Post on 04-Jan-2016
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
Methane CH4
Tetrachloromethane, CCl4
C
Hx
x
x x Cx xx
x
H
H
H
HH
H
H
C
Cl
Cx xx
x
x
x
x x
Cl
ClCl Cl
ClCl
Cl
Molecules
Silane SiH4
Silicon tetrachloride, SiCl4
Si
Hx
x
x x Six xx
x
H
H
H
HH
H
H
Si
Cl
Six xx
x
x
x
x x
Cl
ClCl Cl
ClCl
Cl
Molecules
Hydrogen H2
Chlorine, Cl2
Fluorine, F2
+xxH H H H
Bond pair
+xx
Cl Cl Cl Cl
x x
x x
xx
x x
x x
xx
Single bond
+xx
F F F F
x x
x x
xx
x x
x x
xx
Molecules
Oxygen, O2
Nitrogen N2
Hydrogen chloride, HCl
+x xO O O Ox
xx
xx
x
x x
xx
Double bond
+x
N N N Nxxx x
xx
x
xx
Triple bond
+xH Cl H Cl
x x
x x
x xx
x x
x x
xx
Molecules
Ammonia NH3
Phosphorus trichloride, PCl3
H N HN
H
Hx
x
x
Hx
Lone pair
H
PxClxx
xx
xx
x
Cl xx
xx
x x
x
Cl
xx
xx
xx
PxClxx
xx
xx
x
Cl xx
xx
x x
x
Cl
xx
xx
xx
Molecules
Water, H2O
Carbon dioxide CO2
H
O HO
H
Hx
x x
x x
x
x x
x x
x
Lone pair
x
xx
xx
xx xxO C O O C O
Molecular formulae
Step Water 1 Write the electronic arrangement
of constituent elements in the molecule.
H 1
O 2,6
2 Write the number of electrons required to obtain octet structure.
1 H
2 O
3 Interchange the numbers. 1 H
= H2
2 O
=O1
4 Simple the ratio. H2O（omit the “1”）
Molecular formulae
Step Tetrachlromethane 1 Write the electronic
arrangement of constituent elements in the molecule.
C 2,4
Cl 2,8,7
2 Write the number of electrons required to obtain octet structure.
4 C
1 Cl
3 Interchange the numbers. 4 C
= C
1 Cl
=Cl4
4 Simple the ratio. 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.
Cx
x
x x Cx
x
x xCx
x
x x
Cx
x
x x
Cx
x
x x
Cx
x
x xCx
x
x xCx
x
x x
Cx
x
x x
Cx
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 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.
silicon
oxygen
Giant covalent structure Diamond and quartz are both giant covalent
compounds. Their melting points (diamond: ~3500C) 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.