1 CHEMISTRY 161 Chapter 9 Chemical Bonding I

2 IONIC COMPOUDS COVALENT COMPOUNDS REVISION ELECTRONEGATIVITY LEWIS SYMBOLS

3 OHH non-bonding, or lone pair of electrons bonding pair of electrons Lewis considers only valence electrons H2OH2O single – double – triple - quadrupole

4 1. formal charge 2. concept of resonances LEWIS STRUCTURES 3. exceptions to the octet rule 4. bond energies

5 structures with the lowest formal charges are likely to have the lowest energy formal charge = #valence electrons _ #unshared electrons _ 1/2#shared electrons { } {} indication of the extent to which atoms have gained or lost electrons in 1. formal charge

6 all possible Lewis structures with stable electronic configurations for HCN and HNC Calculate formal charge for this one FC on C= /2 (8) = 0 _ 1/2#shared electrons {} Formal charge = #valence electrons _ #unshared electrons { } HCN.. HNCHCN

7 All possible Lewis structures with stable electronic configurations for HCN and HNC. Calculate formal charge for this one FC on C= /2 (8) = 0 _ 1/2#shared electrons {} Formal charge = #valence electrons _ #unshared electrons { } FC on N = /2 (6)= 0 HCN.. HNCHCN

8 All possible Lewis structures with stable electronic configurations for HCN and HNC. Calculate formal charge for this one FC on C= /2 (6)= -1 _ 1/2#shared electrons {} Formal charge = #valence electrons _ #unshared electrons { } HNC.. HNCHCN

9 All possible Lewis structures with stable electronic configurations for HCN and HNC. Calculate formal charge for this one FC on C= /2 (6)= -1 _ 1/2#shared electrons {} Formal charge = #valence electrons _ #unshared electrons { } FC on N= /2 (8) = +1 HNC.. HNCHCN

THIS IS THE MOST LIKELY FORMAL CHARGES HNC HN HCNC OZONE

11 2. RESONANCES OOOOOO NO 3 - N: 1s 2 2s 2 2p 3 O: 1s 2 2s 2 2p 4 plus one extra electron for negative charge

13 experiment shows all three bonds are the same any one of the structures suggests one is different! OO O N bond angles pm

14 modify the description by blending the structures blending of structures is called resonance OO O N bond angles pm

15 use a double headed arrow between the structures electrons involved are said to be DELOCALIZED over the structure. blended structure is a RESONANCE HYBRID RESONANCE N OO O OO O NN OO O

16 We use a double headed arrow between the structures.. RESONANCE N OO O OO O NN OO O OO O N

17 CO 3 2- NO 2 -

18 1. more than 8 electrons around central atom 3. molecules with unpaired electrons 2. less than an octet around central atom 3. Exceptions to the octet rule

19 elements in rows 3 and following can exceed octet rule F S F F F F F SF 6 S F F F F F F 1. more than 8 electrons around central atom participation of d electrons

20 Lewis structure for SF 6 F has seven 1s 2 2s 2 2p 5 S has six 1s 2 2s 2 2p 6 3s 2 2p 4 SF 2 SF 4 SF 6 PF 3 PF 5 NF 3 NF 5 ClO 4 - SO 4 2- I3-I3-I3-I3-

21 2. less than an octet around central atom BeH 2 AlF 3 BF 3 resonances NH 3 (dative bond) Lewis acids Lewis base

22 3. molecules with unpaired electrons FREE RADICALS NO but not NO -

23 4. BOND ENERGIES HCl

24 1. formal charge 2. concept of resonances SUMMARY 3. exceptions to the octet rule 3.1. excess of electrons 3.2. missing electrons 3.3. radicals 4. bond energies

25 Homework Chapter 9 pages , problem sets