1. Chem II
Chapter 9:

2. Why is this important?
the size and shape of a molecule of a particular substance, together with the strength and polarity of its bonds, largely determine the physical and chemical properties of that substance.

4. The geometry of a molecule includes a description of the arrangements of the atoms in the molecule.
Remember: we are dealing with covalent bonded (polar and nonpolar) compounds that consist of molecules…not ionic crystals

5. Since atoms are always in motion, when we talk about shapes of molecules or polyatomic ions, we mean the shape based on the average location of the atoms

6. BF3 Molecules have shapes and sizes
that are defined by the angles and distances
between the nuclei of their component atoms
BF3
The overall shape of a molecule is determined by its bond angles

7. Molecular shape is only meaningful when there are at least three atoms
If there are only two atoms, the
molecule must always be linear in shape as the centers of the 2 atoms will
always be in a straight line. H Cl

8. AX2 molecules AX3 molecules CO2 H2O NH3 SnCl3-
Molecules with a central atom (“A”) surrounded by two or more atoms of the same type (“X”) are denoted AXn
AX2
molecules
AX3
molecules
CO2
H2O
NH3
SnCl3-

9. 5 fundamental shapes for
Axnmolecules

10. The possible shape of the AXn molecule is determined by the value of n
AX2 molecules can be either linear or bent
Three possible shapes for AX3 molecules

11. Can we predict the exact shape?
To determine the shape, we use a simple theory called VSEPR (often read as "vesper"), which stands for Valence Shell Electron Pair Repulsion.(≈90% accuracy)
VSEPR theory states the shapes of different AXn molecules or ions depends on the number of “electron domains” surrounding the central “A” atom

12. Electron domains are the regions of electron density where electron pairs are most likely to be found
Each nonbonding pair (referred to as a lone pair), single bond, and multiple bond produces an electron domain around the central atom
The Lewis structure of NH3 has
four electron domains around
the central nitrogen atom. (3 bonding pairs & one lone pair)

13. The number of electron pairs surrounding an atom, both bonding and nonbonding, is called its steric number.
Steric number = (number of lone pairs on central atom) + (number of atoms bonded to central atom)

15. Electron domains are negatively charged and repel each other
In other words, electron domains try to
stay out of each other’s way
VSEPR theory is based upon the idea that
the best arrangement of a given number
of electron domains is the one that
minimizes the repulsions among them

16. The "AXE method" of electron counting is commonly used when applying the VSEPR theory
A represents the central atom and always has an implied subscript one
The X represents the number of bonds between the central atoms and outside atoms
Note:Multiple covalent bonds (double,triple, etc) count as one X.
E represents the number of lone electron pairs surrounding the central atom

19. We will use electron domain geometry To determine molecular geometry
The molecular geometry describes the arrangement of the atoms only and not the lone pairs of electrons

21. Molecular geometry is linear
Molecules with 2 electron domains on central atom
Electron domain geometry is linear
Bonding domains = 2 Non-bonding domains = 0
Molecular geometry is linear

22. Linear Molecular
geometry

23. Molecular geometry is trigonal planar
Molecules with 3 electron domains on central atom
Electron domain geometry is trigonal planar
Bonding domains = 3 Non-bonding domains = 0
Molecular geometry is trigonal planar

24. Trigonal planar
geometry

25. Molecular geometry is Bent
Molecules with 3 electron domains on central atom
Electron domain geometry is Trigonal planar
Bonding domains = 2 Non-bonding domains = 1
Molecular geometry is Bent

27. Molecular geometry is Tetrahedral
Molecules with 4 electron domains on central atom
Electron domain geometry is Tetrahedral
Bonding domains = 4 Non-bonding domains = 0
Molecular geometry is Tetrahedral

28. In geometry, a tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex.
A regular tetrahedron is one in which the four triangles are regular, or "equilateral",

30. Molecular geometry is Trigonal pyramidal
Molecules with 4 electron domains on central atom
Electron domain geometry is Tetrahedral
Bonding domains = 3 Non-bonding domains = 1
Molecular geometry is Trigonal pyramidal

31. Trigonal pyramid geometry

32. Molecular geometry is Bent
Molecules with 4 electron domains on central atom
Electron domain geometry is Tetrahedral
Bonding domains = 2 Non-bonding domains = 2
Molecular geometry is Bent

34. Expanded valence shells
Although the octect rule is useful for understanding bonding in molecules, certain atoms can expand their valence shells to accommodate extra electrons
Third row elements (“Period 3” e.g., Al, Si, P, S, Cl) and higher often have more than four valence shell orbitals filled with Lone Pairs and/or Bond Pairs; this is called"expanded valence".
This means more than 8 electrons can surround a third-row element. P

35. Elements have a greater tendency to expand their valence:
ClF3
PCl5

36. FC of sulfur = 6 − 0 −½(12)
FC of sulfur = 0
Sulfuric acid

38. Using VESPR to determine molecular geometry
Draw the Lewis dot structure for the molecule and count the total number of electron densities or steric number (single bonds + multiple bonds + lone pair electrons + unpaired electrons) For molecules or ions that have resonance structures, you may use any one of the resonance structures. #1 #2
Determine electron-density geometry by arranging electron densities to minimize repulsions #3
Using only bonding pairs from the electron-density geometry, determine molecular geometry

40. The lone pair electron domain is spread out more
Lone pairs repel more than bond pairs

41. Triple bonds repel other bonding-electrons more strongly than double bonds.
Double bonds repel other bonding-electrons more strongly than single bonds.

42. VESPR model can be extended to predict the shapes of complex molecules

44. Bond polarity is a measure of how equally the electrons are shared in a bond
As the difference in electronegativity between the two atoms increases, so does the polarity

47. The quantitative measure of the amount of charge separation is called the dipole moment
The overall dipole moment of a molecule is the sum of its bond dipoles.

48. In CO2 the bond dipoles are equal in magnitude but exactly oppose each other. The overall dipole moment is zero.

50. Why are dipoles and bond polarity important?
Molecular polarity affects physical properties such as boiling and melting points

51. The VESPR model is a simple tool for predicting the shapes of molecules but does not explain why bonds exist between atoms
Valence-bond theory pictures individual atoms, each with its own orbitals and electrons coming together and forming covalent bonds of the molecule.
Bonding involves the overlap of valence orbitals on the central atom with those of the surrounding atoms.

52. Review
Electron orbital notation identifies the valence orbitals

55. sigma (σ ) bonds are single covalent bonds
sigma (σ ) bonds are single covalent bonds. They result from the overlap of two s orbitals, an s and a p orbital, or two head-to-head p orbitals.

57. pi (π) bonds result from the sideways overlap of p orbitals, and pi orbitals are defined by the region above and below an imaginary line connecting the nuclei of the two atoms
pi bonds never occur unless a sigma bond has formed first, and are always part of a double or triple bond.

58. Whenever there is a double bond it is made up of one sigma (direct orbital overlap) bond and one pi (lateral orbital overlap) bond.

61. Triple bonds have two pi bonds arranged at 90º to one another brought about by the lateral overlap of one pair of py orbitals and one pair of pz orbitals.

62. Problem #1
The experimental shapes and bond angles do not correspond to the theoretical expectation The 'p' orbitals are oriented at 90º to one another and yet there are few molecules that show a bond angle of 90º
Problem #2
The valence electrons in the s-orbital cannot bond without interfering with the p orbitals.

64. Hybridization is a model that allows us to combine the atomic orbitals and then produce four degenerate orbitals to be used for bonding.

65. Hybridization is the ‘mixing’ or ‘blending’ of standard atomic orbitals to form new bonding orbitals that will accommodate the spatial requirements in a molecule.
The hybrid orbitals are formed by combining s, p and d orbitals

66. How does hybridization occur?
Example: methane (CH4), 1 Carbon binds with 4 Hydrogens. The carbon atom itself has only 2 electrons available for bonding in the 2p subshell.
In order for 4 hydrogens to bind there need to be 4 electrons available for bonding, which cannot be achieved with the current orbital arrangement.

67. The pull of a hydrogen nucleus results in an electron being excited from the 2s subshell into the 2p subshell, where it is available for bonding.
This leads to the creation of a new ‘hybridized orbital’, called sp3.

69. Sigma bonds are generally formed through hybridization:
Pi bonds are formed by unhybridized
p-orbitals

70. hybridized orbitals are notated as sp, sp2, sp3, spx, etc.
sp2 refers to a hybrid orbital being constructed from one s orbital and two p orbitals.

71. Valence-bond theory will correspond with the
VESPR geometry

74. The End