In this part you will learn:
IGCSE COURSEBOOK p51-56
3.2 Why do atoms form bonds
3.3 the covalent bond
3.4 covalent compound
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
The reaction between sodium and chlorine
Page 3
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 4
The reaction between sodium and chlorine
Sodium and chlorine are both elements.
When sodium is heated and placed in a jar
of chlorine, it burns with a bright flame.
The result is a white solid that has to be
scraped from the sides of the jar. It looks
completely different from the sodium and
chlorine.
The word equation for the reaction is:
sodium + chlorine  sodium chloride
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
The reaction between magnisium and oxygen
The word equation for the reaction is:
magnisium + oxygen  magnisium oxide
Page 5
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 6
Why do atoms form bonds?
 In another hand……
▲ Neon: the unreactive gas
used in light tubes for advertising.
▲ Welding is often carried out in an
atmosphere of argon, which will not react
with hot metals (unlike oxygen).
 This is because the atoms of noble gas(group 0)have a very stable
arrangement of electrons in the outer shell. This makes the noble gases
unreactive.
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 7
Why do atoms form bonds?
electron shells
period
1
period 2-7
helium atom: full
outer shell of 2
electrons – stable
2
stable outer shell of electrons
1
2
2---->7
8
neon atom: full
outer shell of 8
electrons – stable
2+8
argon atom: outer
shell of 8 electrons
– stable
2+8+8
 And that gives us the answer to our question:
Atoms bond with each other in order to gain a stable arrangement of
outer-shell electrons.
 In other words, they bond in order to gain 8 electrons in their outer shell
(or 2, if they have only one shell).
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 8
How sodium atoms gain a stable outer shell
1. lose electron
Na - e- ------> Na+ positive ion/ cation
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 9
How chlorine atoms gain a stable outer shell
2. gain elcetron
Cl + e- -----> Cl-
negative ion/anion
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 10
Chemical bonding: The strong forces that hold
atoms(or ions) together.
①covalent bonding
bonding.
②ionic bonding
③metallic
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 11
Practice
1. Draw a diagram to show how these atoms gains
a stable outer shell of 8 electrons: [Group Work]
a a calcium atom
b a fluorine atom
c an aluminium atom
d a sulfur atom
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 12
2.Draw a diagram to show how magnesium reacts
with oxygen[individual Work]
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 13
3.Sharing electrons （for gaining a stable outer shells)
▲ Atoms of nonmetals do not give
up electrons to gain
a full shell, because
they would have to
lose so many. It
would take too much
energy to overcome
the pull of the
positive nucleus.
 When two non-metal atoms react together, both need to gain
electrons to achieve stable outer shells. They manage this by sharing
electrons.
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Hydrogen
 A hydrogen atom has only one shell, with one
electron. The shell can hold two electrons. When
two hydrogen atoms get close enough, their shells
overlap and then they can share electrons. Like
this:
Page 14
▲ A model of the
hydrogen molecule.
The molecule can also
be shown as H–H. The
line represents a single
bond.
So each has gained a full shell of two electrons, like helium atoms.
The bond between the atoms
 Each hydrogen atom has a positive nucleus. Both nuclei attract the
shared electrons – and this strong force of attraction holds the two
atoms together. This force of attraction is called a covalent bond.
 A single covalent bond is formed when atoms share two electrons.
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Molecules
 The two bonded hydrogen atoms above form a
molecule.
 A molecule is a group of atoms held together
by covalent bonds.
 Since it is made up of molecules, hydrogen is a
molecular element. Its formula is H2. The 2 tells
you there are 2 hydrogen atoms in each
molecule.
 Many other non-metals are also molecular. For
example:
iodine, I2
oxygen, O2 nitrogen, N2
chlorine, Cl2 sulfur, S8
phosphorus, P4
 Elements made up of molecules containing two
atoms are called diatomic. So iodine and oxygen
are diatomic. Can you give two other examples?
Page 15
▲ A model of the
hydrogen molecule.
The molecule can also
be shown as H–H. The
line represents a single
bond.
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 16
Chlorine
 A chlorine atom needs a share in one more electron, to obtain a
stable outer shell of eight electrons. So two chlorine atoms bond
covalently like this:
 Since only one pair of electrons is shared,
the bond between the atoms is called a
single covalent bond, or just a single bond.
You can show it in a short way by a single
line, like this: Cl-Cl.
▲ A model of the chlorine molecule.
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 17
Oxygen
 An oxygen atom has six outer electrons, so needs a share in two more.
So two oxygen atoms share two electrons each, giving molecules with
the formula O2. Each atom now has a stable outer shell of eight
electrons:

 Since the oxygen atoms share two pairs of
electrons, the bond between them is called a
double bond. You can show it like this: O==O.
▲ A model of the
oxygen molecule.
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page 18
Nitrogen
 A nitrogen atom has five outer electrons, so needs a share in three
more. So two nitrogen atoms share three electrons each, giving
molecules with the formula N2. Each atom now has a stable outer shell
of eight electrons:
 Since the nitrogen atoms share three
pairs of electrons, the bond between them
is called a triple bond. You can show it like
this: N
N.
▲ A model of the
nitrogen molecule.
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Covalent Compounds
 In a molecular compound, atoms of different elements
share electrons.
 The compounds are called covalent compounds.
Here are three examples.
Covalent Compound
Description
a molecule of hydrogen
chloride
The chlorine atom shares
one electron with the
hydrogen atom.
Both now have a stable
arrangement of electrons in
their outer shells: 2 for
hydrogen (like the helium
atom) and 8 for chlorine (like
the other noble gas
atoms).
Page 19
Most are
molecular …
Most non-metal
elements and
their compounds
exist as
molecules.
Model of the Molecule
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Covalent Compound
Description
The oxygen atom shares
electrons with the two
hydrogen atoms.
All now have a stable
arrangement of electrons in
their outer shells: 2 for
hydrogen and 8 for oxygen.
a molecule of water
The carbon atom shares
electrons with four hydrogen
atoms.
All now have a stable
arrangement of electrons in
their outer shells: 2 for
hydrogen and 8 for carbon.
molecule of methane
Page 20
Model of the Molecule
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Page21
More examples of covalent compounds
 This table shows three more examples of covalent
compounds. Each time:
 the atoms share electrons, to gain stable outer shells
 repulsion between pairs of electrons dictates the shape
of the molecule.
Covalent Compound
a molecule of ammonia
Description
Each nitrogen atom shares
electrons with three
hydrogen atoms.
So all four atoms now have
a stable arrangement of
electrons in their outer
shells: 2 for hydrogen and 8
for nitrogen.
The molecule is shaped like
a pyramid.
Model of the Molecule
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Covalent Compound
Description
The carbon atom shares
electrons with three hydrogen
atoms and one oxygen atom.
Look at the shape of the
molecule: a little like methane,
but changed by the presence of
the oxygen atom.
a molecule of methanol
a molecule of carbon
dioxide
The carbon atom shares all four
of its electrons: two with each
oxygen atom. So all three
atoms gain stable shells.
The two sets of bonding
electrons repel each other. They
move as far apart as they can,
giving a linear molecule.
All the bonds are double bonds,
so we can show the molecule
like this: O = C = O.
Page 22
Model of the Molecule
4.1 – Compounds
4.2 – Atoms
& Mixtures
Forming Bonds
4.3 – Ionic
4.4 –
4.5 – Covalent 4.6 – Covalent 4.7 – Tonic 4.8 – Covalent 4.9 – Metals &
Bond
More Ions
Bond
Compounds vs. Covalent
Structures
Non-Metals
Covalent Compound
a molecule of ethane
Description
Model of the Molecule
Look how each carbon atom
shares its four electrons this
time.
It shares two with two
hydrogen atoms. and two
with another carbon atom,
giving a carbon-carbon
double bond.
So the molecule is usually
drawn like this:
H
H
C == C
H
Page 23
H
practice: P56
C
practice: P57
C
Assignment：
Use diagram to show the covalent bonding of Cl2、O2、N2、H2O、CO2、
C2H4、NH3、CH4、C2H5OH
Suggested extension work:
Making diagram models to show the formation of different molecules
Extension activity: Stretch and prepare for A Level
For a deeper understanding of covalent bonding:
https://pbslmcontrib.s3.amazonaws.com/WGBH/arct15/SimBucket/Simulations/chemthinkcovalentbonding/content/index.html
Another extension activity: Learners who are interested in the 3-D shapes of
the molecules could try building their own molecules using this desktop app:
http://molview.org/
Learners draw the structures in 2-D and they are converted to rotatable 3-D
models.