1. VSEPR Theory
Demos
sp3 Bonding Balloons
Videodisk Unit 3

2. Valence Shell Electron Pair Repulsion
VSEPR Theory
Valence Shell Electron Pair Repulsion
Electrons repel each other in a molecule
The structure around a given atom is determined by minimizing electron-pair repulsions (getting pairs of electrons as far apart as possible)
Think about 300 students in the gym, and everyone swinging their arms to make sure they have space on all sides in every direction
Helps to predict the 3-D geometry of molecules
sp3 Bonding Balloons Demo
4 balloons tied together.

3. Steps for Determining Shape
Draw the dot diagram for the compound
Most compounds follow the octet rule but not all of them (note: only the central atom can break the octet rule. Boron breaks the rule)
Determine how many total pairs of electrons are on the CENTRAL atom
Determine how many bonds the central atoms is involved in. (each bond, whether single, double or triple, counts as one shared pair)
Look at number of bonds (shared pairs) versus unshared (lone) pairs of electrons and assign shape

4. Linear
CO2 O C O
Total Pairs = 4 Shared pairs = 4 Unshared (lone) pairs = 0
*A linear shape occurs any time only 2 atoms bond

5. BH3 H B H H Trigonal Planar
Total Pairs = 3 Shared Pairs = 3 Unshared (lone) Pairs = 0

6. Tetrahedral
CH4 H H C H H
Total Pairs = 4 Shared Pairs = 4 Unshared (lone) Pairs = 0

7. NH3 H N H H Trigonal Pyramidal
Non-bonding pairs push harder then bonding pairs, thus, bond angles get smaller.
In trigonal pyramidal, the bond angles are 107o instead of 109.5o.
Total Pairs = 4 Shared pairs = 3 Unshared (lone) pairs = 1

8. H2O H O H Bent Total Pairs = 4 Shared Pairs = 2
In bent, the bond angles are 105o instead of 109.5o.
Total Pairs = 4
Shared Pairs = 2
Unshared (lone) pairs = 2

9. Bond Angles
Bond angles are determined by the number of non-bonding pairs of electrons which push the bonding pairs closer together
Linear 180°
Trigonal Planar 120°
Tetrahedral °
Trigonal Pyramidal 107.3°
Bent °

10. Polar Covalent Bonds

11. Covalent Bonding
A covalent bond is a shared pair of electrons electrostatically attracted to the positive nuclei of two atoms. - + - +
The atoms achieve a stable outer electron arrangement (a noble gas arrangement) by sharing electrons.

12. Covalent Bonding
Both nuclei try to pull the electrons towards themselves - + - +
This is like a tug-of-war where both sides are pulling on the same object.
It creates a strong bond between the two atoms.

13. Covalent Bonding Picture a tug-of-war:
If both teams pull with the same force the mid-point of the rope will not move.

14. Pure Covalent Bond H e H
This even sharing of the rope can be compared to a pure covalent bond, where the bonding pair of electrons are held at the mid-point between the nuclei of the bonding atoms.

15. Polar Covalent Bonding
What if it was an uneven tug-of-war?
The team on the right are far stronger, so will pull the rope harder and the mid-point of the rope will move to the right.

16. Polar Covalent Bond
A polar covalent bond is a bond formed when the shared pair of electrons in a covalent bond are not shared equally.
This is due to different elements having different electronegativities.

17. Polar Covalent Bond I δ- δ+ H e.g. Hydrogen Iodide e
If hydrogen iodide contained a pure covalent bond, the electrons would be shared equally as shown above.
However, iodine has a higher electronegativity and pulls the bonding electrons towards itself
(winning the tug-of-war)

18. Polar Covalent Bond I δ- δ+ H e.g. Hydrogen Iodide e
This makes iodine slightly negative and hydrogen slightly positive. This is known as a dipole.

19. Polar Covalent Bond C Cl δ+ δ-
In general, the electrons in a covalent bond are not equally shared. δ- δ+
e.g. C Cl
2.5
3.0
Electronegativities
Polar Bonds (1.5 minutes)
δ- indicates where the bonding electrons are most likely to be found.

20. Polar and Nonpolar Covalent Bonds
Although all covalent bonds involve sharing of electrons, they differ widely in the degree of sharing
We divide covalent bonds into nonpolar covalent bonds & polar covalent bonds
Bonds are polarized along a continuum from covalent to ionic depending on the difference in electronegativity. A polar covalent bond has some amount of partial + and – charges at either end.
A-A δ+A-B δ A+B-
|____________________________________________________________|
Covalent Polar Covalent Ionic

21. Polar Covalent Bond Consider the polarities of the following bonds:
Electronegativities
Difference
C Cl
0.5
P H
O H
1.3
C Cl δ- δ+
O H δ- δ+
P H
Increasing Polarity

22. Polar Vs. Non Polar Properties
Polar Covalent
Pure (non-polar) Covalent
Soluble in water (sugar), or other polar solutes
Think about sugar and water.
Insoluble in water or other polar solutes
Think about oil and water

23. Network Solids A compound with all covalent bonds
Highest m.p. of all substances
Examples: Carbon exhibits the most versatile bonding of all the elements: diamond structure, graphite structure, fullerene structure, nanotubes
diamond
graphite

24. Network Solids
Diamond structures consists of tetrahedral carbons in a 3-dimensional array
Graphite structures consist of trigonal planar carbons in a 2-dimensional array. The layers in graphite are able to slide past each other easily.
Fullerenes consist of 5 and 6 member carbon rings fused into icosahedral spheres of at least 60 carbons
Nanotubes are long hollow tubes constructed of fused C6 rings

25. Network Solids Other examples include
Silicates: covalent atomic solids of Si and O; includes quartz, pyroxene, asbestos, talc, mica
Boron: metalloid almost always found in compounds with O
borax = Na2[B4O5(OH)4]8H2O
kernite = Na2[B4O5(OH)4]3H2O
colemanite = Ca2B6O115H2O
Pyrex

26. Molecular Solids
A crystalline molecular solid has molecules arranged in a particular conformation held together by intermolecular forces (van der Waals forces).
In water, H2O, the molecules are arranged in a regular three-dimensional structure for ice.
Other examples of crystalline molecular solids are table sugar, C12H22O11, and sulfur, S8.
Chapter 13

27. Polymers and Proteins
Substances composed of molecules characterized by the multiple repetition of one or more species of atoms or groups of atoms covalently linked to each other in amounts sufficient to provide a set of properties that do not vary markedly with the addition of one or a few of the repeating units.

28. Common Polymers
Polymers are common in nature. Wood, rubber, cotton, silk, proteins, enzymes, and cellulose are all examples of polymers
A wide variety of synthetic polymers have been produced, largely from petroleum based raw materials. These include polyurethane, teflon, polyethylene, polystyrene, and nylon.

29. Polymer & Protein Structure
Molecular Mass:
High molecular mass structures
Extremely large molecular weights are to be found in polymers/proteins with very long chains.
Molecular Shape:
Chain molecules are usually straight chains
These chains may bend, coil and kink, leading to extensive intertwining and entanglement of neighboring chain molecules.
These random coils and molecular entanglements are responsible for many of the important characteristics of such as strength, flexibility and molecular function.
Molecular structure and function are inseparable