1. Lecture /11/05

2. Quiz H2O CH3 H3O+ NO3- Draw the Lewis structure for SO32-.
Which of the following is isoelectronic with NH3?
H2O CH3 H3O+ NO3-

3. Violations of the Octet Rule
Only elements with shells of 3 and greater can have more than an octet.
Electrons are put in the d subshell

4. Violations of the Octet Rule
H and He
Transition metals
Too few electrons
B in BF3
Too many electrons
Expanded octet
Odd number of electrons (radicals)

5. Electronegativity ()
Ability of an atom to attract electrons to itself
No values for He, Ne, or Ar since there are no known molecules with these elements

6. Bond Polarity
Greater the electronegativity difference between atoms, the more polar the bond
Non-polar Covalent Bond:
Atoms share the electrons equally
Polar Covalent Bond:
Electrons shared unequally
0 <  < ~1.7
Partial + and partial - on atoms
Ionic Bond:
~1.7 < 

7. Bond Polarity and polarity is “reversed.” Which bond is more polar?
O—H O—F  
OH is more polar than OF
and polarity is “reversed.”

8. For each pair, which has a more polar bond?
a) H-F or H-I b) B-C or B-F c) C-Si or C-S

9. Polar covalent bonds have more energy than non-polar covalent bonds
BOND ENERGY
“pure” bond 339 kJ/mol calc’d
real bond 432 kJ/mol measured

10. Formal Charge
Charge on a bonded atom if the bonding electrons are assumed to be shared equally
F.C. = valence e-s – [# of lone pair e-s + ½ # of bond e-s]
Sum of F.C. always equals the net charge of molecule

11. Formal Charge
Can be used to determine the most likely structure among potential Lewis structures
Pauli electroneutrality principle:
Smaller F.C. are more favorable than larger ones
As close to zero as possible
Negative F.C. should be on the more electronegative atoms
Like charges should not be on adjacent atoms

12. Examples
Calculate formal charges of the atoms in:
OH-
NH4+

13. Which is the most likely Lewis structure?
OCN-
Which is the most likely Lewis structure? • O N C • O N C • O N C

14. Which is the most likely Lewis structure?
SCN-
Which is the most likely Lewis structure? • S N C • S N C • S N C