CHM 101
Sinex
Let’s examine the melting point of compounds
across two periods.
What is the trend?
Chemical Bonding
What holds things together?
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
compound
melting point
PGCC CHM 101 Sinex
NaCl MgCl2 AlCl3 SiCl4
PCl3
SCl6
Cl2
801
-112
-51
-102
714
193
-69
high
low
CRC Handbook of Chemistry and Physics, 1995
Bonding
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
Write out the electron configuration for
the following atoms and ions:
He
H-
Ne
Ca +2
Ar
O-2
Kr
Br-
Which noble gas is isoelectronic with each ion?
• Lewis dot structures show the valence
electrons around at atom and for most
molecules and compounds a complete octet
for the elements
N
Al
• most monatomic ions have an electron
configuration of noble gases
1s2 2s2 2p 5
NaCl
F
+ e- à
F
1s2 2s2 2p 6
Ne
This is the formation of an ionic bond.
Na
+
Cl
-
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
1
CHM 101
Sinex
What about the distance
between the atoms in a bond?
NaCl
Na + Cl-
d = 281 pm
Cl2
Cl-Cl
d = 199 pm
Draw the Lewis dot structures for the
following compounds:
ionic
MgO
covalent
HCl
CaCl2
Na2S
H2O
CH4
What property can be used to tell
when a bond will ionic or covalent?
Electronegativity
Some exceptions to the Octet Rule
The electronegativity difference - ∆EN = ENhigher – EN
BF3
lower
Chlorides of Period 2
PCl5
compound
LiCl
BeCl2
BCl3
CCl4
NCl3
OCl2
Cl2
∆EN
2.2
1.6
1.1
0.6
0
0.6
0
PCl3
SCl6
Cl2
1.0
0.6
0
Chlorides of Period 3
Compound
2.2
∆EN
1.9
1.6
large difference
Using electronegativities
to determine bond type
∆EN > 1.7 ionic bond - transfer
∆EN < 1.7 covalent bond - sharing
So we have a range of electronegativity difference of
0 to 1.7 for sharing an electron pair.
1.3
small difference
Is the sharing of electrons
in molecules always equal?
non-polar
bond
X
Y
∆EN = 0
X
Y
∆EN = 0.3
ENY > ENX
X
Y
polar bond
X
Y
X
Y
Which element
is more
electronegative?
0 < EN < 1.7
increasing polarity of bond
SF 6
NaCl MgCl2 AlCl3 SiCl4
∆EN = 0.6
∆EN = 0.9
∆EN = 1.2
Direction of electron migration
2
CHM 101
Sinex
BF3 – a planar molecule
B
Ball & stick
F
Space-filled
More sharing examples
O2
4.0
2.0
Share until octet is complete.
O
O
OO
negative
O O
double bond (2 pairs)
N2
top
side
positive
Electrostatic potential maps
N
N
NN
N N
N
N
triple bond (3 pairs)
Spartan ‘02
Some more sharing examples
Bond Energy
Is breaking a bond an endothermic or exothermic process?
NH3
H N H
H
X + X
F2
single bond
BE = 142 kJ/mole
O2
double bond
BE = 494
N2
triple bond
BE = 942
increasing bond strength
X2 + energy à
octet complete
NH4+
normal covalent bond
(each atom supplies
an electron)
NH3 + H+ à N H4 +
H+
H N H
H
coordinate covalent bond
(the pair of electrons
from the same atom)
http://wulfenite.fandm.edu/Data%20/Table_6.html
TiO2
CH4
NaI
CS 2
CO2
KCl
AlCl3
CsF
HBr
Using the EN trends to predict bond type
Increasing EN
Increasing EN
Type of bond? – I, PC, or NC
105
Db
NO
107
Bh
RbF
FeS
H2 S
Modified fromhttp://www.cem.msu.edu/~djm/cem384/ptable.html
3
CHM 101
Sinex
Show the direction of electron
migration (
) in the following.
Draw the Lewis dot structures
CO2
NH2-
C–H
H–F
H3O+
C=O
CO
C – Cl
Rank the bond polarity (1-most … 3-least)
HCN
H2CO
As-H
N-H
P-H
(C in center)
Here is the electrostatic potential map for H2CO.
Comparison of Bonding Types
ionic
Show the electron
migration on this
planar molecule.
covalent
ions
molten salts
conductive
H
C
transfer of
electrons
O
H
blue – positive
valence
electrons
high mp
∆EN > 1.7
red - negative
molecules
nonconductive
sharing of
electrons
low mp
∆EN < 1.7
How is this molecule different than BF3 ?
100% covalent
A
Bonding spectrum
B
A
100% ionic
A+
B
B-
Increasing ∆EN
Increasing polarity
Transfer
4