1. Drawing Lewis Structures and VSEPR A Tutorial on Writing Lewis Dot Structure

2. OCTET RULE: atoms will gain, lose or share electrons to fill their valence shell (noble gas) use only valence electrons in drawing Lewis dot.

3. STEPS / RULES TO LEWIS DOT: Determine total valence electrons for all atoms. ~Watch for negative/positive/polyatomic ions. Position least electronegative atom in the centre. ~central atom usually first in formula (unless H/ F). CH 2 ClF, SeCl 2, O 3 (CO 2, NH 3, PO 4 3- ) Write other atoms around central with line bond. ~line represents 2 e - (must be subtracted from total) Se – Cl Cl –

4. Place electrons around each atom to fill valence. Too few electrons - give each atom a complete octet by adding double or triple bonds. SeCl 2 : 6 +7 + 7 = 20- 4              Se – Cl Cl –

5. Draw the Lewis structure for CO 2 : CO 2 : 4 + 6 + 6 = 16 C – O O – - 4           – –

6. S – O O – – O Draw the Lewis structure for SO 4 2- : SO 4 2- : 6 + (4)6 + 2 = 32 - 8                     2-

7. BeCl 2 : 2 + 7 + 7 = 16 – Cl Cl –           Be - 4 Ionic Bond Cl        +2 Be Cl       

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

9. AXE method: A: Central atom X n : # of bonding pairs E n : # lone pairs

10. Draw the Lewis structure for BF 3 : BF 3 : 3 + (3)7 = 24           - 6 – F F – B F –      *Unusual (Does not have octet) 3 – bonding pairs (X 3 ) 0 – lone pairs Trigonal Planar– 120 o bond angle Central Atom with only bonding pairs:

11. Draw the Lewis structure for CH 4 : Draw the Lewis structure for PCl 5 : Draw the Lewis structure for SF 6 : 4 – bonding pairs (X 4 ) 0 – lone pairs Tetrahedral – 109.5 o 5 – bonding pairs (X 5 ) 0 – lone pairs Trigonal Bipyramidal – 90 o and 120 o 6 – bonding pairs (X 6 ) 0 – lone pairs Octahedral – 90 o

12. Central Atom with LONE pairs: Lone pairs have a greater repulsion force and distort the predicted bond angles. H 2 S : 1 + 1 + 6 = 8 – H H –    S - 4 2 – bonding pairs (X 2 ) 2 – lone pairs (E 2 ) Bent – 104.5 o bond angle *Predict Linear

13. Draw the Lewis structure for PF 3 : PF 3 : 5 + (3)7 = 26           - 6 – F F – P F –      *Predict Trigonal Planar 3 – bonding pairs (X 3 ) 1 – lone pairs Trigonal Pyramidal – 107 o 

14. Treat double/triple bonds as single bonds: CO 2 : 4 + 6 + 6 = 16 - 2 Double Bonds 2 – bonding pairs (X 2 ) 0 – lone pairs Linear – 180 o bond angle   C – OO –         ––