1. Drawing Lewis Structures and VSEPR

2. OUTCOME QUESTION(S): C12-2-07 PERIODIC TRENDS Vocabulary & Concepts
Identify factors and account for periodic trends among the properties of elements, and relate to electron configurations.
Include: atomic / ionic radii, ionization energy, and
electronegativity.
Construct Lewis dot diagrams for various compounds and use VSEPR to predict bond shape from the electron arrangement.
Vocabulary & Concepts 
Line Bond

3. *Valence electrons determine atom reactivity*
Aluminum
Fluorine
3 F-
Al+3

4. S Ca Electron Dot Diagrams (Lewis Dot)
A 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
Since valence electrons do all the work – the Lewis dot is only concerned with them

5. The central atom is supposed to have the lowest EN value
Constructing a Lewis Dot Diagram:
1. Determine total valence electrons for all atoms.
~ watch for negative/positive ion
SeCl2, CH2ClF, CO2, NH41+, PO43-
= 20
2. Position first atom in the centre.
~ usually first in formula (unless H or F)
The central atom is supposed to have the lowest EN value
3. Join other atoms with line bonds to the central.
Cl – Se
– Cl

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

7. Draw the Lewis structure for CO2: CO2 : 4 + 6 + 6 = 16 - 4 = 12
Not “full”      – – 
O – C
– O       8 8 8 4 8 8
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. Be sure to account for extra or lost electrons in a ionic compound
Draw the Lewis structure for SO42- :
SO42- : 6 + (4)6 + 2 = 32
- 8 = 24
Be sure to account for extra or lost electrons in a ionic compound 2-  S
– O
O –   
Charged Compound? – put structure into brackets and write the charge on the right.

9. MgCl2 : = 16
- 4 = 12
Ionic Bond
Note: a complete octet for the metal is not necessary here – valence electrons are given away  
Cl – Mg
– Cl 2+ Mg 1- Cl  1- Cl 
Ionic Compound? – put each atom into brackets and write the charge on the ions.

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 sites
and lone pair electrons around the central atom

11. A line bond counts as 1 pair of electrons
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 pair of electrons
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. This is just a different way of representing the same information
AXE method:
A: Central atom
Xn: # of bonding sites
En: # lone pairs
This is just a different way of representing the same 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 sites
0 – lone pairs
Trigonal Planar

15. Draw the Lewis structure for CH4:
4 – bonding sites (X4)
0 – lone pairs
Tetrahedral
Draw the Lewis structure for PCl5:
5 – bonding sites (X5)
0 – lone pairs
Trigonal Bipyramidal
Draw the Lewis structure for SF6:
6 – bonding sites (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 sites
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 sites
1 – lone pairs 
Trigonal Pyramidal
Notice again the distorted shape from the lone pair of electrons

18. Double and triple bonds still counted as ONE bonding site
- 4  
O – – C
– O –
Double Bonds  
2 – bonding sites
0 – lone pairs
Linear
Double and triple bonds still counted as ONE bonding site

19. CAN YOU / HAVE YOU? C12-2-07 PERIODIC TRENDS Vocabulary & Concepts
Identify factors and account for periodic trends among the properties of elements, and relate to electron configurations.
Include: atomic / ionic radii, ionization energy, and
electronegativity.
Construct Lewis dot diagrams for various compounds and use VSEPR to predict bond shape from the electron arrangement.
Vocabulary & Concepts 
Line Bond