Week 9/Tu: Lecture Unit ‘21’
Exam 2: thermodynamics in reactions and atomic structure
Unit 21: Lewis Structures, II
-- multiple bonds
-- formal charge
-- “resonance”
Unit 22: VESPR
-- Electron Arrangements
-- 3D shapes
Issues: ?
No Homework this week
of the 2nd exam
© DJMorrissey, 2o12
Tacoma Narrows Bridge ripped apart by resonances
Nov. 7th 1940
Week 9/Tu: SXFY
Lewis Structures for the fluorides of sulfur …
SF2
F S
F
S
Remember Water ?
F
F
S2F2
F S S
F
Remember Hydrogen
Peroxide ?
F S
F
SF4
F F
SF6
© DJMorrissey, 2o12
F S
F
F
No Oxygen analog
F
F
No Oxygen analog
F
Fluorine (and oxygen) can
attack lone pairs of electrons
in orbitals with n > 2
Week 9/Tu: COX
Lewis Structures for three oxides of carbon …
CO
CO2
CO32-
C
O
C
O
O
O
C
O
O
C
Moved electron !
3 bonds to Oxygen
O
2 bonds to each Oxygen
i.e., “normal”
O
O
C Valence Electrons:
O Count
O C 4 for C O C
3*6 for O
O
2 O
extra for charge state O
24 eArbitrary assignment of electrons
– called resonance – indicates another flaw in model
1 1/3 Bond to each Oxygen !
© DJMorrissey, 2o12
Week 9/Tu: CHXOY
Lewis Structures for combinations of C, H & O
CH4O
H
HC O
H H
H
C
CH2O
CHO2-
H
H C
O
O
© DJMorrissey, 2o12
O
H C
O
O
Week 9/Tu: SOX
Lewis Structures for three oxides of sulfur …
SO2
O S
(or O3 ?)
O S
O
O
O
Count Valence Electrons
SO32-
O S
O
S
O
O
O S
O
O
Count Valence Electrons
SO42-
O
3*6 for O
6 for S
2 extra for charge state
26 e-
O S
O
© DJMorrissey, 2o12
O S
O
4*6 for O
6 for S
2 extra for charge state
32 e-
O
O
Week 9/Tu: NOX “Formal Charge”
Lewis Structures for some of the oxides of nitrogen …
O N
NO
O N
or
Formal charge = Valence e- - (non-bonding e-) – ½ (bonding e-)
Oxygen FC = 6 – 4 - 4/2 = 0
Nitrogen FC = 5 – 3 – 4/2 = 0
2
N2O
2
0
N O N
or
6 – 3 – 4/2 = +1
5 – 4 – 4/2 = -1
1
2
1
N O N
or
O N N
© DJMorrissey, 2o12
O N N
Week 9/Tu: NO2 Structures
Lewis Structures for some of the oxides of nitrogen …
NO2
O N
O N
O
O
NO2© DJMorrissey, 2o12
O N
O N
N2O4
O
O
Combine two NO2 molecules
by a weak N-N bond
FC on N = 5 – 0 – 4/2 = +1
O N
O N
O
O
Week 9/Tu: Lewis Structures: Summary
There are only eight Lewis Structures for all of the Main
Group elements (s, p-blocks). Relatively few structures
with many analogs using other atoms in the same group.
Assign electrons to the atoms as necessary to form octets.
Use Formal Charge to aid in assignment of electrons,
generally you want the lowest formal charges.
The relative chemical stability, bond strength, can be
estimated from the number of bonds in the Lewis structure.
Fluorine (and oxygen) can form bonds to lone pairs of
electrons on “large” central atoms (n>2).
© DJMorrissey, 2o12