1. Lewis Dot Structures

2. Valence Electrons
Electrons in the “outer shell” (Bohr) or the highest energy s and p orbitals (Wave Mechanical) of an atom
Sulfur 1s22s22p63s23p4
Highest Principal Energy Level is 3
2 electrons in 3s, 4 electrons in 3p = 6 valence electrons

3. Valence Electron Patterns
The number of valence electrons can be easily read from the top of the periodic table.

4. Lewis Dot Structures
Show us the NUMBER and TYPES of bonds in each molecule.
We will also be able to tell the SHAPE of the molecules

5. Drawing Lewis Dot Structures
Obtain the TOTAL number of valence electrons in the entire molecules.
Use one pair of electrons to form a bond between each pair of bound atoms. Draw a LINE to represent the bonding pair. (In general, if there is a single element, it is the center atom and all others bond to it).
Arrange the remaining electrons to satisfy the octet/duet rule in the outer atoms.

6. Drawing Lewis Dot Structures…
If there are any remaining electrons, place them on the center atom.
Check to see that there is an octet on EVERY atom (except Hydrogen).
If there are not enough electrons to make octets on all atoms, SHARE!
Make a DOUBLE BOND
If there are still not enough, make a TRIPLE BOND

7. Practice Problems
Draw the Lewis Dot Structures for the following molecules. Make sure:
there are no more electrons around the atoms than the number of total valence electrons
all atoms have obey the octet rule
O2 N2 CO2

8. Answers
O2 N2 CO2
2(6)=12 v.e. 2(5)=10 v.e. 4+2(6)=16

9. Resonance Structures
When two or more correct Lewis Dot Structures can be drawn for a single molecule, they are known as resonance structures.
The following molecules have resonance structures. Can you show how many?
CO2
NO3-1
SO2
CO3-2 3 2

10. Exceptions to the Octet Rule
Every rule has an exception, eh?
BORON
Tends to act more like an ionic compound and forms only 3 bonds
BF3 makes a molecule with 3 single bonds and only 6 electrons around the B
BF3 reacts vigorously with NH3 to make H3NBF3 which further proves the structure

11. Exceptions to the Octet Rule
Any ionic compound
Ionic compounds GIVE and TAKE electrons to form their bonds. The ones that GIVE electrons get an octet or duet from their inner electrons.
Examples:
NaCl
MgCl2

12. Exceptions to the Octet Rule
Paramagnetism
When an object displays paramagnetism, it reacts to a strong magnet, indicating that it has UNPAIRED ELECTRONS
OXYGEN GAS (O2)
The LDS that satisfies the octet rule shows that it has a double bond, yet its observed behavior is paramagnetic.

13. Exceptions to the Octet Rule
Odd numbers of electrons
NITROGEN
NO and NO2 have eleven and fifteen valence electrons, respectively, yet both are observed molecules

14. Exceptions to the Octet Rule
More than an octet
Some molecules have more than an octet
Only if they have d orbitals that electrons can go into
N, O, F CANNOT ever have more than an octet
Examples
IF5 AsF6 IBr3

15. Bond Order
When referring to bond order, it is usually the type of bond.
Single bond = bond order of 1
Double bond = bond order of 2
Higher bond order means:
Stronger bonds (harder to break)
Shorter bond lengths (atoms closer together)

16. Bond Order
When a molecule has resonance structures, the bond order tends to be an average of the resonant structures.
Predict the bond order of each bond in the following molecules that you already wrote resonance structures for:
CO2
SO2
NO3-1
CO3-2 2
1.3
1.5