Chapter 3: 3.5 Molecular Orbital Theory

Question 1

What are the limitations of Valence Bond Theory?

Answer 1

Resonance
Relative energies of electrons in molecules
Fails to predict diamagnetism/paramagnetism

Question 2

Explain the resonance deficiency of VB theory

Answer 2

The assumption of electrons are localized is not correct in the case of resonance structures , as bonding electrons are delocalized throughout possible bonds

Question 3

Explain the relative energies of electrons deficiency of VB theory

Answer 3

Does not provide obvious insight on the relative energies of the electrons

Question 4

Explain the diamagnetism/paramagnetism deficiency of VB theory

Answer 4

Fails to explain certain phenomena such as whether molecules are paramagnetic or diamagnetic

Question 5

In a bonding MO, what happens?

Answer 5

Two orbitals of a plus sign combine, and the electron density increases

Question 6

Are there any nodes in a bonding MO?

Answer 6

No

Question 7

In an antibonding MO, are there nodes?

Answer 7

Yes

Question 8

What is an antibonding MO?

Answer 8

Two orbitals of a minus sign combine, and the electron density decreases

Question 9

Why do nodes occur in antibonding MO?

Answer 9

Destructive interference of wavefunction, causes atoms to repel each other

Question 10

Distinguish between stabilized and destabilized

Answer 10

Stabilized: Lower in energy
Destabilized: Higher in energy

Question 11

The bonding MO is __________ relative to the ______ ________

Answer 11

Stabilized
Atomic orbitals

Question 12

The antibonding MO is ____________ relative to the ______ ________

Answer 12

Destabilized
Atomic orbitals

Question 13

How many electrons can each MO hold?

Answer 13

Maximum 2

Question 14

The number of ___ formed, is always _____ to the total number of ______ ________ combines

Answer 14

MOs
Equal
Atomic orbitals

Question 15

True or False:
Bonding orbitals are always higher in energy than antibonding

Answer 15

False, it’s the opposite

Question 16

How are MOs filled?

Answer 16

From lowest energy to highest energy (Aufbau Principle)

Question 17

What rules are obeyed while filling up MOs?

Answer 17

Aufbau Principle (filled from lowest to highest energy)
Pauli Exclusion Principle (no more than two electrons with opposite spins per MO)
Hund’s rule (Degenerate MOs are distributed with electrons in parallel spin until all MOs are singly occupied)

Question 18

What is the highest energy MO with one or more electrons known as?

Answer 18

Highest Occupied Molecular Orbital (HOMO)

Question 19

What is the lowest energy unoccupied MO called?

Answer 19

Lowest Unoccupied Molecular Orbital (LUMO)

Question 20

When are MOs most effectively formed?

Answer 20

When originating atomic orbitals are close in energy

Question 21

Bonding MOs __________ attraction while antibonding MOs _________ attraction

Answer 21

Increases
Decreases

Question 22

True or False:
For any molecule to be stable (exist), it must have more antibonding electrons than bonding electrons

Answer 22

False, it must have more bonding electrons than antibonding electrons

Question 23

How do we calculate bond order?

Answer 23

(# of bonding electrons - # of antibonding electrons) / 2

Question 24

What does a bond order of 0 mean?

Answer 24

There are no net electrons holding the molecule together, therefore the molecule cannot exist

Question 25

If chemical species are paramagnetic, they are:

Answer 25

Attracted to a magnetic field

Question 26

If chemical species are not attracted to a magnetic field, they are:

Answer 26

Diamagnetic

Question 27

What is the condition for chemical species to be paramagnetic?

Answer 27

They must have one or more unpaired electrons