Chapter 10 Notes

Chemical Bonding II

Molecular Shape,

Valence Bond Theory, and

Molecular Orbital Theory

Sections 10.1 – 10.8

Pg. : 453 - 459

(Sect. 10.2, 10.3, 10.4): 2, 3, 4, 5, 30, 31, 32, 34, 42

(Sect. 10.5): 7, 38 (ignore last part of directions), 40

(Sect. 10.6, 10.7): 11, 65, 66

Ch. 10 Problem Assignment

(Due test day)

I. Artificial Sweeteners: Fooled by Molecular Shape Artificial sweeteners such as aspartame (Nutrasweet) taste sweet like sugar but do not have the calories of sugar. This is because taste and nutritional value are independent properties. The caloric value of food depends on the amount of energy released when the food is metabolized in the body. Many artificial sweeteners are not even metabolized by the body- they just pass straight though.  The taste of a food begins with specialized cells on the tongue that detect different molecules of food. These cells are so sensitive that the tongue can detect one molecule of sugar out of thousands of different molecules in a bite of food. We experience certain tastes when molecules of food fit into a special part of a taste receptor on our tongue, much in the same way a key fits into a lock. Artificial sweeteners “trick” our tongues into tasting sweetness because these molecules mimic the molecular shape of the sugar molecule and fit snuggly into our taste receptors.

- used to determine the 3- D shape of a molecule

- based on the idea that electrons groups (bonds & lone pair e─) repel one another to varying degrees

- the combination of repulsions on the central atom of a molecule determines its 3-D shape

• VSEPR THEORY (VALENCE SHELL ELECTRON PAIR REPULSION):

II. VSEPR Theory: The Five Basic Shapes

*The basic idea behind VSEPR is that repulsions between electron groups determine molecular geometry. The preferred geometry is the one in which the electron groups have the maximum separation (and therefore the minimum energy) possible.

Preview We will first look at molecular geometries where there are two to six electron groups around a central atom and all of the electron groups are bonding groups (single or double bonds). We will then look at what happens to the shapes when some of the electron groups become lone pair electrons.

A. TWO ELECTRONS GROUPS: LINEAR GEOMETRYBond Angles:

180° Other examples:

SiO2

CS2

Si OO

C SS

16

16

B. THREE ELECTRONS GROUPS: TRIGONAL PLANAR GEOMETRY

Bond Angles:

120°

Other examples:

BH3 SO3

B

H

H H

S

O

O O

In CH2O, the bond angles deviate slightly from

120° because there is more electron density in a double bond than in a single bond. Different electron groups repel each other in slightly different ways.

6 24

C. FOUR ELECTRONS GROUPS: TETRAHEDRAL GEOMETRYBond Angles:

109.5°

Other examples:

SiCl4 CF4

Si

Cl

Cl ClCl

C

F

F FF

32 32

D. FIVE ELECTRONS GROUPS: TRIGONAL BIPYRAMIDAL GEOMETRY

Bond Angles: 90°,

120°

Other examples:

SOF4 ClO2F3

S

O

FF

F

F

40 40

Cl

O

FO

F

F

E. SIX ELECTRONS GROUPS: OCTAHEDRAL GEOMETRY

Bond Angles: 90°

Other examples:

SCl6

S

Cl

Cl Cl

Cl

Cl

Cl

48

III. VSEPR Theory: The Effect of Lone Pairs

PreviewWe will now look at molecular geometries where there are four to six electron groups around a central atom and some of the electron groups are lone pairs. Keep in mind that these shapes are all variations of the geometries from the last section, but now one or more bonding groups have been replaced with lone pairs. The difference in shapes results from the fact that lone pair electrons repel other lone pair and bonding electrons to a greater extent than bonding electrons repel one another.

THE ORDER OF ELECTRON PAIR REPULSION:

lone pair - lone pair > lone pair- bonding pair > bonding pair- bonding pairMost repulsive Least repulsive

* Want to be as far away from each other as possible

• In this section, don’t get confused between electron and molecular geometries. • In the last section, the electron and molecular geometries were the same (same name for each). • In this section, they are different.• There are only 5 electron geometries, but there are 11 molecular geometries.

THE DIFFERENCE BETWEEN: Electron Geometry:

Molecular Geometry:

The geometrical arrangement of electron groups

The geometrical arrangement of the atoms

A. FOUR ELECTRON GROUPS WITH LONE PAIRS (TETRAHEDRAL ELECTRON GEOMETRY)

Bond Angles: 107°

TRIGONAL PYRAMIDAL (MOLEC. GEO.): ONE LONE PAIR

Other examples:

PF3 NCl3

P

F FF

N

Cl ClCl

26 26

Bond Angles:

104.5°

BENT (MOLEC. GEO.): TWO LONE PAIRSOther examples:

SCl2 H2S

A. FOUR ELECTRON GROUPS WITH LONE PAIRS (TETRAHEDRAL ELECTRON GEOMETRY)

SCl Cl

SH H

18 8

Summarizing Tetrahedral Electron Geometries

B. FIVE ELECTRON GROUPS WITH LONE PAIRS (BIPYRAMIDAL ELECTRON GEOMETRY)

Bond Angles: 90°,

120°

SEESAW (MOLEC. GEO.): ONE LONE PAIR

Other examples:

SeCl4 IOF3

Se

ClCl

Cl

ClI

OF

F

F

34

Lone pair must go equatorial

34

B. FIVE ELECTRON GROUPS WITH LONE PAIRS (BIPYRAMIDAL ELECTRON GEOMETRY)

Bond Angles: 90°,

120°

T-SHAPED OR LINEAR (MOLEC. GEO.): TWO OR THREE LONE PAIRS

Other examples:

ClF3 I3-

Cl

F

F

FI

I

I

28 28

Lone pairs must go equatorial

C. SIX ELECTRON GROUPS WITH LONE PAIRS (OCTAHEDRAL ELECTRON GEOMETRY)

Bond Angles: 90°

SQUARE PYRAMIDAL (MOLEC. GEO.): ONE LONE PAIR

Other examples:

XeOF4 BrF4-

Xe

OF

F

F

FBr

F

F

F

F

SQUARE PLANAR (MOLEC. GEO.): TWO LONE PAIRS

Lone pairs must go axial

42 36

Summary:

There are 5 ELECTRON Geometries:

LINEAR : 2 e- groups around central atom

TRIGONAL PLANAR : 3 e- groups around central atom

TETRAHEDRAL : 4 e- groups around central atom

TRIGONAL BIPYRAMIDAL : 5 e- groups around central atom

OCTAHEDRAL : 6 e- groups around central atom

These electron geometries are also the MOLECULAR geometriesFor molecules where all e- groups are bonding groups.

Summary:

There are 6 additional MOLECULAR Geometries.These occur when one or more of the e- groups are lone-pair e-

Bent : 3 e- groups around central atom- 1 is a lone-pair 4 e- groups around central atom- 2 are a lone pair

Trigonal Pyramidal : 4 e- groups around central atom- 1 is a lone-pair

Seesaw: 5 e- groups around central atom- 1 is a lone-pair

T-Shaped: 5 e- groups around central atom- 2 are a lone-pair

Square Pyramidal: 6 e- groups around central atom- 1 is a lone-pair

Square Planar: 6 e- groups around central atom- 2 are a lone-pair

IV. VSEPR Theory: Predicting Molecular Geometries

Procedure:

1. Draw a Lewis structure for the molecule & total the valence e─

2. Determine the total number of electron groups around the central atom: lone pairs, single, double, and triple bonds each count as one electron group

3. Determine the number of (1) bonding groups and (2) the number of lone pair groups around the central atom. These should total the sum from Step 2

4. Use the VSEPR Table to determine the electron geometry and the molecular geometry

Use pgs. 414-415 in your text to fill out the table

V. Molecular Shape and Polarity Recall that in Ch 9, we said a bond is polar when there is an uneven sharing of electrons between atoms of different electronegativities.

Now we will determine if an entire molecule is polar by looking at both its bonds and shape. Just because a molecule contains polar bonds does NOT mean it is a polar molecule. We must also look at its molecular geometry.

To Determine if a Molecule is Polar:

1. Draw the molecule with the correct molecular geometry

2. Determine if each bond in the molecule is polar by considering electronegativity differences. If the bond is polar ( ≥ 0.4 ∆EN), draw an arrow over it pointing toward the more EN atom

3. Determine whether the polar bonds add together to form a net dipole moment, which makes the molecule polar (See Table 10.2)

Dipoles do NOT cancel; molecule is polar

O C O

Dipoles cancel; molecule is nonpolar

HO

H

Dipoles do NOT cancel; molecule is polar

Dipoles do NOT cancel; molecule is polar

HN

HH

HC

HH

F F: 4.0 C: 2.5 H: 2.1

A B

C D

E

Always polar

Always polar

Others become polarwhen bonds are NOTidentical

• In Lewis Theory, we use “dots” to represent valence electrons. Although this theory helps us to understand bonding, it is not an actual representation of how bonding between atoms truly occurs. • In the next three sections we will learn about Valence Bond Theory and Molecular Orbital Theory. These theories attempt to explain how bonding actually occurs at the atomic level.

VI. Valence Bond Theory

• e─ reside in atomic orbitals and/or hybridized atomic orbitals

• bonds result when half-filled orbitals overlap

• bonds are localized between atoms

• VALENCE BOND THEORY:

Let’s apply the general concepts of valence bond theory to under-Stand the bonding in H2S:

VII. Valence Bond Theory: Hybridization of Orbitals

Although the overlap of half-filled orbitals adequately explains the bonding in some molecules such as H2S, it cannot explain the bonding in many other molecules. Let’s see how this idea doesn’t predict the bonding of C and H in CH4, a stable molecule that we know forms.

Because C only has two half-filled orbitals, this model would suggest that carbon only

bonds with 2 H’s. Again, we know it bonds with 4.

Some General Statements Regarding Hybridization:

• The number of standard atomic orbitals added together always equals the number of hybrid orbitals formed. In order words, the numbers of orbitals is preserved.

• The combination of orbitals determines the shapes and energies of the orbitals formed. • The specific type of hybridization that forms for a molecule is the type that lowers the overall energy of the molecule.

To better understand bonding in Valence Bond Theory, we must consider hybridization.

a mathematical procedure in which atomic orbitals are combined to form new atomic orbitals called hybrid orbitals

• HYBRIDIZATION:

• One s orbital and three p orbitals hybridize to form four sp3 hybrid orbitals of equal energy.

• Head-on overlap of sp3 orbitals only forms single bonds called σ (sigma) bonds.

A. SP3 HYBRIDIZATION

Now we get a better idea of how CH4 bonds.

Using sp3 hybrid orbitals, C now has four half-filled orbitals

to overlap with 4 H 1s orbitals, each of which is half-filled.

Remember: according to valence bond theory, bonds form

when half – filled orbitals overlap

• One s orbital and two p orbitals hybridize to form three sp2 hybrid orbitals of equal energy.

• Head-on overlap of sp2 orbitals only forms single bonds called σ (sigma) bonds.

B. SP2 HYBRIDIZATION

• One p orbital remains unhybridized used to form a double bond

• Sideways overlap of two unhybridized p orbitals forms pi (π) bonds.

A DOUBLE BOND CONSISTS OF ONE SIGMA AND ONE PI BOND

- Double bonds consist of 1 sigma σ and 1 pi π bond.

- A σ bond forms from head-on overlap of two orbitals.

- A π bond forms from sideways overlap of two p orbitals.

• One s orbital and one p orbital hybridize to form two sp hybrid orbitals of equal energy.

• Head-on overlap of sp orbitals only forms single bonds called σ (sigma) bonds.

C. SP HYBRIDIZATION

• Two p orbitals remains unhybridized used to form triple bonds

• Sideways overlap of four unhybridized p orbitals forms 2 π bonds.

DIFFERENCE IN BETWEEN DOUBLE AND TRIPLE BONDS:

•A double bond contains one sigma and one pi bond•A triple bond contains one sigma and two pi bonds

Double bond

Triple bond

Summary of bond types:

Bond Type # of σ # of π

Single

Double

Triple

1 0

1 1

1 2

Note: Each of the following count as ONE electron group:

A lone pair of e −, a single bond, a double bond, or a triple bond

*An “interior atom” is an atom bonded to more than one other atom

Number of electron

groups around

interior* atom

Hybridization typeElectron geometry

around interior* atom

     

     

     

HOW TO DETERMINE HYBRIDIZATION and MOLECULAR GEOMETRY for A MOLECULE:

4 sp3 tetrahedral

3 sp2 trigonal planar

2 sp linear

Ex. Determine the hybridization type of the interior atoms in each of the following molecules:

C SS

S

O

O O

C

F

F FF

VIII. Molecular Orbital TheoryMolecular orbital theory is different from valence bond theory but also attempts to explain how compounds actually bond at the atomic level. In valence bond theory, we treated orbitals, whether hybrid-ized or not, as being centered on individual atoms. In molecular orbital theory, we will treat orbitals as overlapping the entire molecule, not as “belonging” to individual atoms.

- e- reside in orbitals that span the entire molecule

- molecular orbitals are made from atomic orbitals

• MOLECULAR ORBITAL THEORY:

To form our molecular orbitals, we will take a Linear Combination of Atomic Orbitals (LCAO). This is a mathematical procedure that is often performed via computers.  

As in valence bond theory, when atomic orbitals are combined to form molecular orbitals, the total number of orbitals does not change.

• Molecular orbital theory is a complicated theory that you can

understand at its basic level.

• It can help us to understand if bonds will form between atoms and

which type (double, triple, etc)

• Because we can often come to the same conclusion via simpler

theories, and because of time constraints, we will not study this

theory in more detail at this point.

• The key is to realize that more complicated bonding theories do exist

and they can more accurately explain and predict how bonding

occurs

Let’s first examine how to take a LCAO for s orbitals:

Whenever two atomic orbitals overlap, we get:• a bonding molecular orbital which is lower in energy than the atomic orbitals• an antibonding molecular orbital which is higher in energy than the atomic orbitals• each orbital can hold up to two electrons

We can write molecular orbital energy diagrams to understand bonding in molecules.

• Here is a molecular orbital diagram for H2:

Only valence e- go here

He atom He atom

•Lets draw a molecular orbital diagram for He2:

We know that He2 does NOT form. We can see why by looking at

the bond order:

Bond Order =

• A positive bond order means there are more electrons in bonding than in antibonding orbitals and the overall energy of the electrons is lower in the molecule than in the individual atoms

• A negative or zero bond order indicates that the energy of the electrons is overall higher than it would be if the atoms did not bond, and therefore no bond forms.

(# of e- in bonding MO’s) - (# of e- in antibonding MO’s) 2

Now let’s examine how to take a LCAO for p orbitals to understand how they form molecular orbitals. Remember that p orbitals exist in sets of three.

When three p orbitals from each atom combine, we get six molecular orbitals. Three are bonding and three are anitbonding.

Of the three bonding orbitals, one overlaps head-on and form a sigma bond and two overlap sideways to form two pi bonds.

Sigma bonding andantibonding molecularorbitals From p atomic orbitals

Two pi bonding and antibonding molecular orbitals made fromfour p atomic orbitals

Here’s the part that’s slightly tricky:• For B, C, and N the π MO is lower in energy than the σ MO. The π* MO is lower in energy than the σ* MO.

• For O, F, and Ne, it’s opposite. The σ MO is lower in energy than the π MO are lower in energy. BUT the π* MO is lower in energy than the σ* MO.

DISCLAIMER:

MO theory is a very useful method that adequately explains bonding and other chemical and physical properties better than most other bonding theories combined.

With the aid of computers, MO theory can be used to calculate molecular orbitals for entire molecules (as seen below for some simple examples).

Keep in mind that we are just touching on the very basics of MO theory here just so that you have been introduced to the concept.

Neither Lewis Theory nor Valence Bond Theory adequately conveys the fact that all of the bonds in ozone (O3) are equal in length and energy.

However, a computer-generated molecular orbital is able to clearly showthat all O-O bonds are equivalent.