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.