symmetry and energy of the atomic orbitals. If they are not orthogonal to one another and not too far away from each other in energy level, atomic orbitals will hybridise.

First, you can have a look at http://www.ch.ic.ac.uk/vchemlib/course/mo_theory/main.html, water is near the middle of the page. I just used google with keywords "water molecular orbital" and picked the first choice.