35 slides very nice ppt good for 11 class

1. Chemical Bonding
and
Molecular Structure
By: Navneet singh
class 11(sa1 Ot)
04/09/2015 Bonding and Structure 1

2. 04/09/2015 Bonding and Structure 2
Chemical Bonding
and
Molecular Structure
• 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
polarity, bond order, bond strength

3. 04/09/2015 Bonding and Structure 3
Chemical Bonding
Problems and questions —
• How is a molecule or polyatomic
ion held together?
• Why are atoms distributed at
strange angles?
• Why are molecules not flat?
• Can we predict the structure?
• How is structure related to
chemical and physical properties?

4. 04/09/2015 Bonding and Structure 4
Most bonds are somewhere in between.
Forms of Chemical Bonds
• There are 2 extreme forms of connecting
or bonding atoms:
• Ionic—complete transfer of electrons
from one atom to another
• Covalent—electrons shared between
atoms

5. 04/09/2015 Bonding and Structure 5
IonicIonic
BondsBonds
Ionic compounds
- essentially complete electron
transfer from an element of low
IE (metal) to an element of high
electron affinity (EA) (nonmetal)
Na(s) + 1/2 Cl2(g) → Na+
+ Cl-
→ NaCl (s)
- NON-DIRECTIONAL bonding
via Coulomb (charge) interaction
- primarily between metals
(Grps 1A, 2A and transition metals)
and nonmetals (esp O and halogens)

6. 04/09/2015 Bonding and Structure 6
Covalent Bonding
Covalent bond is the sharing of the VALENCE
ELECTRONS of each atom in a bond
Recall: Electrons are divided between core
and valence electrons.
ATOM core valence
Na 1s2
2s2
2p6
3s1
[Ne] 3s1
Br [Ar] 3d10
4s2
4p5
[Ar] 3d10
4s2
4p5
Br Br

7. 04/09/2015 Bonding and Structure 7
Valence Electrons1A1A
2A2A 3A3A 4A4A 5A5A 6A6A 7A7A
8A8A
Number of valence electrons is
equal to the Group number.

8. 04/09/2015 Bonding and Structure 8
Covalent Bonding
The bond arises from the mutual attraction of
2 nuclei for the same electrons.
HB+ HA
HB
HA
A covalent bond is a balance
of attractive and repulsive forces.
6_H2bond.mov

9. 04/09/2015 Bonding and Structure 9
Bond Formation
A bond can result from a “head-to-head” overlap
of atomic orbitals on neighboring atoms.
H H Cl
••
•
•
••
Cl
••
•
•
••
+
Overlap of H (1s) and Cl (2p)
This type of overlap places bonding electrons in a
MOLECULAR ORBITAL along the line between
the two atoms and forms a SIGMA BOND (σ).

10. 04/09/2015 Bonding and Structure 10
Sigma Bond Formation by Orbital Overlap
••↑ ••
sigma bond ( σ)
↓↑↑+
HH
Two s Atomic Orbitals (A.O.s) overlap to form
an sσ (sigma) Molecular Orbital (M.O.)
6_H2pot.mov

11. 04/09/2015 Bonding and Structure 11
Sigma Bond Formation by Orbital Overlap
••↑ ••
sigma bond ( σ)
↓↑↑+
HH
↑↓
↑↑
Two s A.O.s overlap to
from an s σ M.O.
Similarly, two p A.O.s
can overlap end-on to
from a pσ M.O.
e.g.
F2

12. 04/09/2015 Bonding and Structure 12
ElectronElectron
Distribution inDistribution in
MoleculesMolecules
• Electron distribution
is depicted with
Lewis electron
dot structures
• Electrons are
distributed as:
• shared or BOND
PAIRS and
• unshared or LONE
PAIRS.
G. N. Lewis
1875 - 1946

13. 04/09/2015 Bonding and Structure 13
Bond and Lone Pairs
• Electrons are distributed as shared or BOND
PAIRS and unshared or LONE PAIRS.
••
H Cl
••
•
•
This is a LEWIS ELECTRON DOT structure.
shared or bond pair
Unshared or
lone pair (LP)

14. 04/09/2015 Bonding and Structure 14
• This observation is called the OCTET RULE
Rules of Lewis StructuresRules of Lewis Structures
• No. of valence electrons of an atom =
Group number
• Except for H
(and atoms of 3rd and higher periods),
#Bond Pairs + #Lone Pairs = 4
• For Groups 5A-7A (N - F),
no. of BOND PAIRS = 8 - group No.
• For Groups 1A-4A (Li - C),
no. of BOND PAIRS = group number

15. 04/09/2015 Bonding and Structure 15
2. Count valence electrons
H = 1 and N = 5
Total = (3 x 1) + 5
= 8 electrons or
1. Decide on the central atom; never H.
Central atom is atom of lowest affinity for electrons.
In ammonia, N is central
Building a Dot Structure
Ammonia, NH3
4 pairs

16. 04/09/2015 Bonding and Structure 16
4. Remaining electrons form
LONE PAIRS to complete
octet as needed.
3. Form a sigma bond
between the central atom
and surrounding atoms.
H H
H
N
Building a Dot Structure
H
••
H
H
N
3 BOND PAIRS and 1 LONE PAIR.
Note that N has a share in 4 pairs (8 electrons), while
each H shares 1 pair.

17. 04/09/2015 Bonding and Structure 17
Step 2. Count valence electrons
S = 6
3 x O = 3 x 6 = 18
Negative charge = 2
TOTAL = 6 + 18 + 2 = 26 e-
or 13 pairs
Step 1. Central atom = S
10 pairs of electrons are left.
Sulfite ion, SO3
2-
Step 3. Form sigma bonds
O O
O
S

18. 04/09/2015 Bonding and Structure 18
Remaining pairs become lone pairs,
first on outside atoms
then on central atom.
Sulfite ion, SO3
2-
(2)
Each atom is surrounded by an octet of electrons.
•
• O O
O
S
••
••
•• ••
••
••
•
•
•
•
•
•
NOTE - must add formal charges (O-
, S+
) for complete dot diagram

19. 04/09/2015 Bonding and Structure 19
Carbon Dioxide, CO2
1. Central atom = __C____
2. Valence electrons = _16_ or _8_ pairs
3. Form sigma bonds.
O OC
•
• O OC•• ••
••••
•
•
This leaves __6__ pairs.
4. Place lone pairs on outer atoms.

20. 04/09/2015 Bonding and Structure 20
•
• O OC•• ••
••••
•
•
Carbon Dioxide, CO2 (2)
4. Place lone pairs on outer atoms.
•
• O OC•• ••
••••
•
•
•
• O OC•• ••
•
•
The second bonding pair forms a pi (π) bond.
5. To give C an octet, form DOUBLE BONDS
between C and O.

21. 04/09/2015 Bonding and Structure 21
SO3
H2CODouble and even
triple bonds are
commonly
observed for C,
N, P, O, and S
•
• O OC•• ••
•
•
C2F4

22. 04/09/2015 Bonding and Structure 22
Sulfur Dioxide, SO2
1. Central atom = S
2. Valence electrons = 6 + 2*6 = 18 electrons
or 9 pairs•
• O OS••
••
••
••••
•
•
•
• O OS••
••
••
••••
•
•
bring in
left pair
OR bring in
right pair
3. Form pi (π) bond so that S has an octet
— note that there are two ways of doing this.

23. 04/09/2015 Bonding and Structure 23
Sulfur Dioxide, SO2
•
• O OS••
••
••
••••
•
•
bring in
left pair
OR bring in
right pair
•
• O OS••
••
••
••
•
•
•
• O OS••
••
••
••
•
•
Equivalent structures
called:
RESONANCE
STRUCTURES
The proper Lewis structure
is a HYBRID of the two.
A BETTER representation of SO2
is made by forming 2 double bonds
O = S = O
Each atom has
- OCTET
- formal charge = 0

24. Bonding and Structure 24
Urea (NHUrea (NH22))22COCO
1. Number of valence electrons = 24 e-
2. Draw sigma bonds.
CN N H
HH
H
O
4. Place remaining electron pairs on oxygen
3. Complete C atom octet with double bond.
CN N H
HH
H
O
Leaves 24 - 14 = 10 e- pairs.
and nitrogen atoms.

25. 04/09/2015 Bonding and Structure 25
Violations of the Octet Rule
Usually occurs with:
Boron
BF3
SF4
elements of higher periods.

26. 04/09/2015 Bonding and Structure 26
Boron Trifluoride
• Central atom = B
• Valence electrons = 3 + 3*7 = 24
or electron pairs = 12
• Assemble dot structure
F••
•
•
••
F
F
B
••
••
•
•
•
•
•
•
•
•
The B atom has a share
in only 6 electrons (or 3
pairs). B atom in many
molecules is electron
deficient.

27. 04/09/2015 Bonding and Structure 27
Sulfur Tetrafluoride, SF4
• Central atom = S
• Valence electrons = 6 + 4*7 = 34 e-
or 17 pairs.
• Form sigma bonds and distribute
electron pairs.
F
••
•
•
••
F
F
S
••
••
•
•
•
•
•
• F••
••
••
••
••
5 pairs around the S5 pairs around the S
atom. A commonatom. A common
occurrence outside theoccurrence outside the
2nd period.2nd period.

28. 04/09/2015 Bonding and Structure 28
Formal charge = Group no.
- 1/2 (no. bond electrons)
- (no. of LP electrons)
Formal Atom Charges
• Atoms in molecules often bear a charge (+ or -).
• The most important dominant resonance structure
of a molecule is the one with formal charges
as close to 0 as possible.

29. 04/09/2015 Bonding and Structure 29
04 - (1/ 2)(8) - 0 =
6 - (1/ 2)(4) - 4 = 0
Carbon Dioxide, CO2
At OXYGEN
O C O
•
•
• •
•
•
• •
At CARBON

30. 04/09/2015 Bonding and Structure 30
C atom
charge is 0.
6 - (1/ 2)(6) - 2 = +1
6 - (1/ 2)(2) - 6 = -1
Carbon Dioxide, CO2 (2)
O C O
•
•
• •
•
•
• •
An alternate Lewis structure is:
AND the corresponding
resonance form
+
O C O
•
•
• •
•
•
• •
+

31. 04/09/2015 Bonding and Structure 31
• REALITY: Partial charges calculated
by CAChe molecular modeling
system (on CD-ROM).
+1.46
-0.73 -0.73
Carbon Dioxide, CO2 (3)
Which is the predominant resonance structure?
O C O
•
•
• •
•
•
• •
OR
O C O
•
•
• •
•
•
• •
O C O
•
•
• •
•
•
• •
+
+
Answer ?
Form without formal charges is
BETTER - no +ve charge on O

32. 04/09/2015 Bonding and Structure 32
Boron Trifluoride, BF3
F••
•
•
••
F
F
B
••
••
•
•
•
•
•
•
•
•
What if we form a B—F double
bond to satisfy the B atom octet?

33. 04/09/2015 Bonding and Structure 33
Boron Trifluoride, BF3 (2)
• To have +1 charge on F, with
its very high electron affinity is
not good. -ve charges best
placed on atoms with high EA.
• Similarly -1 charge on B is bad
• NOT important Lewis structure
fc = 7 - 2 - 4 = +1 Fluorine
F••
••
F
F
B
••
••
•
•
•
•
•
•
•
•
fc = 3 - 4 - 0 = -1 Boron
+

34. 04/09/2015 Bonding and Structure 34
A. S=C=N
Thiocyanate ion, (SCN)
-
Which of three possible resonance structures
is most important?
-0.52 -0.32-0.16
Calculated partial charges
ANSWER:
C > A > B
C. S-C N
B. S=C - N

35. Thank yo u
allfo r watching .
Subm issio n to : Ms
Navde e p Kaur
04/09/2015 Bonding and Structure 35