7. MO theory

Question 1

Explain VB vs MO theories

Answer 1

VB - hybrids - localised bonds between atoms (resonance)

MO - atomic orbitals delocalised (over whole atom)??

Question 2

What are the assumptions in MO theory?

Answer 2

• linear combination of atomic orbitals (LCAO)
• orbitals can’t be invented nor destroyed

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Question 3

Explain linear combination of atomic orbitals (LCAO)

Answer 3

Question 4

What is the equation of in phase and out of phase combination of atomic orbitals?

Answer 4

Question 5

What is the graphical representation of orbital combination in H₂ when a bond is being formed?

Answer 5

In phase orbitals overlap - constructively interfere - increase e density between nuclei - stable - 1 bonding orbital formed

YOU START WITH 2 ORBITALS (H 1s, H 1s), YOU END WITH 2 ORBITALS (bonding / antibonding) -> another orbital is out of phase (one + another -) - destructive ineterference 0 overlap equals to 0 - decreased wavefunction - antibonding orbital - orbitals puch apart

Question 6

What is the radial wavefunction of in phase combination of orbitals?

Answer 6

Bonding orbital

Question 7

What is the radial wavefunction of out of phase combination of orbitals?

Answer 7

Antibonding orbital

Question 8

What are the symmetry labels?

Answer 8

Symmetry labels characterise the orbitals symmetry - correspond to s, p, d, f… orbital shapes

sigma or pi

Question 9

Explain sigma symmetry label

Answer 9

Completely symmetrical orbital → sigma symmetry - sigma orbitals

Question 10

Explain parity labels

Answer 10

Inversion centre in the molecule:

g orbital (gerade) - phase is the same after inversion around inversion centre of MO

u orbital (ungerade) - phase changes after inversion around inversion centre of MO

Atomic orbitals can also be classified as u / g

Question 11

What are MO energy level diagrams?

Answer 11

Question 12

What does the MO energy level diagram fro H₂ looks like?

Answer 12

Bonding orbital comes down - e placed - E has come down (compared to single atoms and molecule)

Antibonding orbital goes up and receives an *

Question 13

What is bond order and how to calculate it?

Answer 13

Half filled - 1/2

Question 14

Explain using MO energy diagram and bond order why He doesn’t form molecules

Answer 14

Not stable - both bonding and antibonding orbitals filled

Question 15

Explain using MO energy diagram and bond order how Li₂ is formed

Answer 15

E in bonding orbitals - E decreases - stable