1. Covalent Bonding Topics covered Homework Covalent bonding
Electronegativity
Bond polarity
Dipole moments
Homework
Exercises 8.33, 35, 39, 40
Read

2. Covalent Bonding
In order for covalent bonds to exist, the attractive forces between molecules must exceed the repulsive ones
Atoms are held together because the nuclei are mutually attracted to the electrons between them

3. Lewis Structures
The formation of covalent bonds can be shown using Lewis symbols
Shared pairs of electrons are shown as dashes (single, double, or triple) +  or

4. Lewis Structures 3 +  or  2 or

5. Covalent Bonding
For the nonmetals, the number of valence electrons on a neutral atom is the same as the group number
You can predict that 7A elements, such as F, would form one covalent bond to have an octet; 6A elements, such as O, would from two bonds; 5A elements, such as N, would form three bonds; and 4A elements, such as C, would from 4 bonds
These predictions are true in many compounds, but keep in mind that these are just guidelines and that there are many exceptions

6. Multiple Bonds
Sharing a single pair of electrons constitutes a single bond (single line drawn)
When two pairs of electrons are shared between two atoms it is a double bond (two lines are drawn)
When three pairs are shared it is a triple bond (three lines are drawn)

7. Multiple Bonds
As a general rule, the distance between bonded atoms decreases as the number of shared electron pairs (bond order) increases

8. Electronegativity
The ability of an atom in a molecule to attract electrons to itself is called electronegativity
We use electronegativity to estimate whether a given bond will be nonpolar covalent, polar covalent, or ionic
Electronegativity is related to electron affinity and follows the same trends across the periodic table

9. Electronegativity

10. Bond Polarity
A nonpolar covalent bond is one in which electrons are shared equally between two atoms
In a polar covalent bond one of the atoms exerts a greater attraction for the bonding electrons and the sharing is unequal

11. Bond Polarity
We can use the difference in electronegativity to gauge the polarity of the bonding between atoms
Examples
F2, = 0, nonpolar covalent
HF, = 1.9, polar covalent
LiF, = 3.0, ionic
Examples
F2, = 0, nonpolar covalent
HF, = 1.9, polar covalent

12. Sample Problem Which bond is more polar: Answer: B-Cl or C-Cl?
P-F or P-Cl?
Indicate in each case which atom has the partial negative charge
Answer:
B-Cl, chlorine has partial negative charge
P-F, fluorine has the partial negative charge

13. Dipole Moments
Sometimes the concentration of negative charge on one side of a polar covalent bond can lead to an overall partially negative charge on a molecule
A molecule in which the center of positive and negative charge do not coincide is said to be a polar molecule

14. Dipole Moments We can represent the polarity of molecules in two ways
The symbol  is the lower-case Greek letter “delta” and denotes “partially” in chemistry

15. Dipole Moments
Polarity helps determine many of the properties of substances that we observe on the macroscopic level
Polar molecules align themselves with each other and with ions
These interactions account for many properties of solids, liquids, and solutions

16. Dipole Moments
Whenever two electrical charges of equal magnitude but opposite sign are separate by a distance a dipole is established
The quantitative measure of the magnitude of a dipole is called its dipole moment ()
For equal and opposite charges separated by a distance r,  = Qr

17. Dipole Moments
Dipole moments are usually reported debyes (D), a unit that equals 3.34 x C-m
For molecules, we usually measure in units of electronic charge e, 1.60 x C, and distance in angstoms
To make things easier, use the following equation with built-in conversions:

18. Sample Problem
The distance between the centers of the H and Cl atoms in HCl (called bond length) is 1.27 Å. (a) Calculate the dipole moment, in D, that would result if the charges on the H and Cl atoms were 1+ and 1-. (b) The experimental dipole of HCl is 1.08 D. What are the partial charges, in e, of H and Cl?
Answer:
(a) 6.08 D
(b) e

19. Sample Problem
The dipole moment of chlorine monofluoride is 0.88 D. The bond length of the molecule is 1.63 Å. (a) Which atom is expected to have the negative charge? (b) What is the charge on that atom, in e?
Answer:
(a) F
(b) e