Chapter 8: Molecular Orbital Theory

Question 1

paragmagnetic

Answer 1

• molecules with unpaired electrons that interact with a magnetic field

Question 2

diagmagnetic

Answer 2

• molecules without unpaired electrons that are only weakly influenced by the magnetic field

Question 3

wave superposition

Answer 3

• key feature of waves where they can be added or subtracted from one another to produce new wave forms

Question 4

What happens when waves superimpose? (3)

Answer 4

• they combine to produce a wave with larger or smaller amplitude at various points
• superposition changes location of electron density
• covalent bond forms when atomic wavelengths superimpose and there is increased electron density between the two nuclei

Question 5

bonding MO (2)

Answer 5

• molecular orbital formed through constructive interference

- electron density increases in the region between the nuclei where a bond should exist

Question 6

antibonding MO (3)

Answer 6

• molecular orbital formed through destructive interference
• reduces electron density between nuclei so it works against bonding
• in some cases, nodal planes can form between nuclei in antibonding orbitals

Question 7

What does MO stand for?

Answer 7

• molecular orbital

Question 8

What does AO stand for?

Answer 8

• atomic orbital

Question 9

What happens to energy during the superposition of 1s orbitals in hydrogen? (2)

Answer 9

• constructive interference: lowers energy

- deconstructive interference: raises energy

Question 10

How does orbital overlap affect energy? (2)

Answer 10

• orbitals with greater overlap have greater differences in energy between bonding and antibonding MOs
• as 1s orbitals are brought closer together, the energy gap between bonding and antibonding orbitals increases

Question 11

What happens to antibonding orbitals with differing internuclear distance?

Answer 11

• energy of antibonding orbital increases as the internuclear distances decrease

Question 12

What happens to bonding orbitals with differing internuclear distance?

Answer 12

• energy of bonding orbital decreases as the internuclear distances decrease

Question 13

How does the potential energy of the H-H bond vary with internuclear distance?

Answer 13

• repulsion increases the potential energy of short internuclear distances

Question 14

energy well

Answer 14

• shows the internuclear distance for which the system has the lowest energy

Question 15

molecular orbital (MO) wavefunctions

Answer 15

• describe the wave-like behaviour of electrons in a molecule as a whole
• can be approximated as the superimposition of atomic orbital wavefunctions (AOs) with constructive and destructive interference

Question 16

What are the 4 rules for forming MOs

Answer 16

1. total number of MOs formed is always equal to the total number of AOs interacting
2. when two AOs are combined, the resulting bonding MO has fewer nodes than the resulting antibonding MO
3. the more the AOs overlap, the larger the energy gap between the resulting bonding and antibonding orbitals
4. only AOs of similar energy interact significantly (1s+1s not 1s+2s)

Question 17

What is the probability of finding an electron at a particular point x, proportional to?

Answer 17

• the wavefunction squared

Question 18

What happens to the wavefunction phase at each node?

Answer 18

• the phase of the wavefunction changes at each node

Question 19

What is the probability of finding an electron at a node?

Answer 19

• zero

Question 20

How do the bonding MO and antibonding MO energies compare to the atomic 1s orbitals? (2)

Answer 20

• bonding MO is lower in energy relative to the atomic 1s orbitals
• antibonding MO is higher in energy by almost the same amount

Question 21

In general, what is the trend between nodes and energy in MOs?

Answer 21

• MOs with more nodes will have higher energies than MOs with fewer nodes

Question 22

orbital interaction diagram

Answer 22

• relative energies of the atomic orbitals and the bonding and antibonding MOs are typically illustrated by this diagram

Question 23

How are the symmetry of the orbitals represented in the orbital interaction diagram? (3)

Answer 23

• each MO is labelled with the appropriate symbol
• σ if the orbital has cylindrical symmetry
• π if the orbital has a plane of symmetry containing the bond

Question 24

How can we tell whether the orbital is bonding or antibonding in an orbital interaction diagram? (2)

Answer 24

• an asterisk indicates antibonding orbitals

- σ is a bonding orbital and σ* is an anitbonding orbital

Question 25

How can we tell which AOs the MO originates from?

Answer 25

• the AO labels are subscripts

Question 26

σ

Answer 26

• bonding MO, called the sigma orbital

Question 27

σ*

Answer 27

• anitbonding MO, called the sigma star orbital

Question 28

π

Answer 28

• bonding MO, called the pi orbital

Question 29

π*

Answer 29

• antibonding MO, called the pi star orbital

Question 30

What is the formula for determining bond order of a diatomic molecule?

Answer 30

bond order= (e_bonding - e_antibonding)/2

- where e_bonding is the number of electrons in bonding orbitals and e_antibonding is the number in antibonding orbitals

Question 31

What is the procedure for filling orbital interaction diagrams with electrons? (2)

Answer 31

1. calculate the total number of electrons in the atomic orbitals
2. fill the MOs with the number of electrons calculated in step 1:
   - fill the lowest energy orbitals first (Aufbau principle)
   - place a max of two spin-paired electrons in each MO (Pauli exclusion principle)
   - place electrons in degenerate MOs singly, with the same spin, before pairing them (Hund’s rule)

Question 32

highest occupied molecular orbital (HOMO) (2)

Answer 32

• highest energy molecular orbital that contains electrons

- can be fully occupied or partially occupied

Question 33

lowest occupied molecular orbital (LOMO) (2)

Answer 33

• molecule orbital second highest in energy

- empty at ground state