HOMO in the MO diagram of C₂

Question

Identify the HOMO in the MO diagram of C₂

a) σ (2s orbitals)

b) σ (2pz orbitals)

c) π (2px and 2py)

d) π* (2px and 2py)

e) σ* (2pz)

please help me

Answer 1

Hi Reina S.,

As you may know, HOMO is shorthand for "highest (energy) occupied molecular orbital".

So, in order to find what that might be, first you have to start filling in the bonds for C₂ . The first bond C-C forms is the σ (a), for 2 electrons. The next is π (c), that soaks up 4 electrons maximum. That leaves you with two more electrons, which can go into the σ (b) bonding orbital. That's all the electrons available, a total of 4 bonds between the C nuclei. That last one is the HOMO, and although it is stable with respect to isolated carbon atoms, it is less stable than forming σ bonds out to H nuclei (with hybidization, which you will learn about). That's why carbon forms lots of big molecules, bristling with H on the surface. Carbon is unusual in being even able to form 4 bonds between two atoms (C-C); I believe tungsten can also do that, though it prefers to metallic-bond.

-- Cheers, -- Mr. d.