1. CHEMICAL BONDING Set 3 Cocaine
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2. Credits
Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website
Other information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

3. Electron distribution is depicted with Lewis (dot) structures
This is how you decide how many atoms will bond covalently! (In ionic bonds, it was decided with charges)
Lewis Structures
G. N. Lewis

4. Bond and Lone Pairs
Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. • •• H Cl
shared or
bond pair
lone pair (LP)
This is called a LEWIS structure.

5. Note that each atom has a single, unpaired electron.
Bond Formation
A bond can result from an overlap of atomic orbitals on neighboring atoms. Cl H •• • +
Overlap of H (1s) and Cl (2p)
Note that each atom has a single, unpaired electron.

6. Review of Valence Electrons
Remember from the quantum chapter that valence electrons are the electrons in the OUTERMOST energy level… that’s why we did all those electron configurations!
B is 1s2 2s2 2p1; so the outer energy level is 2, and there are 2+1 = 3 electrons in level 2. These are the valence electrons!
Br is [Ar] 4s2 3d10 4p5 How many valence electrons are present?

7. Review of Valence Electrons
Number of valence electrons of a main (A) group atom = Group number

8. Steps for Building a Dot Structure
Ammonia, NH3
1. Decide on the central atom; never H. Why?
If there is a choice, the central atom is atom of lowest affinity for electrons. (Most of the time, this is the least electronegative atom…in advanced chemistry we use a thing called formal charge to determine the central atom. But that’s another story!) Therefore, N is central on this one
2. Add up the number of valence electrons that can be used.
H = 1 and N = 5
Total = (3 x 1) + 5
= 8 electrons / 4 pairs

9. Building a Dot Structure
3. Form a single bond between the central atom and each surrounding atom (each bond takes 2 electrons!) H N
4. Remaining electrons form LONE PAIRS to complete the octet as needed (or duet in the case of H). H •• N
3 BOND PAIRS and 1 LONE PAIR.
Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair.

10. Building a Dot Structure
Check to make sure there are 8 electrons around each atom except H. H should only have 2 electrons. This includes SHARED pairs. H •• N
6. Also, check the number of electrons in your drawing with the number of electrons from step 2. If you have more electrons in the drawing than in step 2, you must make double or triple bonds. If you have less electrons in the drawing than in step 2, you made a mistake!

11. Carbon Dioxide, CO2 1. Central atom = 2. Valence electrons =
3. Form bonds.
C 4 e- O 6 e- X 2 O’s = 12 e- Total: 16 valence electrons
This leaves 12 electrons (6 pair).
4. Place lone pairs on outer atoms.
5. Check to see that all atoms have 8 electrons around it except for H, which can have 2.

12. Carbon Dioxide, CO2
C 4 e- O 6 e- X 2 O’s = 12 e- Total: 16 valence electrons
How many are in the drawing?
6. There are too many electrons in our drawing. We must form DOUBLE BONDS between C and O. Instead of sharing only 1 pair, a double bond shares 2 pairs. So one pair is taken away from each atom and replaced with another bond.

13. Double and even triple bonds are commonly observed for C, N, P, O, and S
H2CO
SO3
C2F4

14. Now You Try One! Draw Sulfur Dioxide, SO2

15. Homework Questions
1) Does a Lewis structure tell which electrons come from which atoms? Explain.
2) Give a rationalization for the octet rule in terms of orbitals.
3) Write Lewis structures that obey the octet rule for each of the following:
A. HCN B. PH3 C. CHCl3 D. NH4+
E. H2CO F. SeF2 G. CO2 H. O2
I. HBr
Except for HCN and H2CO, the first atom listed is the central atom. For HCN and H2CO, carbon is the central atom.