Can anyone teach me how to write configuration for hetero-atomic molecules using MOT. Eg. $\sf H_2O$

Question

abhisar · Answer

@Kainui

abhisar · Answer

@JFraser

abhisar · Answer

I am doing it like 
$\sigma 1s^2 ~\sigma^*1s^2~\sigma 2s^2~\sigma ^*2s^2 (\pi 2p_x^1 \pi2p_y^1)$

abhisar · Answer

But according to this there are 2 unpaired electrons and thus water should be para-magnetic but water is actually diamagnetic.

abhisar · Answer

@aaronq

anonymous · Answer

Your problem is here in the $\sigma ^*2s$ orbital's placement. It's actually higher in energy than the $\pi$ orbitals.
$\sigma 1s^2 ~\sigma^*1s^2~\sigma 2s^2~\sigma ^*2s^2 (\pi 2p_x^1 \pi2p_y^1)$

So the correct placement should be:

$\sigma 1s^2 ~\sigma^*1s^2~\sigma 2p_z^2(\pi 2p_x^2 \pi2p_y^2)~\sigma ^*2p_z^0 $

Which has only paired electrons, thus diamagnetic like you're looking for.

Also note that I changed them from s to $p_z$. Remember these $\sigma$ molecular orbitals came from two p orbitals not s orbitals! Here's the difference in a picture! |dw:1439133799765:dw|