Chem 'o' level slides

1. Chemistry Matters –
Chapter 7
Covalent Bonding and
Metallic Bonding
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2. 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 called a covalent bond.
After bonding, each atom attains the electronic
configuration of a noble gas.
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3. What are Molecules?
When atoms combine by sharing electrons,
molecules are formed.
A molecule is a group of two or more
non-metal atoms held together by
covalent bonds.
Covalent bonds, like ionic bonds, are
STRONG bonds.
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4. Molecules of Elements
Many non-metallic elements exist as
molecules made up of two or more identical
atoms which bond by sharing electrons in
order to become stable.
Examples:
Hydrogen (H2), chlorine (Cl2), oxygen (O2),
nitrogen (N2), fluorine (F2), bromine (Br2).
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5. Example 1: Two hydrogen atoms share
a pair of electrons so that
Hydrogen (H2) each atom has two
electrons in its valence
shell. As a result, both
A hydrogen atom has atoms have a stable duplet
one valence electron. configuration.
H (1) H (1) H – H or H2
Single covalent bond
consisting of 2 shared 5
electrons.

6. Hydrogen
The sharing of two electrons forms a single
covalent bond.
A single covalent bond or a single bond is usually
represented by a single line ‘–’ in the structural formula.
A structural formula shows A molecular formula only
how the atoms are arranged shows the number of each
and the bond between them. kind of atom in a molecule.
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7. Example 2:
Chlorine
A chlorine atom has seven
valence electrons.
To attain an octet structure, two chlorine
atoms combine to share a pair of electrons
(two electrons).
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8. Chlorine
Like hydrogen, chlorine has a single covalent bond.
Q. How many electrons are shared in a single
covalent bond? Two
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9. Oxygen
An oxygen atom has It needs two more electrons to
six valence electrons. form a stable octet structure.
Each oxygen atom shares two of its
electrons with another oxygen atom.
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10. Oxygen
4 electrons are shared between two oxygen atoms.
Such a bond is called a double covalent bond.
O=O
A double bond is represented by ‘==’ in a structural
formula.
Q. How many electrons are shared in a double covalent
bond? 10
Four electrons (two pairs of electrons)

11. Let’s Practice!
Q. What can you deduce about the bonding
in this particle?
• It is a molecule in which the atoms are
bonded by a single covalent bond.
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12. Let’s Practice!
Q. The valency of an atom is the number
of electrons from each atom that is shared.
What is the valency of each atom here?
One.
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13. Let’s Practice!
Q. How many electrons are shared the a single
covalent bond?
Two.
Examples of this kind of molecule – Cl2,
F2, Br2, I2.
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14. Let’s Practice!
Q. Give the names and molecular formulae of
molecules of the type shown here.
Fluorine (F2), chlorine (Cl2), bromine
(Br2), iodine (I2).
(Group VII elements)
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15. Molecules of Compounds
(Molecules are always covalent, not ionic!)
When 2 or more different types of atoms form
covalent bonds, a molecular compound or
covalent compound is formed.
Water (H2O), methane (CH4),, ammonia (NH3)
and carbon dioxide (CO2)are examples of
covalent compounds.
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16. Example 1: Water
Water is formed by the reaction of hydrogen with
oxygen such that all three atoms attain noble gas
configurations.
Each water molecule contains two single covalent bonds.
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17. Water molecule (H2O)
+ →
The hydrogen atoms are now electronically like helium
and the oxygen atom like neon.
Two hydrogen – oxygen (H – O) 105°.
single covalent bonds are
formed.
The water molecule is V-shaped,
with the H-O-H bond angle of
105°. 17

18. Example 2: Methane
Methane (CH4) contains carbon and hydrogen.
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.
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19. Methane, CH4
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20. Example 3: Ammonia, NH3
Q. How many electrons does each hydrogen atom need
to become stable? 1
Q. How many electrons does each nitrogen atom need to
become stable? 3
Ammonia molecule, NH3 (with 20
3 single covalent N-H bonds)

21. Example 4: Carbon dioxide (CO2)
8p 6p 8p
Each carbon Each
atom shares oxygen
two electrons 8p 6p 8p atom
each with shares two
two oxygen electrons.
atoms.
Two double covalent (C=O) bonds
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22. Different ways of representing the carbon dioxide
molecule.
A carbon dioxide molecule (CO2) contains two
double covalent bonds. Each double bond has 4
shared electrons or 2 pairs of shared electrons. 22

23. Example 4: Carbon dioxide (CO2)
Each carbon atom shares two electrons each
with two oxygen atoms.
Each oxygen atom shares two electrons.
23

24. Structures of Covalent
Substances
• Simple (small)
There are 2 molecules
forms in which
covalent (2) Giant structures
substances exist:
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25. Simple (small) Covalent
Molecules
Also known as simple molecular
structures.
Most covalent substances exist in this form
e.g.
Hydrogen (H2), Oxygen (O2), nitrogen (N2),
chlorine (N2), Iodine (I2), Methane (CH4), water
(H2O), carbon dioxide (CO2), Hydrogen
chloride (HCl), ammonia (NH3).
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26. Ex 1: Simple Covalent Molecules
Methane (CH4)
In a molecule of methane,
CH4, the four C–H
covalent bonds are strong.
However, weak van der Waals’
forces between methane
molecules hold them together
loosely. Therefore, methane
exists as a gas at room
temperature and pressure. 26

27. Simple Covalent Molecules have low
melting & boiling points
Little energy is
needed to overcome
the weak forces
between molecules.
(In melting or boiling which are
PHYSICAL CHANGES, no
chemical bonds are broken;
e.g. in methane C-H bonds
are NOT broken).
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28. Ex 2: Simple Covalent Molecules
Iodine (I2)
Within each iodine
molecule, the iodine atoms
are held together by strong
covalent bonds.
Between the iodine
molecules, there are
only weak van der
Waals’ forces holding
the molecules together.
Weak force 28

29. Q. Why does iodine sublime easily
when heated gently?
Sublimation is a physical
change and only bonds
between iodine molecule are
broken. The intermolecular
(van der Waals’ forces are
weak, so little energy is
needed to overcome them.
Weak force between molecules
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30. Physical Properties of Simple
Molecular (Covalent) Compounds
1. Volatile – low melting and boiling points
2. Soluble in organic solvents; most are
insoluble in water.
3. Cannot conduct electricity in any state.
(Some exceptions are: carbon (graphite), ammonia,
ethanol, sugar, chlorine, hydrogen chloride, sulphur
dioxide.)
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31. Q. Why are many covalent substances
liquids or gases at room temperature?
This is because of the weak van der
Waal’s forces between the molecules, so
little energy is needed to overcome them
during changes of state.
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32. Q. Why do covalent substances not
conduct electricity even when molten
or in aqueous solution?
Covalent substances contain neutral
molecules. There are no mobile, charged ions
or electrons to conduct electricity.
(Exceptions: solid Carbon, in the form of graphite,
conducts electricity. Hydrogen chloride, sulfur
dioxide and ammonia react with water to form
solutions that conduct electricity. )
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33. Giant Molecular (Covalent)
Structures
Examples:
Diamond Silicon(IV) oxide
Graphite (silicon dioxide)
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34. Giant Covalent (Molecular)
Structures Tetrahedral
structure
Ex 1: Diamond
It is a form (allotrope) of
carbon.
Each carbon atom
is bonded to 4 other
carbon atoms by
strong covalent
bonds.
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35. How to draw the diamond structure.
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36. Physical properties of diamond
1. Diamond is a very hard substance. It is not
easily scratched or worn out.
2. It has high melting point (3500°C) and high
boiling point (4800 °C).
3. Diamond does not conduct electricity.
4. It is insoluble in water.
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37. Q. Why is Diamond hard?
A crystal of diamond contains
millions of carbon atoms joined
by strong covalent bonds.
A large amount of energy is
required to break these strong
covalent bonds. This explains
why diamond is so hard.
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38. Q. Why is the melting point of
diamond high?
A large amount of
energy is required to
break the millions of
strong covalent
bonds so that the
atoms are separated.
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39. Q. Why diamond does not conduct
electricity?
All the valence electrons of
the carbon atoms are used
for bonding.
There are therefore no free
electrons that are available
move through the structure to
conduct electricity.
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40. Uses of diamond
Synthetic diamonds produced under high
pressures and temperatures are used at the
tips of drills and other cutting tools.
They are used for drilling, grinding and
polishing very hard surfaces.
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41. Graphite
Graphite is another form (allotrope) of carbon.
It is made of layers of carbon atoms. 41

42. Graphite - Structure
Within each layer, each
carbon atom forms strong
covalent bonds (C – C)
with three other carbon
atoms.
The atoms form rings of six
carbon atoms that are joined
together to form two-
dimensional flat layers.
Each layer is a giant molecule.
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43. Graphite - Structure
The layers of carbon
atoms are held
together by weak van
der Waals’ forces.
Strong covalent bond (C-C)
between carbon atoms 43

44. Why does graphite have high
melting and boiling points?
In graphite, the (C-C)
bonds within each
layer are strong and
difficult to break.
Hence, graphite has high melting and boiling points.
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45. Why is graphite soft and
slippery?
The forces of
attraction between the
layers of carbon are
very weak.
The layers can slide over
each other. This explains
why graphite is soft and
slippery.
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46. Why is graphite able to conduct
electricity?
In graphite, each
carbon atom has one
outer (valence) shell
electron that is not
used to form covalent
bonds, and can move
through the layer to
conduct electricity!
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47. Why is graphite able to conduct
electricity?
These electrons are
delocalised (free) , that
is, they can move along
the layers from one
carbon atom to the next
when graphite is
connected to a battery.
Hence, graphite is a good conductor of electricity.
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48. Uses of graphite
1. As a dry lubricant.
2. As brushes for electric motors (to reduce
friction).
3. To make inert electrodes for electrolysis.
4. Baked with clay to make pencil lead.
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49. Silicon (IV) oxide
Sand is actually silicon
(IV) oxide.
It is also commonly
known as silica.
The formula of silicon
(IV) oxide is SiO2. It
is also known as
silicon dioxide.
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50. Silicon (IV) oxide - structure
Each silicon atom is
bonded to four oxygen
atoms in a tetrahedral
structure and
each oxygen atom is
bonded to two silicon
atoms.
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51. Why does Silicon (IV) oxide
have a high melting point?
This is because the silicon and
oxygen atoms are all held together
by strong covalent bonds in a giant
covalent (molecular) structure.
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52. Writing the formulae of covalent
compounds
Ex 1:
What is the chemical formula of carbon dioxide?
We assume the first element named (carbon)
contains only one atom unless otherwise stated.
The word ‘dioxide’ means ‘two oxygen atoms’.
So the formula is CO2.
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53. Ex 2: What is the chemical formula
of dinitrogen monoxide?
Dinitrogen means 2 nitrogen atoms.
Monoxide means 1 oxygen atom.
So the formula is N2O.
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54. Ex 3: What is the chemical formula
of dinitrogen tetroxide?
Dinitrogen means 2 nitrogen atoms.
Tetroxide means 4 oxygen atoms.
So the formula is N2O4.
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55. Formulae of covalent substances are not
always so easily derived.
Chemical formulae of some common covalent
substances which cannot be derived from their
names include:
ammonia (NH3),
hydrogen peroxide (H2O2),
methane (CH4),
ozone (O3),
water (H2O).
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