• CHM 2045• Molecular Geometry & Chemical Bonding Chapter 10

Using Lewis Theory to PredictMolecular Shapes

• Lewis theory says that these regions of electron groups should repel each other

• This idea can then be extended to predict the shapes of molecules– the position of atoms surrounding a central atom will be

determined by where the bonding electron groups are– the positions of the electron groups will be determined by

trying to minimize repulsions between them

2

VSEPR Theory• Electron groups around the central atom will be

most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory

• The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

Electron Group Geometry• There are five basic arrangements of electron

groups around a central atom

• Each of these five basic arrangements results in five different basic electron geometries

• For molecules that exhibit resonance, it doesn’t matter which resonance form you use – the electron geometry will be the same

4

Linear Electron Geometry• When there are two electron groups around the

central atom, they will occupy positions on opposite sides of the central atom

• This results in the electron groups taking a linear geometry

• The bond angle is 180°

5

Trigonal Planar Electron Geometry• When there are three electron groups around the

central atom, they will occupy positions in the shape of a triangle around the central atom

• This results in the electron groups taking a trigonal planar geometry

• The bond angle is 120°

6

Tetrahedral Electron Geometry• When there are four electron groups around the

central atom, they will occupy positions in the shape of a tetrahedron around the central atom

• This results in the electron groups taking a tetrahedral geometry

• The bond angle is 109.5°

7

Tetrahedral Geometry

8

Trigonal Bipyramidal Electron Geometry

• When there are five electron groups around the central atom, they will occupy positions in the shape of two tetrahedra that are base-to-base with the central atom in the center of the shared bases.

• This results in the electron groups taking a trigonal bipyramidal geometry

9

Trigonal Bipyramidal Geometry

10

Octahedral Electron Geometry• When there are six electron groups around the

central atom, they will occupy positions in the shape of two square-base pyramids that are base-to-base with the central atom in the center of the shared bases

• This results in the electron groups taking an octahedral geometry

• All positions are equivalent• The bond angle is 90°

11

Octahedral Geometry

12

Octahedral Geometry

13

Molecular Geometry• The actual geometry of the molecule may be

different from the electron geometry• When the electron groups are attached to

atoms of different size,• when the bonding to one atom is different than

the bonding to another, • When there are lone pairs since they occupy

space on the central atom, but are not “seen” as points on the molecular geometry

14

The Effect of Lone Pairs• Lone pair groups “occupy more space” on the

central atom

• Relative sizes of repulsive force interactions isLone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair

• This affects the bond angles, making the bonding pair – bonding pair angles smaller than expected

15

Bond Angle Distortion from Lone Pairs

16

Bond Angle Distortion from Lone Pairs

17

18

Molecular Geometry

Example 1Which molecule or ion has a trigonal planar molecular geometry?a)PCl3

b)AsF3

c) HCNd)HCCHe)CO3

2–

Molecular GeometryExample 2Which molecule or ion has a trigonal pyramidal molecular geometry?a)CO3

2–

b)SO3

c) BF3

d)C2H4

e)SO32–

Molecular Geometry

Example 3What is the bond angle in a linear molecule or ion?a)90°b)109°c) 120°d)180°

Molecular Geometry

Example 4The approximate H—C—C bond angle in ethane, C2H6, isa)60°.b)90°.c) 109°.d)120°.e)180°.

Molecular Geometry

Example 5What is the H—O—H bond angle in water?a)180°b)120°c) 90°d)109°e)slightly less than 109°

Dipole moment ,the measure of charge separation

• A molecule has a dipole moment when there is charge separation

• The strength of the dipole moment depends on the difference in the electronegativity of the atoms in the molecule

Dipole moment ,the measure of charge separation

• Dipole moment can be shown with an arrow with a cross on one end:

• An arrow indicates the direction in which the electrons concentrate.

Molecule Polarity

The O─C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule.

26

Molecule Polarity

The H─O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.

27

Predicting Polarity of Molecules1. Draw the Lewis structure and determine the

molecular geometry2. Determine whether the bonds in the

molecule are polara) if there are no polar bonds, the molecule

is nonpolar3. Determine whether the polar bonds add

together to give a net dipole moment

28

Decide whether the following molecules Are polar

polarnonpolar

1. polar bonds, N-O2. asymmetrical shape 1. polar bonds, all S-O

2. symmetrical shape

TrigonalBent Trigonal

Planar2.5

29

Polar or NonpolarExample 6Which of the following molecule is polar?

a)CF4

b)SO2

c) CS2

d)C2H4

e)C6H6

Polar or NonpolarExample 7Which of the following compounds is nonpolar?

a)XeF2

b)HClc) SO2

d)H2Se)N2O

Dipole MomentExample 8Which of the following molecules has a dipole moment?a)NF3

b)CCl4

c) SiCl4

d)SF6

e)BF3

Dipole MomentExample 9For which molecule or ion does the nitrogen atom have the positive end of the dipole moment?a)NH4

+

b)Ca3N2

c) HCNd)AlNe)NO

Valence Bond Theory

• Linus Pauling and others applied the principles of quantum mechanics to molecules

• They reasoned that bonds between atoms would occur when the orbitals on those atoms interacted to make a bond

• The kind of interaction depends on whether the orbitals align along the axis between the nuclei, or outside the axis

34

Valence Bond Theory

• Linus Pauling and others applied the principles of quantum mechanics to molecules

• They reasoned that bonds between atoms would occur when the orbitals on those atoms interacted to make a bond

• The kind of interaction depends on whether the orbitals align along the axis between the nuclei, or outside the axis

35

Valence Bond Theory – Hybridization• One of the issues that arises is that the number

of partially filled or empty atomic orbitals did not predict the number of bonds or orientation of bonds – C = 2s22px

12py12pz

0 would predict two or three bonds that are 90° apart, rather than four bonds that are 109.5° apart

36

Hybridization• To adjust for these inconsistencies, it was

postulated that the valence atomic orbitals could hybridize/mix before bonding took place

– one hybridization of C is to mix all the 2s and 2p orbitals to get four orbitals that point at the corners of a tetrahedron

Unhybridized C Orbitals Predict the Wrong Bonding & Geometry

38

Hybridization• Some atoms hybridize their orbitals to maximize

bonding• Hybridizing is mixing different types of orbitals

in the valence shell to make a new set of degenerate orbitals– sp, sp2, sp3, sp3d, sp3d2

• Same type of atom can have different types of hybridization– C = sp, sp2, sp3

39

Hybrid Orbitals

• The number of atomic orbitals combined = the number of hybrid orbitals formed

• The number and type of standard atomic orbitals combined determines the shape of the hybrid orbitals

40

Orbital Diagram of the sp3 Hybridization of C

41

Methane Formation with sp3 C

42

sp3 Hybridized AtomsOrbital Diagrams

Unhybridized atom

2s 2p

sp3 hybridized atom

2sp3

C

2s 2p

2sp3

N

• Place electrons into hybrid and unhybridized valence orbitals

as if all the orbitals have equal energy• Lone pairs generally occupy hybrid orbitals

43

Bonding with Valence Bond Theory

• According to valence bond theory, bonding takes place between atoms when their atomic or hybrid orbitals “overlap”

• To interact, the orbitals must either be aligned along the axis between the atoms, or

• The orbitals must be parallel to each other and perpendicular to the interatomic axis

44

Types of Bonds• A sigma (s) bond results when the interacting atomic

orbitals point along the axis connecting the two bonding nuclei

• A pi (p) bond results when the bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the two bonding nuclei

• The interaction between parallel orbitals is not as strong as between orbitals that point at each other; therefore s bonds are stronger than p bonds

45

46

Orbital Diagrams of Bonding

• “Overlap” between a hybrid orbital on one atom with a hybrid or nonhybridized orbital on another atom results in a s bond

• “Overlap” between unhybridized p orbitals on bonded atoms results in a p bond

47

sp2 Hybridized AtomsOrbital Diagrams

Unhybridized atom

2s 2p

sp2 hybridized atom

2sp2

2p

C3 s1 p

2s 2p

2sp2

2p

N2 s1 p

48Tro: Chemistry: A Molecular Approach, 2/e

sp2 hybridization in Ethylene

• Two sp2-hybridized orbitals overlap to form a s bond

• p orbitals overlap side-to-side to formation a pi (p) bond

• sp2–sp2 s bond and 2p–2p p bond result in sharing four electrons and formation of C-C double bond

• Electrons in the p bond occupy regions on either side of a line between nuclei

sp2 hybridization in Ethylene, C 2H4

sp hybridization in Acetylene, C2H2

Hybridization & Molecular Geometry

Hybridization

Example 11The hybridization of the central atom in a molecule is described as sp2. The arrangement in space of the hybrid orbitals about that atom isa) linear.b) trigonal planar.c) tetrahedral.d) trigonal bipyramidal.

Hybridization

Example 12Which of the following statements is incorrect

regarding the water molecule?a)The molecule is polar.b)The hybridization of oxygen is sp3.c) There are two lone pairs and two bonding

pairs on the central atom.d)The molecular geometry is bent.e)The hybridization of hydrogen is sp.

Hybridization

Example13What is the hybridization of Se in SeF6?

a)spb)sp2

c) sp3

d)dsp3

e)d2sp3