1. Chemical Bonding and Molecular Structure (Chapter 9)
Ionic vs. covalent bonding
Molecular orbitals and the covalent bond (Ch. 10)
Valence electron Lewis dot structures
octet vs. non-octet
resonance structures
formal charges
VSEPR - predicting shapes of molecules
Bond properties
electronegativity
polarity, bond order, bond strength
20 Oct 97
Bonding and structure (2)

2. Rules for making Lewis dot structures
1. Count no. of valence electrons
(- don’t forget to include the charge on molecular ions!)
— 2 for # of PAIRS
2. Place a bond pair (BP) between connected atoms
3. Complete octets by using rest of e- as lone pairs (LP)
4. For atoms with <8 e-, make multiple bonds to complete octets
5. Assign formal charges : fc = Z - (#BP/2) - (#LP)
Indicate equivalent (RESONANCE) structures
6. Structures with smaller formal charges are preferred
- consider non-octet alternatives (esp. for 3rd, 4th row)
OCTET RULE: #Bond Pairs + #Lone Pairs = 4
(except for H and atoms of 3rd and higher periods)
#lone pairs at central atom in AXn = {(#e-) - 8*n}/2
20 Oct 97
Bonding and structure (2)

3. Bonding and structure (2)
Sulfur Dioxide, SO2
Rules 1-3 
O—S —O
These equivalent structures
are called: + —
RESONANCE
STRUCTURES.
The proper Lewis structure
is a HYBRID of the two.
Each atom has OCTET . .
. . . BUT there is a +1 and -1 formal charge
20 Oct 97
Bonding and structure (2)

4. Bonding and structure (2)
SO2 (2)
Alternate Lewis structure for SO2 uses 2 double bonds
Sulfur does not obey OCTET rule
BUT the formal charge = 0
O = S = O
This is better structure than O=S+-O-
since it reduces formal charge (rule 6).
3rd row S atom can have 5 or 6 electron pairs
NB: # of central atom lone pairs = (3*6 -8*2)/2 = 1 in both O=S+-O- and O=S=O structures
20 Oct 97
Bonding and structure (2)

5. Thiocyanate ion, (SCN)-
Which of three possible resonance structures
is most important?
A. S=C=N
B. S=C - N
C S-C N
-0.52
-0.32
-0.16
Calculated partial charges
ANSWER:
C > A > B
20 Oct 97
Bonding and structure (2)

6. Bonding and structure (2)
MOLECULAR GEOMETRY
Molecule adopts the shape that minimizes the electron pair repulsions.
VSEPR
Valence Shell Electron Pair Repulsion theory.
Most important factor in determining geometry is relative repulsion between electron pairs.
6_VSEPR.mov
20 Oct 97
Bonding and structure (2)

7. Bonding and structure (2)
No. of e- Pairs
Around Central
Atom
CAChe
image
Example
Geometry
180o
linear 2
F—Be—F F
120o
planar trigonal B 3 H
tetrahedral
109o C 4
20 Oct 97
Bonding and structure (2)

8. Structure Determination by VSEPR
Ammonia, NH3
There are 4 electron pairs at the corners of a tetrahedron. H N
lone pair of electrons
in tetrahedral position
The ELECTRON PAIR GEOMETRY is tetrahedral.
20 Oct 97
Bonding and structure (2)

9. Bonding and structure (2)
VSEPR - ammonia
Ammonia, NH3 H N
lone pair of electrons
in tetrahedral position
Although the electron pair geometry is tetrahedral . . .
. . . the MOLECULAR GEOMETRY
— the positions of the atoms
— is PYRAMIDAL.
20 Oct 97
Bonding and structure (2)

10. Bonding and structure (2)
AXnEm notation
a good way to distinguish between
electron pair and molecular geometries
is the AXnEm notation
where:
A - atom whose local geometry is of interest (typically the CENTRAL ATOM)
Xn - n atoms bonded to A
Em - m lone pair electrons at A
NH3 is AX3E system  pyramidal
(NB this notation not used by Kotz)
20 Oct 97
Bonding and structure (2)

11. 1. Draw electron dot structure
VSEPR - water
Water, H2O
H - O - H ••
1. Draw electron dot structure
2. Count BP’s and LP’s = 4
3. The 4 electron pairs are at the corners of a tetrahedron.
The electron pair geometry is TETRAHEDRAL.
20 Oct 97
Bonding and structure (2)

12. Bonding and structure (2)
VSEPR - water (2)
H - O - H ••
Although the electron pair geometry is TETRAHEDRAL . . .
. . . the molecular geometry is bent.
H2O - AX2E2 system - angular geometry
20 Oct 97
Bonding and structure (2)

13. 1. Draw electron dot structure
Formaldehyde, CH2O
VSEPR - formaldehyde • C H O
1. Draw electron dot structure
2. Count BP’s and LP’s:
At Carbon there are 4 BP but . . .
3. These are distributed in ONLY 3 regions.
Double bond electron pairs are in same region.
There are 3 regions of electron density
Electron repulsion places them at the corners of a planar triangle. • C H O
Both the electron pair geometry and the molecular geometry are PLANAR TRIGONAL  120o bond angles.
H2CO at the C atom is an AX3 species
20 Oct 97
Bonding and structure (2)

14. 109o because both the C and O
VSEPR - Bond Angles
Methanol, CH3OH H ••
Define bond angles 1 and 2
Angle 1 = H-C-H = ?
Angle 2 = H-O-C = ?
Answer:
H—C—O—H ••
Angle 1 H
Angle 2
109o because both the C and O
atoms are surrounded by 4 electron pairs.
6_CH3OH.mov
AXnEm designation ?
at C
at O
AX4 = tetrahedral
AX2E2 = bent
20 Oct 97
Bonding and structure (2)

15. Bonding and structure (2)
VSEPR - bond angles (2)
Acetonitrile, CH3CN H 1
H—C—C 2 •• N
Define bond angles 1 and 2
Angle 1 = ?
109o
Angle 2 = ?
180o
Why ? :
The CH3 carbon is surrounded by 4 bond charges
The CN carbon is surrounded by 2 bond charges
AXnEm designation ?
at CH3 carbon
at CN carbon
AX4 = tetrahedral
AX2 = linear
20 Oct 97
Bonding and structure (2)

16. Bonding and structure (2)
What about: STRUCTURES WITH CENTRAL ATOMS THAT DO NOT OBEY THE OCTET RULE ?
PF5
BF3
SF4
20 Oct 97
Bonding and structure (2)

17. Geometry for non-octet species also obey VSEPR rules
Consider boron trifluoride, BF3
The B atom is surrounded by only 3 electron pairs.
Bond angles are 120o
Molecular Geometry is
planar trigonal
BF3 is an AX3 species
20 Oct 97
Bonding and structure (2)

18. Compounds with 5 or 6 Pairs Around the Central Atom
6_VSEPR.mov F
Trigonal bipyramid
120°
90° P
5 electron pairs
AX5 system F
Octahedron
90° S
6 electron pairs
AX6 system
20 Oct 97
Bonding and structure (2)

19. Sulfur Tetrafluoride, SF4• •• S
Number of valence e- = 34
No. of S lone pairs =
{ b.p. - 3x4 l.p.(F)}
= 1 lone pair on S
There are 5 (BP + LP)
e- pairs around the S
THEREFORE:
electron pair geometry ? F • S F • S
= trigonal bipyramid OR
AX4E system. Molecular geometry ?
20 Oct 97
Bonding and structure (2)

20. Sulfur Tetrafluoride, SF4 (2)
90°
axial F • S
120°
equatorial
Lone pair is in the equatorial position because it requires more room than a bond pair.
Molecular geometry of SF4 is “see-saw”
Q: What is molecular geometry of SO2 ?
20 Oct 97
Bonding and structure (2)

21. Bonding with Hybrid Atomic Orbitals
- Carbon prefers to make 4 bonds as in CH4
But atomic carbon has an s2p2 configuration
Why can it make more than 2 bonds ?
6_CH4.mov
4 C atom orbitals hybridize to form four
equivalent sp3 hybrid atomic orbitals.
20 Oct 97
Bonding and structure (2)

22. Orbital Hybridization
BONDS SHAPE HYBRID REMAIN e.g.
s2p2 
2 linear {2 x sp & 2 p’s} C2H2
3 trigonal {3 x sp2 & 1 p} C2H4 planar
4 tetrahedral {4 xsp3 } CH4
20 Oct 97
Bonding and structure (2)

23. Multiple Bonds s and p Bonding in C2H4­¯ 2p 2s
3 sp2
hybrid
orbitals p
orbital
C atom orbitals are COMBINED
(= re-hybridized) to form orbitals
better suited for BONDING
The extra p orbital electron on each C atom overlaps the p orbital on the neighboring atom to form the p bond.
The 3 sp2 hybrid orbitals
are used to make the C-C
and two C-H  bonds H C
sp2
120°
6_C2H4-sg.mov
6_C2H4-pi.mov
6_C2H4.mov
20 Oct 97
Bonding and structure (2)

24. Consequences of Multiple Bonding
Restricted rotation around C=C bond in
1-butene = CH2=CH-CH2-CH3.
See Butene.Map in ENER_MAP in CAChe models. 27
233
E (kJ/mol)
C-C=C angle (o)
P Photo-rotation
about double bonds
lets us see !!
20 Oct 97
Bonding and structure (2)

25. Bonding and structure (2)
Bond Properties
What is the effect of bonding and structure on molecular properties ?
- bond order
- bond length
- bond strength
- bond polarity
- MOLECULAR polarity
Buckyball in HIV-protease, see page 107
20 Oct 97
Bonding and structure (2)

26. Bonding and structure (2)
Bond Order
the number of bonds between a pair of atoms. H C N
triple, BO = 3 1 s
and 2 p
CH2CHCN
Acrylonitrile
single
BO = 1 1 s
double, BO = 2 1 s
and 1 p
20 Oct 97
Bonding and structure (2)

27. Bonding and structure (2)
Bond Order (2)
Fractional bond orders occur in molecules with resonance structures.
Consider NO2-
Bond
order =
Total #
of e -
pairs used for a type of bond
of bonds of that type
Bond order in NO2- =
3 (e - pairs in N-O bonds)
2 (N - O bonds)
N-O bond order in NO2- = 1.5
20 Oct 97
Bonding and structure (2)

28. Bond Order and Bond Length
Bond order is related to two important bond properties:
(a) bond strength
as given by DE
745 kJ
414 kJ
123 pm
110 pm
Formaldehye
(b) Bond length
- the distance between the nuclei of two bonded atoms.
20 Oct 97
Bonding and structure (2)

29. Bonding and structure (2)
Bond Length
- depends on size of bonded atoms:
Molecule R(H-X)
H- F 104 pm
H- Cl 131 pm
H- I 165 pm
- depends on bond order.
Molecule R(C-O)
CH3C- OH 141 pm
O=C=O 132 pm
C O pm
20 Oct 97
Bonding and structure (2)

30. Bonding and structure (2)
Bond Strength
Bond Dissociation energy (DE) - energy required to break a bond in gas phase.
See Table 9.5
BOND STRENGTH (kJ/mol) LENGTH (pm)
H—H
C—C
C=C
CºC
NºN
The GREATER the number of bonds (bond order)
the HIGHER the bond strength and the SHORTER the bond.
20 Oct 97
Bonding and structure (2)

31. Bonding and structure (2)
Bond Strength (2)
Bond Order Length Strength
HO—OH pm 210 kJ/mol
O=O kJ/mol ?
303 kJ/mol
O3 (g)  3 O(g)
HOW TO CALCULATE ?
Hrxn = {3xHf(O) - Hf(O3)} = {3x } = 605 kJ/mol
2 O-O bonds in O3  DE (O3) = 605/2 = kJ/mol
20 Oct 97
Bonding and structure (2)

32. Bonding and structure (2)
Bond Polarity
HCl is POLAR because it has a positive end and a negative end (partly ionic).
Polarity arises because Cl has a greater share of the bonding electrons than H.
Calculated charge by CAChe:
H (red) is +ve (+0.20 e-)
Cl (yellow) is -ve (-0.20 e-).
(See PARTCHRG folder in MODELS.)
20 Oct 97
Bonding and structure (2)

33. Bonding and structure (2)
Bond Polarity (2)
Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a “pure” covalent bond.
BOND ENERGY
“pure” bond 339 kJ/mol calculated
real bond 432 kJ/mol measured
Difference kJ/mol.
This difference is the contribution of IONIC bonding
It is proportional to the difference in
ELECTRONEGATIVITY, c.
20 Oct 97
Bonding and structure (2)

34. Bonding and structure (2)
Electronegativity, c
c is a measure of the ability of an atom in a molecule to attract electrons to itself.
Concept proposed by
Linus Pauling ( )
Nobel prizes:
Chemistry (54), Peace (63)
See p. 425; 008vd3.mov (CD)
20 Oct 97
Bonding and structure (2)

35. Bonding and structure (2)
Electronegativity, c
Figure 9.7
F has maximum c.
Atom with lowest c is the center atom in most molecules.
Relative values of c determines BOND POLARITY (and point of attack on a molecule).
20 Oct 97
Bonding and structure (2)

36. Therefore OH is more polar than OF
Bond Polarity
Which bond is more polar ? (has larger bond DIPOLE)
O—H O—F
c H
2.1
O F
c(A) - c(B) Dc
0.5
(O-H) > (O-F)
Therefore OH is more polar than OF
Also note that polarity is “reversed.”
20 Oct 97
Bonding and structure (2)

37. Bonding and structure (2)
Molecular Polarity
Molecules—such as HCl and H2O— can be POLAR (or dipolar).
They have a DIPOLE MOMENT.
Polar molecules turn to align their dipole with an electric field.
20 Oct 97
Bonding and structure (2)

38. Predicting molecular polarity
A molecule will be polar ONLY if
a) it contains polar bonds
AND
b) the molecule is NOT “symmetric”
Symmetric molecules
20 Oct 97
Bonding and structure (2)

39. Molecular Polarity: H2O
Water is polar because:
a) O-H bond is polar
b) water is non-symmetric
The dipole associated with polar H2O
is the basis for absorption of microwaves
used in cooking with a microwave oven
20 Oct 97
Bonding and structure (2)

40. Bonding and structure (2)
Carbon Dioxide
CO2 is NOT polar even though the CO bonds are polar.
Because CO2 is symmetrical the BOND polarity cancels
The positive C atom is why water attaches to CO2
CO2 + H2O  H2CO3
20 Oct 97
Bonding and structure (2)

41. NON-symmetric molecules
Molecular Polarity in
NON-symmetric molecules
B—F bonds are polar
molecule is symmetric
B—F, B—H bonds polar
molecule is NOT symmetric
Atom Chg. 
B +ve
H +ve
F -ve
B +ve
F -ve
BF3 is NOT polar
HBF2 is polar
20 Oct 97
Bonding and structure (2)

42. Fluorine-substituted Ethylene: C2H2F2
C—F bonds are MUCH more polar than C—H bonds.
(C-F) = 1.5, (C-H) = 0.4
CIS isomer
both C—F bonds on same side
 molecule is POLAR.
TRANS isomer
both C—F bonds on opposite side
 molecule is NOT POLAR.
20 Oct 97
Bonding and structure (2)

43. Chemical Bonding and Molecular Structure (Chapter 9)
Ionic vs. covalent bonding
Molecular orbitals and the covalent bond (Ch. 10)
Valence electron Lewis dot structures
octet vs. non-octet
resonance structures
formal charges
VSEPR - predicting shapes of molecules
Bond properties
electronegativity
polarity, bond order, bond strength
20 Oct 97
Bonding and structure (2)