Molecular Orbital Theory

Question 1

State was is meant by a constructive interference.

Answer 1

An in-phase combination of atomic orbitals - ADD.

Question 2

State was is meant by destructive interference.

Answer 2

An out-of-phase combination of atomic orbitals - SUBTRACT.

Question 3

How can we tell if a molecular orbital is sigma?

Answer 3

If you rotate around the internuclear axis and the wave function remains the SAME. No nodal planes.

Question 4

How can we tell if a molecular orbital is pi?

Answer 4

Orbital changes sign as we rotate by 180/pi. Single nodal plane.

Question 5

How can we tell if a molecular orbital is § (delta)?

Answer 5

Orbital changes sign as we rotate by 90. Two nodal planes.

Question 6

What determines what atomic orbitals will combine?

Answer 6

Quantum mechanics.

Question 7

How can we tell if a molecular orbital is even (g) with respect to inversion?

Answer 7

Wave function does NOT change sign with respect to the centre of inversion.

Question 8

How can we tell if a molecular orbital is uneven (u) with respect to inversion?

Answer 8

Wave function DOES change sign with respect to the centre of inversion.

Question 9

To what molecules do g/u labels apply?

Answer 9

Molecules with a centre of symmetry i.e. ONLY homonuclear diatomics.

Question 10

State the bond order equation.

Answer 10

BO = 1/2 (number of bonding electrons - number of antibonding electrons)

Question 11

State the 5 rules for forming MOs.

Answer 11

1. The same number of MOs form as the number of AOs combined.
2. Only AOs of the correct symmetry combine.
3. The closer in energy the AOs are, the stronger the interaction.
4. The size of AOs is also a factor in determining how strong the interaction is.
5. Each MO is formed from a particular combination of AOs, with the contribution from each AO determined by how close in energy they are to the MO energy.

Question 12

Draw an MO diagram with labels and annotation.

Answer 12

See poster on wardrobe

Question 13

State the 3 ions of O2.

Answer 13

O2 - : superoxide
O2 2- : peroxide
O2 + : not so common

Question 14

How did MO diagrams explain the paramagnetism of O2?

Answer 14

1. Lewis structures didn’t predict unpaired electrons in O2 yet liquid oxygen is attracted to a magnet? Therefore it is paramagnetic.
2. MO diagram of O2 shows the unpaired electron which accounts for the paramagnetism of O2.

Question 15

What is s-p mixing? (3).

Answer 15

1. Orbitals of the same symmetry repel each other.
2. 2og + 3og repel each other, pushing the 3og orbital to higher energy than the 1piu* orbital.
3. This stabilises the 2og + 2ou* orbitals whilst destabilising the 3og + 3ou* orbitals.

Question 16

State the energy of the AOs for homonuclear diatomics.

Answer 16

AOs are at the same energy.

Question 17

State the energies of the AOs for heteronuclear diatomics.

Answer 17

AOs are at different energies. The more electronegative atom’s AO is at lower energy.

Question 18

What needs to be considered for heterodiatomic MO diagrams?

Answer 18

We need to look at the relative energies of the AOs to decide which CAN combine e.g. sometimes the 1s of one atom combines with the 2s of the other rather than the 1s.