1. Polar Bonds & Molecules

2. Objectives
Describe how electronegativity values determine the distribution of charge in a polar molecule
Describe what happens to polar molecules when they are placed between oppositely charged metal plates
Evaluate the strength of intermolecular attractions compared with the strength of ionic and covalent bonds
Identify the reason why network solids have high melting points

3. Important Vocabulary Nonpolar covalent bond Polar covalent bond
Polar bond
Polar molecule
Dipole
Van der Waals forces
Dipole interactions
Dispersion forces
Hydrogen bonds
Network solids

4. Bond Polarity
Electronegativity is used to predict what kind of bond will form between two different atoms
Atoms can share electrons equally or unequally due to different electronegativity values
The bonding pairs of electrons in covalent bonds are pulled, as in a tug-of-war

5. Electronegativity Chart

6. Nonpolar Covalent Bonds
When two atoms share electrons equally, a nonpolar covalent bond is formed
Molecules of hydrogen (H2), oxygen (O2), and nitrogen (N2) have nonpolar covalent bonds
Diatomic ________ molecules are also nonpolar

7. Polar Covalent Bonds
When two atoms share electrons unequally, a polar covalent bond is formed
It is also known as a polar bond
The _____ electronegative atom attracts electrons more strongly and gains a slightly negative charge
The ______ electronegative atoms has a slightly positive charge
If the difference for electronegativity is greater than or equal to 2.0, an ionic bond will form

8. Predicting Bond Type

10. Polarity in Relation to Bond Strength
How do you think polarity affects bond strength?

11. Practice Problems N and H F and F Ca and Cl Al and Cl H and Br
Which type of bond will form between each of the following pairs of atoms?
N and H
F and F
Ca and Cl
Al and Cl
H and Br
Li and O
C and O
Cl and F

12. Partial Charges
Having partial charges indicates that the shared pair of electrons will be closer to the atom with the greater electronegativity
Thus making one end more negative and the other end more positive
They are fractions of charge ranging between 0 and +1.6 x or 0 and -1.6 x 10-19
The amount of the partial charge depends on the electronegativity differences between the elements involved

13. Polar Molecules Have positive and negative ends
A molecule in which one end has a partial positive charge and the other end has a partial negative charge is called a dipole
When polar molecules are placed between oppositely charged plates, they tend to become oriented with respect to the positive and negative plates
For example: HF molecule
We emphasize the nature of the dipole using the Greek letter delta to mean partial
Hδ+Fδ-

15. Intramolecular vs. Intermolecular
Intramolecular Forces
Intermolecular Forces
Are forces within ONE molecule
Nonpolar covalent
Electrons shared equally
Polar covalent
Electrons shared unequally
Ionic
Electrons are transferred
Are forces between MULTIPLE molecules
Van der Waals Forces
London Dispersion Forces
Hydrogen Bonding
Dipole-Dipole

16. Attractions Between Molecules
Molecules can attract each other by a variety of forces
Intermolecular attractions are weaker than either ionic or covalent bonds
However, these forces are still very important
For example, intermolecular attractions help to determine if a molecular compound is a gas, liquid, or solid

17. Van der Waals Forces Are an umbrella term for intermolecular forces
They are named after Dutch chemist Johannes van der Waals ( )
Van der Waals forces consist of strong forces such as dipole-dipole interactions and hydrogen bonds, and weak London dispersion forces

18. Dipole-Dipole Interactions
Occur when polar molecules are attracted to one another
The electrical attraction involved occurs between the oppositely charged regions of polar molecules
Dipole interactions are similar to ionic bonds but they are much weaker
They also include hydrogen bonds

19. Hydrogen Bonds
Are attractive forces in which a hydrogen covalently bonded to a very electronegative atom is also weakly bonded to an unshared electron pair of another electronegative atom
Hydrogen bonding always involves hydrogen
For example: water

20. London Dispersion Forces
Are the weakest of all Van der Waals forces
They occur between non-polar molecules
They are believed to result from weak, momentary attractions called instantaneous dipoles
An instantaneous dipole is formed when electrons moving in a molecule get “off balance” for an instant so that for an instant partial charges appear.
This leads to short-live attractions
For example: halogen diatomic molecules attract each other mainly by dispersion forces

22. Intermolecular Attractions & Molecular Properties
The physical properties of a compound depend on the type of bonding it displays

23. Network Solids
Are solids in which all of the atoms are covalently bonded to each other
Melting a network solid would require breaking covalent bonds throughout the solid