Valence and MO Theories

Question 1

Valence Bond Theory

Answer 1

A covalent bond forms when orbital of two atoms overlap, and a pair of electrons is localized in the region between the atoms

Question 2

3 Principles of Valence Bond Theory

Answer 2

1. The pair of electrons has opposite spins
2. Good, in-phase overlap leads to strong bonding interactions
3. Orbitals can hybridize to more appropriate shapes and orientations, to maximize overlap.

Question 3

Molecular Orbital Theory and basic principle

Answer 3

A bonding approach in which quantum mechanics is applied to molecules

The Schrödinger equation is solved for electrons in a molecule, resulting in molecular orbitals

Question 4

Molecular Orbitals Similarities to Atomic Orbitals

Answer 4

They can each hold two electrons w/ opposite spins
The square of the molecular orbital wave function gives the probability function

Question 5

Features of MO diagrams

Answer 5

• Number of MOs = AOs
• Combining 2 AOs gives a bonding MO and an antibonding MO
• In ground state configurations, electrons occupy lowest energy MO upwards
• Bond strength can be measured by the calculated bond order
• Bond order = 1/2(no. of bonding es - no. of non-bonding es)

Question 6

How do you calculate Bond Order?

Answer 6

Calculate from Molecular Order Theory
BO = 1/2(MO-MO*)
= 1/2 (bonding e - antibonding e)

Question 7

Sigma Bond

Answer 7

End-to-end overlap of orbitals with electron density along axis of bond from MO Theory

single bonds and first overlap in multi-bonds

Question 8

Pi bond

Answer 8

side-to-side overlap of orbitals, with electron density above and below axis of bond in MO theory

extra overlaps in multiple bonds

Question 9

Limitations of Hybridization Schemes

Answer 9

• VBT does not always match experimental data
• Don’t work for d-orbitals
• Misleading when molecules have resonance

Question 10

Hybridization

Answer 10

A hypothetical process so tetrahedral geometry can be modified to create hybrid orbitals when bonding occurs

Question 11

MO Triangle shape and Nonmetals

Answer 11

Sigma
Pi pi
Sigma
Pi pi

B2 C2 N2

Question 12

MO oval shape and Nonmetals

Answer 12

Sigma
Pi pi
Pi pi
Sigma

O2 F2

Question 13

Hybridization Rules

Answer 13

Number of hybrid orbitals = number of orbitals mixed

Type of hybrid orbitals varies on the types of atomic orbitals mixed (s + p = sp)

Question 14

Bond Order Trend

Answer 14

Positive ion = low bond order
Negative ion = high bond order

Fewest antibonding electrons

Question 15

Bond Length Trend

Answer 15

Reverse of bond order trend

positive ion = longest bond length

negative ion = shortest bond length