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Question 1

Molecular Orbital (MO) theory vs. Valence Bond (VB) theory.

Answer 1

MO theory predicts bond order without the need of resonance structures

Question 2

MOs are similar to atomic orbitals, but.

Answer 2

But with more than a single nucleus (all atoms, not just bonded pairs).

Question 3

How are molecular orbitals created?

Answer 3

Atomic orbitals of all atoms in the molecule both constructively and destructively interfere to make new wavefunctions, called “molecular orbitals”.

Question 4

What are the types of atomic orbital mixing?

Answer 4

1) Antibonding: mixing out of phase.

2) Bonding: mixing in-phase.

Question 5

Why are bonding orbitals more favorable than anitbonding orbitals?

Answer 5

Place less e- density between the nuclei, are closer to the nuclei, thus lowering energy.

Question 6

What conditions must be met for succesful bonding?

Answer 6

For orbital overlap to lead to bonding distance, symmetry, and energy must be considered.

1. distance: atoms must get close enough, but not close enough that e- or nuclei repel each other.
2. symmetry: orbital regions must have the same phase.
3. energy: orbital energies should be similar looking at size and effective nuclear charge.

Question 7

If conditions for bonding are met:

Answer 7

MOs provide lower energy configuration of e-s than of free atoms in orbitals, thus
the molecule has lower energy than the seperated atoms.

Question 8

Why are antibonding orbitals higher in energy compared to bonding orbitals?

Answer 8

Higher in energy because there is less e- density between the two nuclei. It takes energy to pull an e- away from a nucleus, therefore when the e-s in an antibonding orbital spend less time between the two nuclei they are at a higher energy level.

Question 9

How do you predict bond order with MO diagrams?

Answer 9

Bond order= (# bonding e-s- #antibonding e-s)/2

Question 10

Do antibonding orbitals reduce stability of molecule? (T/F)

Answer 10

True.

Question 11

What does MO diagrams help us understand?

Answer 11

1. Excited state behaviour: photodissociation.

2. stability of molecules.

Question 12

What happens to a H2 molecule if one e- is excited to the antibonding orbital?

Answer 12

H2 molecule has an original bondorder of 1. Once excited, one e- jumps to antibonding orbital, so new bond order is 0.

Question 13

Why doesn’t He2 exist?

Answer 13

Bond order is equal to 0, thus no energetic advantage to bonding (unstable molecule), and 2 He atoms would just collide and move apart.

Question 14

What interactions involve end-on, side-on, and face-on overlap.

Answer 14

end-on overlap: sigma interaction.
side-on overlap: pi interaction.
face-on overlap: delta interaction.

Question 15

Define symmetry-allowed and symmetry-forbidden combinations.

Answer 15

Symmetry-allowed combinations can occur if the nergies match sufficiently. Symmetry-forbidden combinations occur where orbital lobes don’t match, preventing overlapping. Also if difference in bonding and antibonding interactions add to zero.

Question 16

What is the effect of orbital energy mismatches in H-X?

Answer 16

H-X bonds involve 1s-np overlaps. Higher n, higher energy, larger orbital size. Mismatch in orbital sizes results in less efficient overlap, and will result in weaker bonds, and higher acid acid strength (the weaker the bond the higher the strength of the acid because it can dissociate with least amount of energy).

Question 17

How many nodes passing through all nuclei do sigma, pie, and delta bonds have?

Answer 17

• sigma: none.
• pie: 1.
• delta: 2.

Question 18

Define MOs of homonuclear diatomic molecules.

Answer 18

Molecular orbitals made up of a pair of atomic orbitals with the same energy. Each pair will yield two molecular orbitals.

Question 19

What are and what do subscripts of molecular orbital mean?

Answer 19

Subscripts g and u indicate symmetry with respect to inversion through center of symmetry. Subscript g (gerade=even) for symmetric orbitals, subscript u (ungerade=odd) for antisymmetric orbitals.

Question 20

What is the relationship between enregy of subshells and effective nuclear charge?

Answer 20

Difference in energy between subshells increases as effective nuclear charge increases.

Question 21

Define MOs of heteronuclear diatomic molecules.

Answer 21

Interaction of atomic orbitals of different energies occurs between in heteronuclear diatomic molecules.

Question 22

Why do interactions become less likely between atomic orbitals of great energy difference?

Answer 22

As the energy separation of contributing orbitals increases, the mixing coefficients become more different in such a way that the bonding MO is more similar to the lower energy AO and the antibonding orbital is more similar to the higher AO. Also the stabilization of the bonding MO and the destabilization of the antibonding orbital decreases.

Question 23

When is sp mixing beocme significant?

Answer 23

Becomes significant when s & p are close in energy.

Question 24

Which elements experience sp mixing?

Answer 24

Diatomic homonoclear molecules Li, Be, B, C, N experience sp mixing.

Question 25

Which elements do not experience sp mixing?

Answer 25

Diatomic homonuclear molecules O, F, Ne do not experience sp mixing.

Question 26

Why does likelihood of sp mixing decrease?

Answer 26

Going across the period, 2s-2p mixing decreases because these atomic orbitals become more widely separated in energy as aZ increases.

Question 27

Describe MO of Li2.

Answer 27

Has configuration 1sigma-g^2 corresponding to Li-Li single bond. Gas phase studies support this prediction. The molecule has a bond length of 70pm, and dissociation energy of 110.

Question 28

Describe MO of Be2.

Answer 28

Has 2 e-s in a bonding orbital and 2-s in antibonding orbital, thus this molecule is expected to not exist or very unstable with respect to dissociation.

Question 29

Describe MO of B2.

Answer 29

Has unpaired e-s in the bonding pie orbitals, so it should be paramagnetic. Without sp mixing, B2 would be predicted to be diamagnetic with all e-s paired.

Question 30

How are MO energies measured?

Answer 30

Ultraviolet hotoelectron spectroscopy (UV-PES).

Question 31

How does UV-PES work?

Answer 31

A UV photon provides enough energy to eject an e- from a molecular orbital, with kinetic energy. E-s have an ionization energy from a particular orbital. Forms diagram of two of three possible ejections.