1. CHM 2045 Molecular Geometry & Chemical Bonding Chapter 10

2. Using Lewis Theory to Predict Molecular 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

3. 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

4. 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

5. 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°

6. 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°

7. 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°

8. Tetrahedral Geometry

9. 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

10. Trigonal Bipyramidal Geometry

11. 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°

12. Octahedral Geometry

13. Octahedral Geometry

14. 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

15. The Effect of Lone Pairs
Lone pair groups “occupy more space” on the central atom
Relative sizes of repulsive force interactions is
Lone 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

16. Bond Angle Distortion from Lone Pairs

17. Bond Angle Distortion from Lone Pairs

19. Molecular Geometry
Example 1 Which molecule or ion has a trigonal planar molecular geometry? a) PCl3 b) AsF3 c) HCN d) HCCH e) CO32–

20. Molecular Geometry Example 2
Which molecule or ion has a trigonal pyramidal
molecular geometry?
a) CO32–
b) SO3
c) BF3
d) C2H4
e) SO32–

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

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

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

24. 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

25. 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.

26. 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.

27. 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.

28. Predicting Polarity of Molecules
1. Draw the Lewis structure and determine the molecular geometry 2. Determine whether the bonds in the molecule are polar a) if there are no polar bonds, the molecule is nonpolar 3. Determine whether the polar bonds add together to give a net dipole moment

29. Decide whether the following molecules Are polar
Trigonal
Bent
Trigonal
Planar
2.5
1. polar bonds, N-O
2. asymmetrical shape
1. polar bonds, all S-O
2. symmetrical shape
polar
nonpolar

30. Polar or Nonpolar Example 6 Which of the following molecule is polar?
a) CF4
b) SO2
c) CS2
d) C2H4
e) C6H6

31. Polar or Nonpolar Example 7
Which of the following compounds is nonpolar?
a) XeF2
b) HCl
c) SO2
d) H2S
e) N2O

32. Dipole Moment Example 8 Which of the following molecules has a dipole
a) NF3
b) CCl4
c) SiCl4
d) SF6
e) BF3

33. Dipole Moment Example 9 For which molecule or ion does the nitrogen
atom have the positive end of the dipole
moment?
a) NH4+
b) Ca3N2
c) HCN
d) AlN
e) NO

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
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

36. 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 = 2s22px12py12pz0 would predict two or three bonds that are 90° apart, rather than four bonds that are 109.5° apart

37. 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

38. Unhybridized C Orbitals Predict the Wrong Bonding & Geometry

39. 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

40. 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

41. Orbital Diagram of the sp3 Hybridization of C

42. Methane Formation with sp3 C

43. sp3 Hybridized Atoms Orbital Diagrams
Place electrons into hybrid and unhybridized valence orbitals as if all the orbitals have equal energy
Lone pairs generally occupy hybrid orbitals
Unhybridized atom
sp3 hybridized atom C        2s 2p
2sp3 N         2s 2p
2sp3

44. 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

45. 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

47. 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

48. sp2 Hybridized Atoms Orbital Diagrams
Unhybridized atom
sp2 hybridized atom C
3 s
1 p        2s 2p
2sp2 2p N
2 s
1 p         2s 2p
2sp2 2p
Tro: Chemistry: A Molecular Approach, 2/e

49. sp2 hybridization in Ethylene
Two sp2-hybridized orbitals overlap to form a s bond
p orbitals overlap side-to-side to formation a pi () bond
sp2–sp2 s bond and 2p–2p  bond result in sharing four electrons and formation of C-C double bond
Electrons in the  bond occupy regions on either side of a line between nuclei

50. sp2 hybridization in Ethylene, C 2H4

51. sp hybridization in Acetylene, C2H2

52. Hybridization & Molecular Geometry

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

54. Hybridization
Example 12
Which 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.

55. What is the hybridization of Se in SeF6? a) sp b) sp2 c) sp3 d) dsp3
Example13
What is the hybridization of Se in SeF6?
a) sp
b) sp2
c) sp3
d) dsp3
e) d2sp3