CHEMICAL BONDING II

MOLECULAR ORBITAL THEORY

DO NOWPick up handout.Get out homework handout.

HOMEWORK ANSWERS #1 a. BrF5 VE = 42; 6 e- pairs: 5 bonded pairs and 1 lone pair;

shape: square pyramid

b. C2H2 VE = 10; 2 e- pairs: 2 bonded pairs; shape: linear

c. BF3 VE = 24; 3 e- pairs: three bonded pairs; shape: trigonal planar

d. SO2 VE = 18; 3 e- pairs: two bonded pairs and one lone pair; shape: bent

e. SCl2 VE = 20; 4 e- pairs: two bonded pairs and 2 lone pairs; shape: bent

HOMEWORK ANSWERS #2 A. BF3 three e- pairs sp2 hybridization B. CCl4 four e- pairs sp3 hybridization C. AsF5 five e- pairs dsp3 hybridization D. ClO3

-1 four e- pairs sp3 hybridization

HOMEWORK ANSWERS #3 A. BeF3

-1 VE = 24 B. AsF4-1 VE = 34

trigonal planar see saw

C. SO4-2 VE = 32

tetrahedral

HOMEWORK ANSWERS #4 - 64. HBr has the larger dipole moment. Br has a higher

electronegativity that I so there is a greater difference between H-Br than H-I.

5. Molecules with no dipole moment: BCl3 (trrgonal planar with three bonded pairs) and BeCl2 (linear with two bonded pairs).

6. For a water molecule: c. the bonds are polar (eneg difference = 1.4) and the molecule is polar (bent).

MOLECULAR ORBITAL MODELProblems with LE Model

1. Electrons are not always localized as in the VSEPR theory; therefore resonance must be added and explained as best possible.

2. Molecules containing unpaired electrons are not easily dealt with using the localized model.

3. Magnetism is easily described for molecules using the MO theory. (Oxygen is paramagnetic which is unexplained by the localized electron model.)

4. Bond energies are not easily related using the localized model.

MOLECULAR ORBITAL MODEL TERMS TO KNOW: Bonding molecular orbital - an orbital lower in energy than the atomic orbitals from which it is composed (favors formation of molecule) represented by a B.

Antibonding molecular orbital - an orbital higher in energy than the atomic orbitals from which it is composed (favors separated atoms) represented by a A or an * .

Bond order - the difference between the number of bonding electrons and the number of antibonding electrons divided by two. Indicates bond strength.

Be sure you know these terms. It will be tough to

fill out the diagrams without

them.

MOLECULAR ORBITAL MODEL TERMS TO KNOW: Homonuclear diatomic molecules - those composed of two identical atoms.

Heteronuclear diatomic molecules - those composed of two different atoms

Paramagnetism - causes the substance to be drawn into a magnetic field; associated with unpaired electrons.

Diamagnetic - causes the substance to be repelled by the magnetic field; associated with paired electrons.

MOLECULAR ORBITAL MODELMolecular orbitals (which come from atomic orbitals) can hold two electrons with opposite spins.

The square of the molecular wave function indicates molecular probability.

Start with two hydrogen atoms: If we assume that the molecular orbitals can be constructed from the atomic orbitals, the quantum mechanical equations result in two molecular orbitals (MO) where 1sA and 1sB represent the 1s orbitals from the two separated hydrogen atoms.

MO1 = 1sA + 1sB and

MO2 = 1sA - 1sB

An atomic orbital hold an ATOM’s electrons

A molecular orbital is made from

atomic orbitals in a MOLECULE.

MOLECULAR ORBITAL MODEL The three orbital properties of interest are size, shape, and energy.

AT THE RIGHT:

- Each H entered on the outside with its lone 1s electron.

- As they approach each other, their two atomic orbitals blend to form two molecular orbitals.

- One MO is of high energy and one MO is of low energy.

- The electrons will choose the LOW energy route.

- The electrons occupy the lower energy level and thus a bond is formed.

High energy

anti-bonding orbital.

Lower energy

bonding orbital

The two hydrogen electrons

MOLECULAR ORBITAL MODEL This is a Molecular Orbital

Filling Diagram. Notice energy on the Y axis.

One MO is of high energy and one MO is of low energy.

The electrons will choose the LOW energy route.

The electrons occupy the lower energy level and thus a bond is formed. The two elements involved in the bonding

is this example are “A” and “B”.

MOLECULAR ORBITAL MODELDiatomic Helium, He2

Since 4 electrons are involved, the first 2 (↑↓) get to be lazy and go to the low energy state.The other 2 electrons(↑↓) must occupy the higher energy state and thus cancel out the bond.

Therefore He2 does not exist.

One helium atom’s two electrons enter on the left, the other helium’s two electrons on the right.

MOLECULAR ORBITAL MODEL Read through these in your notes:

1. The electron probability of both molecular models is centered along a line passing through the two nuclei. For MO1, the greatest probability is between the two nuclei (like constructive interference) and for MO2, it is on either side of the nuclei (like destructive interference). This is the sigma (σ) bond - will be called the sigma (σ) molecular orbitals.

2. In the molecule, only the molecular orbitals exist. The 1s atomic orbital no longer exists.

3. MO1 is lower in energy than the 1s orbital of a free hydrogen atom. MO2 is higher in energy than the 1s atomic orbital. The formation of a molecule is favored.

MOLECULAR ORBITAL MODEL4. Electrons lower their energies by being attracted by both nuclei.

5. Labels on the molecular orbitals indicate their symmetry (shape), the parent atomic orbital, and whether they are bonding or antibonding.

Bonding H2 orbital - MO1 = σ1sAntibonding H2 orbital - MO2 = σ1s*

6. Molecular electron configurations can be written just like atomic electron configurations. H2 = σ1s

2.

7. Each molecular orbital can hold two electrons with opposite spins.

8. Orbitals are conserved. The number of molecular orbitals is always the same and the atomic orbitals used to make them.

MOLECULAR ORBITAL MODEL Now you are ready! Watch this explanation .

Watch: Introduction to Molecular Orbital Theory

MOLECULAR ORBITAL PRACTICE Get out the “Practice” sheet. We are going to fill in the energy diagram for F2. Info on the next slide.

1. Write a capital “F” on the bottom left and one on the bottom right for the fluorine atoms.

2. Each fluorine has an electron configuration of 1s22s22p5

3. Fill in the 1s orbital with two electrons for both.

F FF

4. Fill in the bonding 1s bonding (σ)orbital with two electrons and the anti-bonding (σ*)orbital with two electrons.

5. Fill in the 2s orbital with two electrons for both fluorines.

6. Fill in the bonding 2s bonding (σ)orbital with two electrons and the anti-bonding (σ*)orbital with two electrons.

7. Fill in the 2p orbitals with the five electrons for both.

7. Start with the blue pi bonds first – fill them with two electrons each. (total 4 electrons)

8. Fill the blue sigma bond next with two electrons (total 6 electrons)

9. Fill the green anti-bonding pi (π*) orbitals (total 10 electrons).

DONE!

MOLECULAR ORBITAL - PRACTICE

That is where you are going to stop with the handout. We will fill in the rest of it on Monday.

Your homework is to do the diagram (A only) on the MO Theory handout for the compounds given – both sides.