CHEMISTRY 161 Chapter 9 Chemical Bonding I www

CHEMISTRY 161 Chapter 9 Chemical Bonding I www. chem. hawaii. edu/Bil 301/welcome. html 1

REVISION LEWIS SYMBOLS IONIC COMPOUDS COVALENT COMPOUNDS ELECTRONEGATIVITY 2

Lewis considers only valence electrons H 2 O H bonding pair of electrons O H non-bonding, or lone pair of electrons single – double – triple - quadrupole 3

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

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

all possible Lewis structures with stable electronic configurations for HCN and HNC H C Formal #valence _ charge = electrons N { H #unshared electrons N _ } { FC on C =4 -0 - 1/2 (8) } 1/2#shared electrons C N =0 6 . . Calculate formal charge for this one H C

All possible Lewis structures with stable electronic configurations for HCN and HNC. H C Formal #valence _ charge = electrons N { H #unshared electrons N _ } { FC on C =4 -0 - 1/2 (8) =0 FC on N =5 -2 - 1/2 (6) =0 } 1/2#shared electrons C N 7 . . Calculate formal charge for this one H C

All possible Lewis structures with stable electronic configurations for HCN and HNC. H C Formal #valence _ charge = electrons N { H #unshared electrons N _ } { FC on C =4 -2 - 1/2 (6) } 1/2#shared electrons N C = -1 8 . . Calculate formal charge for this one H C

All possible Lewis structures with stable electronic configurations for HCN and HNC. H C Formal #valence _ charge = electrons N { H #unshared electrons N _ } { C =4 -2 - 1/2 (6) FC on N =5 -0 - 1/2 (8) N C = -1 = +1 9 . . Calculate formal charge for this one H FC on C } 1/2#shared electrons

H C N H N 0 0 +1 C -1 FORMAL CHARGES THIS IS THE MOST LIKELY H C N OZONE 10

2. RESONANCES O O NO 3 N: 1 s 22 p 3 O O O: 1 s 22 p 4 plus one extra electron for negative charge 11

experiment shows all three bonds are the same O N 128 pm bond angles 120 0 O O any one of the structures suggests one is different! 13

O N 128 pm bond angles 120 0 O O modify the description by blending the structures blending of structures is called resonance 14

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

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

2 CO 3 NO 2 17

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

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

Lewis structure for SF 6 1 s 22 p 5 F has seven S has six 1 s 22 p 63 s 22 p 4 SF 2 SF 4 PF 3 PF 5 SF 6 NF 3 NF 5 Cl. O 4 - SO 42 I 3 - 20

2. less than an octet around central atom Be. H 2 Al. F 3 resonances BF 3 NH 3 (dative bond) Lewis acids Lewis base 21

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

4. BOND ENERGIES HCl 23

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

Homework Chapter 9 pages 347 -359, problem sets 25