1. Lewis Structures Of Covalent Compounds
NAB
Method

2. Information…
Octet rule alone does not allow us to create correct Lewis Structures
Does not tell us where or how to place the bonded electrons
NAB method helps

3. N = “NEEDED”
Calculate the “N” as the sum of the electrons necessary for all elements to achieve an octet
Important exceptions (as usual)
H = 2
Be = 4
B = 6

4. NEEDED
Add together the number of valence electrons for each atom in the molecule if each had a full octet. For example, CF4 There are 5 atoms in the molecule. If each had a full octet, 5 x 8 = 40 e-s are needed

5. A “AVAILABLE”
Calculate “A” as the sum of all valence electrons available to share

6. AVAILABLE ELECTRONS
Add together the number of valence electrons for each atom in the molecule. For example, CF4 Carbon has four valence electrons and each fluorine has seven valence electrons = 4 + 4(7) = 32 available e-

7. “B” BONDS
This is easy
Shared is the difference between NEEDED and AVAILABLE: N - A.
Each bond requires 2 electrons
So B = (N – A) / 2
- B is the number of bonds that will come off the central atom

8. Number of Bonds H: halogens and hydrogen can only have 1 bond
O: oxygen group can have 2 bonds
N: nitrogen group can have 3 bonds
C: carbon group can have 4 bonds
Be and B, Al: too small to hold a full octet; Be forms only 2 bonds and B and Al form only 3 bonds

9. BONDS
For example, CF4 The number of shared electrons is N – A = 40 – 32 = 8 shared electrons 2 electrons per bond, so 8/2 = 4 bonds

10. Steps for Creating Lewis Structure of Molecules
Create a “reasonable” skeleton structure
The least electronegative element will be the central atom
Never H or any of the Halogens
WHY???

11. How are the atoms connected?
Skeleton structure of the molecule:
You'll generally be writing Lewis structures for molecules that have a central atom and surrounded by one or more atoms.

12. How do you choose the central atom?
Many times it's readily apparent:
SiBr4 CH4
H2S CO2

13. Molecules are many times comprised of atoms of lesser electronegativity surrounded by atoms of greater electronegativity.
SiBr4 CH4

14. Write out the elements of the molecule so that the least electronegative elements is in the center surrounded by the other elements. For example, CF4 14

15. Place a covalent bond between the central atom and the outside atoms
Place a covalent bond between the central atom and the outside atoms. Remember each covalent bond contains two
electrons. 15

16. After making bonds between the central atom and the surrounding atom(s), you distribute the remaining electrons in pairs first about the surrounding atom(s) and then distribute any remaining electrons about the central atom

17. The four covalent bonds use eight of the 32 valence electrons in CF4
This uses 24 electrons. There are no electrons left, so this is The Lewis dot structure for CF4
There are 24 valence electrons remaining. Add electrons to the outer atoms as lone pairs to satisfy the Octet Rule. 17

18. Rule 3: Add covalent bonds between the center atom and the outer atoms
Rule 1: NAB
Rule 2: Place the least electronegative element at the center, except for H which is always an outer atom
Rule 3: Add covalent bonds between the center atom and the outer atoms
Rule 4: Add lone pairs to the outer atoms
Rule 5: Add lone pairs to the center atom 18

19. Building Lewis Structures for Covalent Molecules
•Determine the number of electrons needed (N) by each atom to complete its octet
•Add up the total number of valence electrons available (A)
•The number of bonds (B) that come off the central atom (N-A) / 2

20. RESONANCE Consider the Lewis structure of the carbonate ion, CO32-.
The Lewis structure for this ion has a carbon-oxygen double bond, and two carbon-oxygen single bonds. But which of the three oxygens forms the double bond?  There are three possibilities:

21. When more than one Lewis structure can be drawn, the molecule or ion is said to have resonance
We use a double headed arrow to show that individual structures are related by resonance.

22. HYPERVALENCY
Larger atoms can exhibit hypervalency, in which the octet is expanded and there are more than eight electrons in the valence orbitals.
Valence expansion (also known as hypervalency) is allowed for atoms in the 3rd period and below of the periodic table.

24. Hypervalency