Do different types of MOs have different occupancies?

There was a question in my textbook regarding the number of electrons in a molecular orbital.



> 
> The maximum number of electrons in a sigma, pi, and delta molecular orbital are respectively?
> 
> 
> 


Now, I believe that any molecular orbital can be occupied by a maximum of two electrons, and so the answer might be 2 electrons for sigma, 2 electrons for pi, and also 2 for delta orbital.


Is my answer correct, or are there other concepts that need to be applied? Is it true that any molecular orbital can occupy a maximum of only 2 electrons? Or is it possible to have more than 2 electrons for delta molecular orbital?




Firstly, note that the labels $\sigma$, $\pi$, and $\delta$ aren't universally applicable to MOs; it depends on the molecular geometry. These labels are mostly useful for linear molecules. Non-linear molecules often have MOs that are labelled differently. Methane is a decent example. Other examples include water and ammonia.


![MO diagram of methane](https://i.stack.imgur.com/gXDRz.png)
Restricting ourselves to linear molecules, where the labels *do* apply: some of the confusion likely arises because of the "simplest" case of a homonuclear diatomic molecule ($\ce{X2}$), where $\sigma$ MOs are always "alone" and $\pi$ MOs always come in pairs.


![MO diagram of a generic homonuclear diatomic](https://i.stack.imgur.com/5uZCu.png)
So it's tempting to think that $\pi$ MOs can fit four electrons. This is not correct, though. The fact is that there are **two** $\pi$ MOs that happen to have the same energy, and each can fit two electrons.


Every MO can contain up to two electrons, *period*.


(Well, at least at this level of knowledge. If you are doing electronic structure theory, you may find that this statement is oversimplified; but you can work that out for yourself.)


Pictures are taken from Housecroft & Sharpe's *Inorganic Chemistry*, 4th ed.


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