1. Molecular Structure Part I: Lewis Diagrams
LPChem:Wz (after Johannesson)
Ch. 6 – Molecular Structure

2. I. Lewis Diagrams (p. 170 – 175)
LPChem:Wz (after Johannesson)

3. Molecular Compounds: Are made of nonmetals
Nonmetals have high electronegativity, so they do NOT release their electrons.
Two nonmetals share some of their valence electrons (in bonds) to achieve full octets.
The atoms are CO-valent-ly bonded!
LPChem:Wz (after Johannesson)
Shared Valence

4. I. Lewis Dot Structures
Are designed to show the placement of valence electrons in covalently bonded compounds.
Covalent compounds
= Molecules
= Compounds with shared electrons
LPChem:Wz (after Johannesson)

5. I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
This is the total number of electrons available to bond the molecule.
(Remember: inner electrons do not participate in bonding.)
LPChem:Wz (after Johannesson)

6. I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
NO2
If the formula carries a CHARGE, add or subtract electrons accordingly.
LPChem:Wz (after Johannesson)
(e- tally)
1 N × 5e- = e-
2 O × 6e- = + 12e-
17e-
NO2- = 17e- +1e -
= 18e-

7. I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
If the formula carries a CHARGE, add or subtract electrons accordingly:
Negative charge: ADD electrons
Positive charge: SUBTRACT electrons
LPChem:Wz (after Johannesson)

8. CF4 I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
(This is the “electron tally.”)
CF4
LPChem:Wz (after Johannesson)
1 C × 4e- = 4e-
4 F × 7e- = +28e-
32e-

9. NH3 I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
NH3
LPChem:Wz (after Johannesson)
1 N × 5e- = 5e-
3 H × 1e- = + 3e-
8e-

10. SO22- I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
SO22-
LPChem:Wz (after Johannesson)
1 S × 6e- = 6e-
2 O × 6e- = 12e-
Charge = +2e-
20e-

11. SO22- I. Lewis Dot Structures
Step 1: Add up all valence electrons in the compound.
SO22-
LPChem:Wz (after Johannesson)
NOTE: Charged dot structures (like SO22- or PO43-) will always be drawn inside square brackets!

12. I. Lewis Dot Structures F F C F F Step 2: Draw a skeleton structure.
A)The first element in the formula goes in the center.*
B)The second element goes around the first element.
Left, right, top, bottom
(Don’t choose weird angles.)
LPChem:Wz (after Johannesson) F F C F F

13. I. Lewis Dot Structures H2S H S H
A) The first element in the formula goes in the center.*
LPChem:Wz (after Johannesson)
*BUT NOT HYDROGEN
H cannot be the center atom.
If H is first in the formula, skip it.
Put the second element in the center.
Treat H as the second element.
H2S H S H

14. I. Lewis Dot Structures Step 2: Draw a skeleton structure. F C F F F
C) Draw a bond (line) connecting each secondary atom to the center.
(Do not connect secondary atoms to each other.)
LPChem:Wz (after Johannesson) F F C F F

15. I. Lewis Dot Structures Step 3: Calculate remaining electrons.
A) Each bond (line) represents two electrons that are shared between two atoms.
Number of bonds x 2e- = # e- in bonds
B) Subtract the bonded e- from the total.
LPChem:Wz (after Johannesson)

16. I. Lewis Dot Structures Step 3: Calculate remaining electrons. CF4 F C
LPChem:Wz (after Johannesson)
= 32e-
4 bonds × 2e- = - 8e-
24e- C F
These electrons will appear as dots.

17. I. Lewis Dot Structures Step 4: Distribute remaining electrons.
A) Octet rule: every atom needs 8 electrons in its valence.*
A bond is two valence electrons that count as valence for both elements involved (at the same time).
LPChem:Wz (after Johannesson)
CO-valence!

18. I. Lewis Dot Structures S H H2S
A) Octet rule: every atom needs 8 electrons in its valence.*
*BUT NOT HYDROGEN.
H can only have two electrons (because 1s is its only orbital)
When it has a bond, H is “full.”
LPChem:Wz (after Johannesson) S H
H2S
Both H’s are full.
No dots for H!

19. I. Lewis Dot Structures
A) Octet rule: every atom needs 8 electrons in its valence.*
*But not hydrogen.
B) Add electron dots to atoms as needed:
You must use up all the e-’s available.
You may NOT use more e-’s than that!
LPChem:Wz (after Johannesson)

20. I. Lewis Dot Structures Step 4: Distribute remaining electrons. CH4 F
LPChem:Wz (after Johannesson)
= 32e-
4 bonds × 2e- = 8e-
24e- C F
These electrons will appear as dots.

21. I. Lewis Dot Structures Step 4: Distribute remaining electrons.
LPChem:Wz (after Johannesson)
24e- dots:
C: 4 bonds x 2e- = 8 v.e-.
No dots needed on C
F: 1 bond x 2e- = 2 v.e-.
6 dots needed on each F C F

22. I. Lewis Dot Structures Step 4: Distribute remaining electrons.
LPChem:Wz (after Johannesson)
24e- dots:
C: 4 bonds x 2e- = 8 v.e-.
No dots needed on C
F: 1 bond x 2e- = 2 v.e-.
6 dots needed on each F C F

23. I. Lewis Dot Structures Step 4: Distribute remaining electrons.
LPChem:Wz (after Johannesson)
24e- dots:
We used exactly 24e-!
Octet check:
Do all atoms* have 8 v.e-.?
YES! We win.  C F

24. I. Lewis Dot Structures Step 4: Distribute remaining electrons. NH3 H
LPChem:Wz (after Johannesson)
= 8e-
3 bonds × 2e- = -6e-
2e- N H
These electrons will appear as dots.

25. I. Lewis Dot Structures Step 4: Distribute remaining electrons. H N
LPChem:Wz (after Johannesson)
We used exactly 2e-!
Octet check:
Do all atoms* have 8 v.e-.?
N has 8
H has 2
We win.  N H

26. I. Lewis Dot Structures Step 4: Distribute remaining electrons.
C) If you run out of electrons:
Share more!
2 e- short = 1 double bond
4 e- short = 2 double bonds
or 1 triple bond
LPChem:Wz (after Johannesson)

27.  I. Lewis Dot Structures C O Let’s try CO2 1 C × 4e- = 4e-
(e- tally)
1 C × 4e- = e-
2 O × 6e- = + 12e-
16e-
16e- (e- tally)
- 4e- (in 2 bonds)
12e- (as dots)
LPChem:Wz (after Johannesson)
But 12 dots won’t be enough.
We’re 2 e- pairs short.
That means we need 2 more bonds.
(4 bonds total) C O 

28. I. Lewis Dot Structures C O Let’s try CO2 1 C × 4e- = 4e-
(e- tally)
1 C × 4e- = e-
2 O × 6e- = + 12e-
16e-
Here we go again:
16e- (e- tally)
- 8e- (in 4 bonds)
8e- (as dots)
LPChem:Wz (after Johannesson)
We’re 2 e- pairs short.
That means we need 2 more bonds.
(4 bonds total) C O

29. I. Lewis Dot Structures C O Let’s try CO2 Octet check! 1 C × 4e- = 4e-
(e- tally)
1 C × 4e- = e-
2 O × 6e- = + 12e-
16e-
Here we go again:
16e- (e- tally)
- 8e- (in 4 bonds)
8e- (as dots)
LPChem:Wz (after Johannesson)
Octet check!
Remember, each bond counts as 2e- C O
4 dots, 2 bonds
4 bonds
WIN 

30. I. Lewis Dot Structures Step 4: Distribute remaining electrons.
D) If you have left-over electrons:
Make an Expanded Octet
This is a fancy name for “more than 8 electrons on the central atom.”
Central atom must be at an energy level ≥ 3. Expanded octets cannot exist in the 1st or 2nd E.L.
LPChem:Wz (after Johannesson)

31. 4. Distributing Electrons
Make an Expanded Octet
1) If there are simply left-over electrons, put them on the central atom.
No multiple bonds allowed!
The limit for expanded octets is 12 electrons total.
ONLY the central atom gets more than 8 electrons.
LPChem:Wz (after Johannesson)

32. I. Lewis Dot Structures Cl Cl Se Cl Cl Let’s try SeCl4
(e- tally)
1 Se × 6e- = e-
4 Cl × 7e- = + 28e-
34e-
34e- (e- tally)
- 8e- (in 4 bonds)
26e- (as dots)
LPChem:Wz (after Johannesson)
But 26 dots won’t fit! Cl
We’ve got 1 extra e- pair.
That pair goes on Se.
(Se gets 10 e- total)
Each Cl gets an octet. Cl Se Cl Cl

33. 4. Distributing Electrons
Make an Expanded Octet
If there are more than 4 atoms of the secondary element, they still bond to the central atom.
Still no multiple bonds allowed!
The limit is 6 secondary atoms bonded to the central atom (Still 12 e- total)
Still ONLY the central atom gets more than 8 electrons.
LPChem:Wz (after Johannesson)

34. I. Lewis Dot Structures Let’s try PF5 F F F P F F
Even before doing a tally we can tell that P must expand its octet in order to bond 5 F atoms.
What’s your e- tally?
1 P × 5e- = e-
5 F × 7e- = + 35e-
40e-
How many e- in bonds and how many in dots?
40e- (e- tally)
- 10e- (in 5 bonds)
30e- (as dots)
LPChem:Wz (after Johannesson)
The 5 Fs are distributed in a pentagon.
(If there were six, it would be a hexagon.)
Octet check:
P gets 10 e- total
Each F gets an octet. F F F P F F

35. 5. Finishing: A) If your dot structure was for an ion:
Place square brackets around the ion and write the charge outside the brackets:
LPChem:Wz (after Johannesson)

36. ClO4- O O Cl O 1 Cl × 7e- = 7e- 4 O × 6e- = 24e- Charge + 1e- 32e-
LPChem:Wz (after Johannesson) O
O Cl O
Charge e-
32e-
32e-
- 8e-
24e-

37. B) Resonance Structures
5. Finishing
B) Resonance Structures
Some molecules with double bonds can’t be correctly represented by a single Lewis diagram.
The actual structure is an average of all the possibilities.
LPChem:Wz (after Johannesson)

38. B) Resonance Structures
Truth: the electrons are evenly distributed, but there isn’t a way to draw that in a Lewis dot structure. (Half a bond? Half a dot?)
Show all possible structures separated by a double-headed arrow.
LPChem:Wz (after Johannesson)

39. D. Resonance Structures
SO3 has 1 double bond which could be in any of these three places: O
O S O O
O S O
LPChem:Wz (after Johannesson) O
O S O

40. HOMEWORK: Molecular Geometry WS Skyward! 1st two columns ONLY!
(We’ll learn the others next time.)
Skyward!
Do the worksheet first– it will give you the answers to the Skyward problems.
LPChem:Wz (after Johannesson)