[PSUkidGT]

I was wondering if anyone could explain this to me a little or possibly send me a link that could help me out.  I'm about 3 weeks into my Organic Chem I class and I understand everything up to this point, but he recently got into HOMO and LUMO.  In case nobody knows, HOMO stands for Highest Occupied Molecular Orbital and LUMO is Lowest Unoccupied Molecular Orbital.  I understand the concept of HOMO and LUMO, but where I get confused is knowing how may molecular orbitals an element or compound has.  I was told that both diatomic molecules, N2 and O2 have the same amount of molecular orbitals, but not quite sure why.  Any help would be appreciated! 

[Yggdrasil]

The number of molecular orbitals will be equal to the total number of atomic orbitals from the constituent atoms of the molecule.  So, for example, each N atom has 5 atomic orbitals (1s, 2s, 2p_x, 2p_y, and 2p_z) so N₂ has 10 molecular orbitals (σ1s, σ*1s, σ2s, σ*2s, σ2p_z, σ*2p_z, π2p_x, π*2p_x, π2p_y, and π*2p_y).

[rkaminski]

Sorry, Yggdrasil, but you're wrong. Every atom has infinite amount of orbitals (it comes from quantum theory), therefore molecule also has infinite number of orbitals. The point is which of them have lowest energy and can, I could say, participate in chemical bonding or other chemical interactions. And to distinguish which molecular orbital can be HOMO, LUMO or SOMO (semi-occupied molecular orbital, this if for example for radicals) you have to know, first of all, the no. of electrons in molecule.

[Mitch]

Sorry, Yggdrasil, but you're wrong. Every atom has infinite amount of orbitals (it comes from quantum theory), therefore molecule also has infinite number of orbitals. The point is which of them have lowest energy and can, I could say, participate in chemical bonding or other chemical interactions. And to distinguish which molecular orbital can be HOMO, LUMO or SOMO (semi-occupied molecular orbital, this if for example for radicals) you have to know, first of all, the no. of electrons in molecule.

When answering you have to remember at what level the student is asking a question. A theoretical discussion on what orbitals really are is fine between experts in the field, but it'll confuse a 1st semester o-chem student and really isn't necessary at this level.

Yggdrasil is right in the sense that the atomic orbitals he mentioned are the ones actually used for that atom. You can say, "oh but its a mixture of all these higher orbitals too", but from a practical standpoint they can be ignored.

[rkaminski]

Yggdrasil is right in the sense that the atomic orbitals he mentioned are the ones actually used for that atom. You can say, "oh but its a mixture of all these higher orbitals too", but from a practical standpoint they can be ignored.

That is why I wrote:

The point is which of them have lowest energy and can, I could say, participate in chemical bonding or other chemical interactions.

And sometimes occurs that even there higher orbitals have influence on bong lengths or angles, especially in so called hypervalent molecules.

[english]

That is why I wrote:

The point is which of them have lowest energy and can, I could say, participate in chemical bonding or other chemical interactions.

The bolded statement may be more obscure than necessary.  It's better to say that the bonding electrons in their respective molecular orbitals of the molecule of interest are in their ground state, and therefore fill up each molecular orbital following Hund's rules.

This will give you the lowest energy state.