Chapter 6
Chemical Bonding
Part 2: Covalent Bonding
Why do Covalent bonds
form?
When non-metals react with one another,
their atoms share valence electrons.
The bond formed between atoms that
share electrons is known as covalent
bonds
After bonding, each atom attains the
electronic configuration of a noble gas.
What are molecules?
When atoms combined by sharing
electrons, a molecule is formed.
A molecule is a group of 2 or more atoms held
together by covalent bonds.
Molecules of Elements
Many non-metallic Elements exist as
molecules made up of two or more identical
atoms.
Let’s look at how electrons are shared and
arranged within molecules of some elements:
hydrogen, chlorine and oxygen
Hydrogen
H
H
x
H
x
H
A hydrogen atom has 1 valence electron
Two hydrogen atoms share a pair of electrons so that
each atom has 2 electrons in its valence shell.
As a result, both have a stable duplet configuration
Hydrogen
H
H
x
H
x
H
The sharing of 2 electrons form a single covalent
bond.
‘Dot and Cross
Diagram’
HxH
Structural Formula Molecular Formula
H--H
H2
Chlorine
xx
Cl
x
x
Cl
xx
xx
x
Cl
x
Cl
x
x
xx
A chlorine atom has 7 valence electron
To attain an octet structure, 2 chlorine atoms combine
to share a pair of electrons
Like hydrogen, chlorine has a single covalent bond
Chlorine
xx
Cl
x
x
Cl
xx
x
Cl
xx
‘Dot and Cross
Diagram’
x
Cl
x
x
x
Structural Formula Molecular Formula
Cl--Cl
Cl2
Oxygen
xx
O
O
xx
xx
x
x
O
x
x
O
xx
An oxygen atom has 6 valence electrons
It needs 2 more electrons to form a stable octet
structure with Another oxygen atom
4 electrons are shared between 2 oxygen atoms
Such a bond is called a double covalent bond
Oxygen
xx
O
O
xx
x
x
O
xx
‘Dot and Cross
Diagram’
x xx
Ox O
xx
x
x
O
xx
Structural Formula Molecular Formula
Cl = Cl
O2
Molecules of Compounds
When 2 or more different types of atoms form
covalent bonds, a molecular compounds
or covalent compound is formed.
Water, methane and carbon dioxide are
examples of covalent compounds
Water
A water molecule is formed by the reaction of 2
hydrogen atoms with one oxygen atoms.
All 3 atoms in the molecule attain noble gas
configuration.
‘Dot and Cross
Diagram’
x
x
xx
Ox x H
H A
Structural Formula Molecular Formula
O–H
O A
H2 O
Each water molecule contains 2 single
covalent bonds.
Methane
Contains carbon and hydrogen
‘Dot and Cross
Diagram’
A
Structural Formula Molecular Formula
CH4
In a methane molecule, the carbon atom has an
octet configuration while each hydrogen atom
has a duplet configuration
Methane has four single covalent bonds.
Carbon Dioxide
‘Dot and Cross
Diagram’
A
Structural Formula Molecular Formula
CO2
Each carbon atoms shares 2 electrons with a
carbon atom.
Consequently, a carbon dioxide molecule
contains two double covalent bonds
Physical Properties of
Covalent Substances
• Volatility
• Solubility
• Electrical Conductivity
Volatility – melting and boiling
points
• Many simple covalent substances that are
made up of small molecules are liquids or
gases at room temperature
• They have low melting points and low boiling
points.
• Those that are liquids are volatile (i.e.: they
evaporate easily)
Volatility – melting and boiling
points
H
H
H
H
H
H
This occurs as the forces
between molecules are
weak compared to the
covalent bonds within the
molecules.
Very little heat energy is
needed to overcome the
intermolecular forces
(Aka: Van Der Waals
Forces)
Solubility
• Most covalent molecules are insoluble in water
and soluble in organic solutions.
• Exceptions: chlorine, hydrogen chloride,
alcohol and sugar are covalent substances
that are soluble in water.
Solubility
• Most covalent molecules are insoluble in water
and soluble in organic solutions.
• Exceptions: chlorine, hydrogen chloride,
alcohol and sugar are covalent substances
that are soluble in water.
Electrical Conductivity
• Most covalent elements or compounds do
not conduct electricity whether in solid, liquid
or gaseous state.
• As they do not have free-moving ions or
electrons to conduct electricity
• Exceptions:
– Carbon (in form of graphite)
– Hydrogen Chloride, Sulfur Dioxide, Ammonia (react
with water)
Writing Formulae of Covalent
Compounds
• The name of a covalent compounds tells us
– The elements it contains
– The number of each type of atom of a molecule
contains (shown by prefixes)
Prefix
Number of
Mono
1
Di
2
Tri
3
Tetra
4
Penta
5
What is the chemical formula of
carbon dioxide?
• Though process:
– We assume the first element named (in this case,
carbon) contains only one atom unless otherwise
stated.
– The word ‘dioxide’ means ‘2 oxygen atoms’
– Answer: CO2
What is the chemical formula of
dinitrogen monoxide?
‘Dinitrogen’
means ‘2 nitrogen
atoms’
N O
2
‘Monoxide’ means ‘1
oxide’
Notice the subscript ‘1’ is not indicated on the formula.
What is the chemical formula of
dinitrogen tetroxide?
‘Dinitrogen’
means ‘2 nitrogen
atoms’
‘Tetroxide’ means
‘4 oxygen atoms’
N O
2
You should learn the chemical formulae of some common covalent
substances such as ammonia, hydrogen peroxide, methane, ozone,
water.