Covalent bonding
Group IV atoms



Group IV atoms do not lose or gain electrons
in order to obtain the octet structure.
They prefer to share electrons with other
atoms to form molecules.
The electrostatic attraction force between
shared electrons and positive nuclei is called
covalent bond.
Molecules
H
Methane CH4
H
x
H
x
C
x
Hx C
H

x
x
x
H
H
H

x
Tetrachloromethane, CCl4
Cl
Cl
x
Cl
x
C
x
Cl
x
Cl
Cl
x
x
x
Cl
C
x
Cl
Molecules
H
Silane SiH4
H
x
H
x
Si
x
H x Si x H
H

x
x
x
H
H

Silicon tetrachloride, SiCl4
Cl
Cl
x
Cl
x
Si
x
Cl
x
Cl
Cl
x
x
x
Cl
Si
x
Cl
Molecules

Bond pair
Hydrogen H2
H
x +
H
H
xx

Chlorine, Cl2
x
x
Cl
x
H
xx
x +
Cl
x
x
Cl
x
Cl
xx
xx
Single bond
xx

Fluorine, F2
x
x
F
xx
xx
x +
F
x
x
F
xx
x
F
Molecules
xx

Oxygen, O2
x
x
O
xx
x +
x
O
x
x
O
xx
x
x
x
N
O
Double bond
xx


Nitrogen N2
x
x
N
xx
Hydrogen chloride, HCl
H
+ x
N
N
x +
Triple bond
xx
Cl
xx
xx
x
x
H
x
Cl
xx
x
x
Molecules

Lone pair
Ammonia NH3
H
N
x
x
H
H
N
H
x
H
x
H
x
x
x
Cl
x
xx
P
Cl
xx
Cl
x
x
x
x
xx
xx
xx
x
xx
Cl
Cl
xx
xx
x
P
x
x
x
Cl
xx
x
x
xx
x
xx
x
x
Phosphorus trichloride, PCl3
xx

Molecules
Lone pair

Water, H2O
H
xx
x
H

O
xx
H
x
x
xx
O
xx
Carbon dioxide CO2
O
x
x
C
x
x
O
O
x
x
C
x
x
O
x
H
Molecular formulae
Step
1 Write the electronic arrangement
of constituent elements in the
molecule.
2 Write the number of electrons
required to obtain octet structure.
3 Interchange the numbers.
4 Simple the ratio.
Water
H
1
O
2,6
1
H
2
O
1
H
= H2
2
O
=O1
H2O（omit the “1”）
Molecular formulae
Step
1 Write the electronic
arrangement of constituent
elements in the molecule.
2 Write the number of electrons
required to obtain octet
structure.
3 Interchange the numbers.
4 Simple the ratio.
Tetrachlromethane
C
Cl
2,4
2,8,7
4
C
1
Cl
4
C
=C
1
Cl
=Cl4
CCl4
Molecules



Molecules are formed from non-metal
elements.
There are strong covalent bonds within
molecules.
However, only weak van der Waals’ forces
between molecules.
Simple molecular structure

Water, ammonia, dry ice (solid carbon dioxide)
o=c=o
Weak van der Waals’ forces
Simple molecular structure

Iodine
Iodine molecule
Properties of simple molecular
compounds






They are usually liquids (water) and gases (oxygen).
Only a few is solids (iodine and dry ice).
The solids are relatively soft.
Low melting points and boiling points.
Usually they are insoluble in water, but soluble in
1,1,1-trichloroethane.
They are electrical non-conductors (without mobile
ions).
Reasons

They have simple molecular structures with
weak van der Waals’ forces between
molecules.
Carbon atom



Carbon atom do not lose or gain electrons in
order to obtain the octet structure.
It prefers to share electrons with other atoms
to form molecules.
How about in pure carbon? How do carbon
atoms combine together?
Carbon atom

Each carbon atom has 4 outermost shell
electrons, they tend to gain 4 extra electrons
to obtain octet structure. As a result, each
carbon atom forms covalent bonds with 4
other carbon atoms.
x
x
x
x
C
xx
x
C
x
x
x
C
C
x x
x
x
x
x
x
x
C
x
x
x
x
x
x
Cx
x
x
x
C C C
x
xx
x
xx
x
x
x
C
x
x
x
extend infinitely
Diamond



Diamond has a giant
covalent structure with
of a network of strong
covalent bonds.
Each carbon atom is
covalently bonded to 4
other carbon atoms.
Similar for silicon.
Carbon atoms
Covalent bonds
Graphite

Graphite is another form of carbon. It has
very high melting point and boiling point.
It conducts electricity and so it is used as
electrode.
Quartz, SiO2




silicon
oxygen
Silicon and carbon are group IV elements, so
they are similar in structure.
When silicon combine with oxygen, each
silicon atom is covalently bonded to 4
oxygen atoms.
While each oxygen atom is covalently
bonded to 2 silicon atoms.
Therefore, the formula is SiO2.
Giant covalent structure





Diamond and quartz are both giant covalent
compounds.
Their melting points (diamond: ~3500C)
and boiling points are very high.
They are very hard and insoluble in water.
They are electrical insulator (except graphite).
They have giant covalent structure with a
network of strong covalent bonds between
atoms.