1. Chemical Bonding What holds things together?
CHM 101
Sinex
Rayat Shikshan sanstha’s
S.M. Joshi College, Hadapsar, Pune.
Chemistry Department
Chemical Bonding What holds things together?
F.Y. B.Sc.
Dr. R. K. Jadhav
Dept. of Chemistry,
S. M. Joshi College, Hadapsar, Pune.

2. Let’s examine the melting point of compounds across two periods.
What is the trend?
Conductivity - high
Conductivity - low
Chlorides of Period 2
compound
LiCl
BeCl2
BCl3
CCl4
NCl3
OCl2
Cl2
melting point
610
415
-107
-23
-40
-121
-102
Chlorides of Period 3
NaCl
MgCl2
AlCl3
SiCl4
PCl3
SCl6
801
714
193
-69
-112
-51
high
low

3. Bonding Can we explain the melting point behavior across a period?
involves the valence electrons or outermost shell (or highest shell) electrons
for group A elements - the group number tells how many valence electrons
How many valence electrons on N?
Group 5A – 5 valence electrons

4. Bonding…
Lewis dot structures show the valence electrons around at atom and for most molecules and compounds a complete octet for the elements
most monatomic ions have an electron configuration of noble gases N Al F F
1s22s22p5
+ e- 
1s22s22p6 Ne

5. Write out the electron configuration for the following atoms and ions:Ne Ar Kr H-
Ca+2
O-2
Br-
Which noble gas is isoelectronic with each ion?

6. NaCl - + Na Cl Cl2 Cl Cl This is the formation of an ionic bond.
electron transfer
and the formation of ions
Cl2
This is the formation of a covalent bond. Cl Cl
sharing of a pair of electrons
and the formation of molecules

7. What about the distance between the atoms in a bond?
NaCl Na+ Cl- d = 281 pm
Cl2 Cl-Cl d = 199 pm
What property can be used to tell when a bond will ionic or covalent?

8. Draw the Lewis dot structures for the following compounds:
ionic
MgO
CaCl2
Na2S
covalent
HCl
H2O
CH4

9. Some exceptions to the Octet Rule
BF3
PCl5
SF6

10. Electronegativity Chlorides of Period 2 DEN Chlorides of Period 3
The electronegativity difference - DEN = ENhigher – EN lower
Chlorides of Period 2
compound
LiCl
BeCl2
BCl3
CCl4
NCl3
OCl2
Cl2
DEN
2.2
1.6
1.1
0.6
Chlorides of Period 3
Compound
NaCl
MgCl2
AlCl3
SiCl4
PCl3
SCl6
1.9
1.3
1.0
large difference
small difference

11. Using electronegativities to determine bond type
DEN > ionic bond - transfer
DEN < covalent bond - sharing
So we have a range of electronegativity difference of 0 to 1.7 for sharing an electron pair.

12. Is the sharing of electrons in molecules always equal?
non-polar bond X Y
DEN = 0
Which element is more electronegative? X Y
DEN = 0.3
increasing polarity of bond X Y
DEN = 0.6
ENY > ENX X Y
DEN = 0.9
polar bond
0 < EN < 1.7 X Y
DEN = 1.2
Direction of electron migration

13. BF3 – a planar molecule Space-filled F 4.0 B 2.0 Ball & stick negative
top
side
positive
Electrostatic potential maps
Spartan ‘02

14. O2 N2 More sharing examples O O O N N N N
Share until octet is complete. O O O
double bond (2 pairs) N2
octet complete N N N N
triple bond (3 pairs)

15. Bond Energy F2 single bond BE = 142 kJ/mole O2 double bond BE = 494
Is breaking a bond an endothermic or exothermic process?
X energy  X X
F2 single bond BE = 142 kJ/mole
O2 double bond BE = 494
N2 triple bond BE = 942
increasing bond strength

16. NH3 NH4+ Some more sharing examples H N H H H+ H N H H NH3 + H+  NH4+
normal covalent bond
(each atom supplies an electron)
NH3 H N H H
NH4+
NH H+  NH4+ H+
coordinate covalent bond
(the pair of electrons from the same atom) H N H H

17. Type of bond? – I, PC, or NC
TiO2
CH4
NaI
CS2
CO2
KCl
AlCl3
CsF
HBr

18. Using the EN trends to predict bond type
Increasing EN
Increasing EN
105 Db
107 Bh NO
RbF
FeS
H2S

19. Draw the Lewis dot structures
CO2
NH2-
H3O+ CO
HCN
H2CO
(C in center)

20. Show the direction of electron migration ( ) in the following.
C – H
H – F
C = O
C – Cl
Rank the bond polarity (1-most … 3-least)
As-H N-H P-H

21. Here is the electrostatic potential map for H2CO.
Show the electron migration on this planar molecule. H
C O H
blue – positive red - negative
How is this molecule different than BF3?

22. Comparison of Bonding Types
ionic
covalent
ions
molecules
molten salts conductive
non-
conductive
valence electrons
transfer of electrons
sharing of electrons
high mp
low mp
DEN > 1.7
DEN < 1.7

23. Bonding spectrum 100% covalent 100% ionic A B A B A+ B- Increasing DEN
Increasing polarity
Transfer