1. Polarity of Covalent Bonds
Lesson 4

2. Nonpolar & Polar 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 2

3. 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-
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Covalent Polar Covalent Ionic 3

4. Nonpolar Covalent Bond
Picture a tug-of-war:
If both teams pull with the same force the mid-point of the rope will not move.
Demos
sp3 Bonding Balloons
Videodisk Unit 3
This even sharing of the rope can be compared to a nonpolar covalent bond, where the bonding pair of electrons are held at the mid-point between the nuclei of the bonding atoms. 4

5. Nonpolar Covalent BondH e H
sp3 Bonding Balloons Demo
4 balloons tied together.
Pure nonpolar covalent bonds occur between non-metals that are same or between non-metals that have electronegativity values that are very close to each other. 5

6. Polar Covalent Bond What if it was an uneven tug-of-war?
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.

7. Polar Covalent Bond C Cl δ- δ+
In general, the electrons in a covalent bond are not equally shared.
e.g. δ- δ+ C Cl
Polar Bonds (1.5 minutes)
δ- indicates where the bonding electrons are most likely to be found. The atom that is closest to fluorine is more electronegative.

8. Polar Covalent Bond I δ- δ+ H e.g. Hydrogen Iodide e
Iodine has a higher electronegativity and pulls the bonding electrons towards itself
(winning the tug-of-war)
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.
This makes iodine slightly negative and hydrogen slightly positive. This is known as a dipole. 8

9. Polar vs. Nonpolar Properties
Polar Covalent
Nonpolar Covalent
Soluble in water (sugar), or other polar solvents
Think about sugar and water.
Insoluble in water or other polar solvents
Think about oil and water

10. Like Dissolves Like Polar solvents dissolve in each other
Nonpolar solvents dissolve each other.
Methanol dissolves in water, but hexane does not dissolve in water.
Hexane dissolves in toluene, but methanol does not dissolve in toluene. 10

11. 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

12. 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.
Chapter 13 12

13. Polymers and Proteins High molecular mass structures
Extremely large molecular weights are to be found in polymers/proteins with very long chains.
Chain molecules are usually straight chains
Chains may bend, coil and kink.
These random coils and molecular entanglements are responsible for many of the important characteristics of such as strength, flexibility and molecular function.

14. In bent, the bond angles are 105o instead of 109.5o.
VSEPR Theory
In bent, the bond angles are 105o instead of 109.5o. 14 14

15. 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 15

16. 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 bond)
Look at number of bonds (shared pairs) versus unshared (lone) pairs of electrons and assign shape 16

17. CO2 O C O Linear Total Pairs = 4 Shared pairs = 4
Unshared (lone) pairs = 0
*A linear shape occurs any time only 2 atoms bond 17

18. BH3 H B H H Trigonal Planar Total Pairs = 3 Shared Pairs = 3
Unshared (lone) Pairs = 0 18

19. CH4 H H C H H Tetrahedral Total Pairs = 4 Shared Pairs = 4
Unshared (lone) Pairs = 0 19

20. NH3 H N H H Trigonal Pyramidal Total Pairs = 4 Shared pairs = 3
Unshared (lone) pairs = 1 20

21. H2O H O H Bent Total Pairs = 4 Shared Pairs = 2
Unshared (lone) pairs = 2 21

22. Molecules with Symmetry
Molecules with a central atom that have NO LONE PAIRS have symmetry. These molecules are linear (with a central atom), trigonal planar, or tetrahedral.
They will be nonpolar even if the bonds have a lot of polarity.
Ex. Carbon dioxide, boron trifluoride, carbon tetrachloride 22

23. Molecules without Symmetry
Molecules that have one or two lone pairs on their central atom do NOT have symmetry.
These molecules have at least some polarity because their bond angles do not match. Lone pairs occupy a lot of space.
Examples of molecules without symmetry are water and ammonia. 23

24. Models of Molecular Compounds
Build the molecules on the following slide and complete the chart in your notes. 24

25. Number of lone pairs on central atom Polarity
Name
Formula
Drawing
Shape
Number of lone pairs on central atom
Polarity
Water
Ammonia
Methane
Nitrogen
Carbon dioxide
Carbon tetraiodide
Hydrogen
Oxygen
monobromide 25

26. Questions
1. Which of the five shapes does not have a representative in the table above?
2. Draw and name an example of a molecule that has this shape.
3. Which molecules are linear?
4. Are all of the linear molecules nonpolar?
5. What is required for a linear molecule to be nonpolar?
6. Which of the molecules have symmetry?
7. What is required for a molecule to be symmetrical?
8. Explain what the tetrahedral, trigonal pyramidal, and bent shape have in common.
9. Explain what the term VSEPR means and how it relates to this activity. 26