Lecture 5: Heteronuclear Diatomic Molecules

Read section 1.17 from your textbook. Work through practice problems to prepare for your exam.

Forming Molecular Orbitals

There are two factors that govern the formation of molecular orbitals from atomic orbitals:
  1. symmetry
  2. energy

Orbitals must have the appropriate symmetry to overlap. When we talk about bonds in diatomic molecules, the bond axis is always the z axis. By symmetry: s orbitals can overlap with s orbitals, pz orbitals can overlap with pz orbitals, and s orbitals can overlap with pz orbitals.

How about energy? In the same row, the valence orbitals decrease in energy from left to right as the electronegativity increases. The atomic orbital ionization energy (the amount of energy it takes to remove an electron from that orbital) give a quantitative energy difference between orbitals.



HF

Let's look at HF. Hydrogen has only one valence atomic orbital, 1s. It takes only 13.6 eV to remove an electron from that orbital, meaning that it is not highly stablilized. A highly stabilized orbital is low in energy. Fluorine has 2s and 2p valence orbitals. The 2s orbital with an ionization energy of 46.4 eV is much too low in energy to interact with the hydrogen 1s. However, the 2p orbitals have an ionization energy of 18.7 eV. They are only 5 eV lower in energy than the hydrogen 1s and can interact.

The F 2pz and the H 1s have the correct symmetry to overlap and make two sigma symmetry molecular orbitals. There are no other orbitals on H so the remaining F 2p orbitals are non-bonding molecular orbitals. There are 5 atomic orbitals (valence orbitals on H and F) and these are combined to form 5 molecular orbitals of HF (sigma bonding, sigma antibonding, and non-bonding).



CO

Carbon monoxide has a more complicated molecular orbital diagram. Carbon and oxygen each have 4 valence orbitals (2s, 2px, 2py, 2pz). Oxygen is more elctronegative than carbon, so its orbitals are more stabilized and lower in energy but the carbon and oxygen orbitals are still close enough to interact.

Let's look at the sigma orbitals first. The sigma symmetry orbitals are C2s, C2pz, O2s, and O2pz. We can combine these 4 atomic orbitals to make 4 molecular orbitals 1, 2, 3, and 4.



There are 2px orbitals on carbon and oxygen that can combine to form a pair of symmetry molecular orbitals and 2py orbitals on each atom that can combine to form another set of molecular orbitals. This gives the full molecular orbital diagram below.