1. Drawing Lewis Structures and VSEPR

2. Draw basic Lewis dot structures of atoms and
compounds.
Using VSEPR, predict bond shape from electron arrangement in Lewis dot diagram.
Additional KEY Terms

3. 3 F- Al+3 As atoms combine in definite proportions: Aluminum Fluorine
Valence electrons determine atom reactivity

4. S Ca Electron Dot Diagrams (Lewis Dot)
Quick way to show bonding electrons (valence)
- atomic symbol represents nucleus and
inner shell electrons
- only valence electrons are used
- dots placed clockwise, ending in pairs S Ca

5. STEPS / RULES TO LEWIS DOT:
1. Position first atom in the centre.
~ usually first in formula (unless H or F)
CH2ClF, SeCl2, O3 (CO2, NH3, PO43-)
2. Join other atoms with line bond to the central.
Cl – Se
– Cl
3. Determine total valence electrons for all atoms.
~ watch for negative/positive ions
The central atom is supposed to be have the lowest EN value

6. 4. Place remaining e- around atoms to fill valence.
SeCl2
= 20
- 4 = 16
Subtract 2 electrons for every line bond formed       
Cl – Se
– Cl         
4. Place remaining e- around atoms to fill valence.
Fill surrounding atoms BEFORE central atom
5. Check that each atom has a complete octet.
Remember: “8 is great, but 2 will do”
– Hydrogen is an exception

7. Draw the Lewis structure for CO2: CO2 : 4 + 6 + 6 = 16 - 4 = 12
Not “full”      –
– O – 
O – C      
Too few electrons? - give central atom a complete octet by creating a double or triple bond.
Create multiple bonds by moving an electron pair from a surrounding atom into a line bond with the central

8. Place brackets around charged structures
Draw the Lewis structure for SO42- :
SO42- : 6 + (4)6 + 2 = 32
- 8 = 24 2-  S
– O
O –   
Place brackets around charged structures

9. MgCl2 : 2 + 7 + 7 = 16 - 4 = 12 Ionic Bond Cl – Mg – Cl 2+ Mg 1- Cl 1- 
Cl – Mg
– Cl 2+ Mg 1- Cl  1- Cl 
Ionic Lewis dots are indicated by using brackets to distinguish the “give/take” nature of the bonding

10. Valence Shell Electron Pair Repulsion Theory
VSEPR
Valence Shell Electron Pair Repulsion Theory
Predicts 3D structures of molecules
Minimizes the electrostatic repulsion between
electrons in valence sublevels
Shape calculated by comparing bonding and lone
pair electrons around the central atom

11. A line bond counts as 1 bonding pair
SeCl2 : = 20
- 4 = 16 
Cl – Se
– Cl
VSEPR shape is determined by counting bonding pairs and lone pairs around the CENTRAL atom only.
A line bond counts as 1 bonding pair
A lone pair is a pair of non-bonding electrons

12. You are given this table – count bonding pairs and lone pairs and read the shape

13. AXE method: A: Central atom Xn: # of bonding pairs En: # lone pairs
This is just a different why of representing the same table information

14. (Does not need a full octet)
Central Atom with only bonding pairs:
Draw the Lewis structure for BF3:
BF3 : 3 + (3)7 = 24
- 6 = 18  
*Exception
(Does not need a full octet)
– F
F – B F – 
3 – bonding pairs
0 – lone pairs
Trigonal Planar

15. Draw the Lewis structure for CH4:
4 – bonding pairs (X4)
0 – lone pairs
Tetrahedral
Draw the Lewis structure for PCl5:
5 – bonding pairs (X5)
0 – lone pairs
Trigonal Bipyramidal
Draw the Lewis structure for SF6:
6 – bonding pairs (X6)
0 – lone pairs
Octahedral

16. Its drawn linear but the lone electrons
Central Atom with LONE pairs:
Lone pairs have a greater repulsion force and distort the predicted bond angles.
H2S : = 8
- 4
– H
H –  S
2 – bonding pairs
2 – lone pairs
Bent
Its drawn linear but the lone electrons
distort the shape

17. Notice again the distorted shape from the lone pair of electrons
Draw the Lewis structure for PF3:
PF3 : 5 + (3)7 = 26
- 6   
– F
F – P F –
3 – bonding pairs
1 – lone pairs 
Trigonal Pyramidal
Notice again the distorted shape from the lone pair of electrons

18. Double and triple bonds are still counting as ONE bonding pair
- 4  
O – – C
– O –
Double Bonds  
2 – bonding pairs
0 – lone pairs
Linear
Double and triple bonds are still counting as ONE bonding pair

19. CAN YOU / HAVE YOU?
Draw basic Lewis dot structures of atoms and
compounds.
Using VSEPR, predict bond shape from electron arrangement in Lewis dot diagram.
Additional KEY Terms