Question

Construct the molecular orbital diagram of He2 using appropriate molecular orbital labels and arrows to represent electrons. a) Sketch each molecular orbital. Explain HOW you identify an antibonding orbital. b) Identify the linear combination of atomic orbitals for each molecular orbital. Explain WHY there are two molecular orbitals. Explain why the He2 molecule is NOT observed experimentally. c)

Answer 1

Antibonding orbitals are the ones with the higher energies. In the diagram attached, the antibonding orbital has been labelled with a * on the top. Higer energy orbitals are located higher up on the energy scale. (REFER diagram.)

The diagrams shows the linear combination of the atomic orbitals. These can be seen on the left and right side of the diagram where individual He atoms are drawn. There are two molecular orbitals because of the fact that each molecular orbital can only accomodate 2 electrons. He has 2 electrons. 2 He has 2*2 = 4electrons. Hence the number of Molecular Orbitals required to fill them up are 2.

The reason for $He_2$ not being observed experimentally is very simple. Remember the formuale :

$Bond Order = \frac{N_b - N_a}{2}$ Where Nb = Number of Bonding orbitals, and Na = Number of antibonding orbitals. ANY compound with Bond Order 0 is chemically unstable, ie is NOT observed experimentally.

For $He_2$ , the number of Nb = 1 and Na = 1, leading to Nb - Na = 0 hence,Bond Order = 0.

This is the reason why $He_2$ is NOT observed.