Molecular Orbitals Chapter 9

Molecular Orbital model This model examines unpaired electrons, bond energies and excited state electrons. Examine the H 2 molecule. When 2 atomic orbitals overlap, two molecular orbitals form. One low energy bonding orbital and one high energy anti-bonding orbital

Electron Probability Distribution The electron probability of both MO and MO* is centered along the line passing between the nuclei. In the MO, the greatest e- probability is between the nuclei. In MO* the greatest e- probability is on either side of the nuclei

Describing the bonding The molecular orbitals are constructed from the atomic 1s orbitals of the H atoms. The resulting molecular orbital may be represented as : MO 1 = 1s a +1s b MO* = 1s a – 1s b

Important points The e- probability is centered along the line passing between the two nuclei.. This type of electron distribution is described as sigma (σ). MO 1 & MO* are sigma molecular orbitals. In this model only molecular orbitals are available for occupation by electrons. Atomic orbitals no longer exist because the molecule, a new entity, has its own set of new orbitals.

Bonding & Anti-bonding

More points MO 1 is lower in energy than 1s of the free H atom. This is the driving force behind molecular formation, and stability. If the electrons are forced to occupy the higher energy MO*, or anti-bonding orbitals, that reduces the stability of the molecule. Bonding MO’s are lower energy than individual atomic orbitals. Anti-bonding MO’s are higher energy than individual atomic orbitals.

Last points Molecular orbitals may be written like electron configurations. σ 1s 2 etc. Each molecular orbitals holds up to 2 electrons with opposite spin. Orbitals are conserved. The number of molecular orbitals will always be the same as the number of atomic orbitals used to construct them.

Bond Order Bond order is defined as bond order = ½ [(# of bonding e-) - (# of anti-bonding e-)]

Examples

Bond order On the left H 2 has as bond order of 1. ½(2bonding electrons-0 anti bonding) It is also diamagnetic. No unpaired electrons On the right, He 2 has a bond order of 0. ½(2 bonding electrons -2 anti bonding electrons) Not stable but still diamagnetic.

More examples Oxygen has a bond order of 2, but it also shows two unpaired electrons. Paramagnetic ½(10-6)=2 The larger the bond order, the greater the bond strength.

Still other examples

Try it Which is stable, He 2 or Li 2 ? With a bond order of 0 versus 1, Lithium is more stable. Which is more stable, O 2, O 2 + or O 2 - ? 2, 2.5 and 1.5 indicate that O 2 + is the most stable.

Paramagnetism Unpaired electrons exhibit paramagnetism. In this picture liquid oxygen is held in a magnetic field until it boils away.