Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
We mentioned before that:
1. Nobel gases such as helium and neon don't react with other elements under normal
circumstances. Why? Because their outermost energy levels are completely filled
with electrons.
2. Nobel gases form monoatomic molecules.
3. Based on the previous any other element in which the outermost energy level is not
completely filled with electron will react with other elements. Why? To
……………………………………………………………………………………….
What do we mean by chemical reaction?
 It is a process in which bonds are formed or bonds are broken.
 Bonds are formed or broken as a result of losing, gaining or sharing electrons.
 If chemical bonds are formed or broken among the toms then no chemical reaction
is taking place and consequently no chemical compounds are formed. What will be
formed? …………………………………………………………………………..
 What is the difference between mixing iron filling with sulphur and heating iron
fillings with sulphur?
………………………………………………………………………………………
………………………………………………………………………………………
……………………………………………………………………………………….
Chemical bonds:
First: Ionic Bond:
1. This bond is usually formed between metals and nonmetals.
Metals
Nonmetals
Characterized by their large volume.
Characterized by their small volume.
Their ionization energy is law, so it is easy
Their electron affinity is high, so it is easy
to lose electrons from the outermost energy to gain electrons to complete the outermost
level.
energy level.
Metals change into cations with identical
Nonmetals change into anions with identical
structure to the nearest Nobel gas.
structure to the nearest Nobel gas.
2. An electronic attraction between cations and anions takes place. This means that
a. The ionic bond has no materialistic existence.
b. The ionic bond is usually formed between the elements of two extreme groups
in the periodic table (the left side which contains metals and the right side
which contains the nonmetals).
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Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
Does the difference in electronegativity between the bonded atoms play a role in forming
the ionic bond?
In order to answer this question you can examine the following table:
Group
1A
2A
3A
Element
Na
Mg
Al
Electronegativity of
0.9
1.2
1.5
element
Chloride compound
NaCl
MgCl2
AlCl3
Electronegativity of
3
chlorine
Difference in
3 – 0.9 = 2.1
3 – 1.2 = 1.8
3 – 1.5 = 1.5
electronegativity
Melting point
810 °C
714 °C
190 °C
Boiling point
Sublimes
1465 °C
1412 °C
Electrical
Very good conductor
Good conductor
Doesn't conduct
conductivity
We observe the followings:
 When the difference in electronegativity increases between the bonded elements
the ionic character (boiling point – melting point – electrical conductivity)
increases.
 When the difference in electronegativity is more than 1.7 the formed compound is
ionic while electronegativity is less than 1.7 the compound is …………………
 When comparing sodium chloride and magnesium chloride to aluminum chloride
we discover that covalent bond character is more apparent than ionic character in
aluminum chloride.
Second: Covalent Bond:
 When the atoms are of the same electronegativity (atoms of the same element) or of
close electronegativity (C = 2.5 and H = 2.1) the binding between atoms is achieved
by sharing electron pairs.
Pure covalent bond:
 It is the bond between two atoms that have the same electronegativity (as in the bond
between two fluorine atoms or two chlorine atoms).
 This is because each atom has the same ability the shared electrons.
 The shared electrons spend the same time in the vicinity of each atom, so the net
charge on each atom is zero.
2
Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
What will happen if the atoms have different electronegativity values?
 The bond is called polar covalent bond.
 One atom has greater ability to attract the shared electrons, so partial negative
charge is formed on this element while the less electronegativity elements will have
partial positive charge.
Exercise:
Explain the type of covalent bond in the following compound:
1. Hydrochloric acid HCl knowing that electronegativity of H = 2.1 – Cl = 3
………………………………………………………………………………………………
………………………………………………………………………………………………
……………………………………………………………………………………………….
Give examples of other polar covalent compounds.
………………………………………………………………………………………………
………………………………………………………………………………………………
………………………………………………………………………………………………..
Theories to explain the formation of the covalent bond:
First: The Octet Rule or Electronic Theory of Valence:
 The theory states that "with the exception of hydrogen, lithium and beryllium the
atoms of all elements tend to reach the octet structure".
 The covalent bond is formed when a number of electrons from the outermost shell of
the atoms are shared.
 Consequently the outermost shell of each atom has 8 electrons through the sharing
of electrons.
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Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
The Inadequacies of Octet rule:
1. The covalent bond in many molecules can't be explained by the octet rule such as the
phosphorus pentachloride molecule in which the phosphorus atom is surrounded by
10 electrons and the boron trifluride molecule in which the boron atoms is
surrounded by 6 electrons only.
2. The octet rule can't explain the properties of some molecules such as the
sterostructure and the angles between the bonds.
The Valence Bond Theory (V.B.T.):
 The theory considered that the electron is not only a negative particle but a material
particle with wave property which can exist in any position in the space surrounding
the nucleus.
 The (V.B.T.) explains the formation of the covalent bond as a result of the overlap
of atomic orbital in one atom which contains single electron with a similar orbital of
another atom.
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Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
 The hydrogen molecule is formed as a result of the overlap of the electron in 1S
orbital of each hydrogen atom.
Hydrogen Fluoride Molecule
How does the (V.B.T.) explain the structure of the methane molecule?
 The formula of methane molecule is CH4.
 The carbon atom bonds with 4 hydrogen atoms by 4 symmetrical bonds in length
and strength.
 The molecule takes the tetrahedron shape and the angle between the bonds = 109.28°
 The electronic structure of the carbon 6C: 1S2 – 2S2 – 2P2.
 The carbon atom has only two orbitals with single electron in each orbital. These
electrons are capable of forming covalent bond.
 But the carbon atom must have four single electrons to bond with four hydrogen
atom with four equal covalent bonds.
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Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
 The solution to that is one electron of the 2S2 is moving to the P orbital so the carbon
atom can have four single electrons to form four covalent bonds with four hydrogen
atoms.
Based on the previous: Are these bonds equal in strength?
 The answer is no because the excited electron from 2S to 2P is different in energy.
What the carbon atom can do?
The answer is the Hybridization.
What is hybridization?
 The overlap between two different orbitals or more of the same atom to produce new
orbitals which are called the hybridized orbitals.
 The hybridization process takes place the orbitals of the same atom which are close
to each other in energy such as (2S and 2P) – (3d, 4s and 4p).
 The shape of the hybridized orbitals differs from those of the pure atomic orbitals
forming them.
 The hybridized orbitals must protrude (be pushed) to the outside to be more capable
of overlapping than the pure atomic orbitals.
The Shape of Hybridized Orbitals
6
Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
The (V.B.T.) explains the angles in the methane molecule:
 Each of the hybridized orbital in the carbon atom contains a negative electron.
 Strong repulsive force occurs.
 The orbitals must go apart as far as possible from the other orbital to decrease the
repulsive force between orbitals.
 When the angle between orbitals = 109° 28' they will be more stable (less repulsive).
 The four equivalent electrons of the four hybridized orbitals of the carbon atom in
the methane molecule can overlap with the four 1S electrons of the four hydrogen
atoms.
Methane Molecule
The following aspects must be considered in the hybridization process:
1. Hybridization process occurs between the orbitals of close energy of the atom.
2. The number of hybridized orbitals = the number of the pure (original) orbitals.
3. When the hybridization process takes place between the S orbitals and the P orbitals
the new hybridized orbitals are called SP orbitals.
4. SP2 means the 1S orbitals is being hybridized with two P orbitals.
Another example using SP2 hybridization in ethylene (ethene) C2H4:
By observing the figure:
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Port Said International Schools
Better Education for Future Generations
Science Department




Grade: 11
Unit 3
The two carbon atoms have been excited.
The three orbitals 2s, 2px, 2py are hybridized to form three hybridized 2sp2 orbitals.
To overcome the repulsive force the angle between the orbitals = 120°.
The 2pz orbital in each carbon atom is not involved in the hybridization process.
Each of these two orbitals lies perpendicular on the plane of the three sp2 orbitals of
each carbon atom.
Two types of bonds are formed:
1. Two sp2 hybrid orbitals on each carbon atom overlap with the 1s in two hydrogen
atom forming the two C – H bonds.
2. This bond is called sigma because the orbitals involved in forming this bond are on
one line.
3. The third sp2 orbital in one carbon atom is overlapping with the similar sp2 orbital in
the second atom to form the C – C bond.
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Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
4. This bond is also called a sigma bond because the sp2 orbitals are on the same line.
5. The 2pz orbital in the two carbon atoms overlap to form a bond called the pi π bond
because both 2pz orbitals parallel to each other.
The Molecular Orbital Theory "MOT":
 The valence bond theory explained the formation of the covalent bond in the
molecule as a result of the overlap of some atomic orbitals in the combined atoms.
 The rest of the atomic hybridized orbitals that don't take part in the formation of
bonds remain as they were in the free atoms.
 The MOT considers the molecule as one unit in which all orbitals in the combined
atoms are mixed or hybridized to form molecular orbitals.
 The atomic orbitals have symbols s, p, d …..etc.
 Also the molecular orbitals have symbols such as sigma𝜎, piπ, and delta𝛿.
First: the sigma (𝝈)bond:
- This bond is formed as a result of the overlap of atomic orbitals which are lining
along an axis. The overlapped orbitals are on one line and called collinear overlap.
Second: the pi (π) bond:
- This bond is formed as a result of the overlap of two atomic orbitals which exist side
by side. These orbitals are parallel to each other and called collateral overlap.
Examine the figure below and explain the bonds in the acetylene molecule:
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Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
From the previous figure we observe the following:
1. After the excitation process, hybridization occurs between one orbital 2px and 1s to
produce two sp orbitals.
2. To overcome the repulsive force between the hybridized orbitals the angle between
orbitals = 180°.
3. We observe that in each carbon atom there are two remaining orbitals 2pz and 2py.
They aren't involved in the hybridization process.
4. In the acetylene molecule we find:
a. One sigma bond: produced from the head-to-head overlap between one sp
orbital on each carbon atom forming the C-C bond.
b. Two pi bonds: between carbon atoms which are produced by the side-to-side
overlap of the two parallel 2py.
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Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
Third: coordination Bond:
A type of covalent bond.
The difference between the covalent bond and the coordination bond is the origin of
the electron pair which is shared between the atoms.
The covalent bond
The coordination bond
The electron pair which forms the ordinary The electron pair of the coordinate bond is a
covalent bond is produced by the two
lone electron pair occupying one orbital
combined atoms "one electron from each
found in one atom called the donor to
atom".
another atom called the acceptor.
An example of a coordinate bond is the one formed between the neutral water
molecules and the positive hydrogen ion to form the hydroxonium ion (H3O) +.
Another example of the coordinate bond is the one formed in the ammonium ion
(NH4)+ when the proton H+ accepts the lone electron pair from the nitrogen atom of
the ammonia molecule.
Forth: Hydrogen Bond:
A bond which connects between the molecules of water.
It is known that water boils at 100° C.
This temperature is considered high for a compound of relative low molecular mass
(18).
The boiling point of the hydrogen sulphide compound which has molecular mass
(34) = -61° C.
The boiling point increases as the molecular weight of the compound increases.
11
Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
The water molecule is a polar molecule, because the oxygen atom has higher
electronegativity than the hydrogen atoms, so the oxygen carries partial negative
charge, while each hydrogen atom carries a partial positive charge.
As a result of the separation of charges in water they are attracted together by the
hydrogen bond or the hydrogen bridge.
Each hydrogen atom becomes a bridge between two oxygen atoms of high
electronegativity.
The water molecules get closer to each other and the hydrogen atoms bind the water
molecules together.
The hydrogen bond is considered a type of intermolecular attraction force.
Why the hydrogen bond is increasing the boiling point of water?
Because the hydrogen bond is binding the water molecules and getting these
molecules close to each other so these molecules need more energy to overcome the
intermolecular attraction force.
You need to consider the following regarding the hydrogen bond:
1. The hydrogen bond is affecting the physical properties.
2. The hydrogen bond is weaker in strength and in length than the covalent bond.
3. The strength of the hydrogen bond depends on the electronegativity of the atoms
involved in forming the covalent molecule.
4. The hydrogen bond comes in many forms, the molecules may form a straight line, a
closed ring or an open net of hydrogen bonds such as in hydrogen fluoride and
water.
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Port Said International Schools
Better Education for Future Generations
Science Department
Grade: 11
Unit 3
Fifth: The Metallic Bond:
1. A bond which is produced from the electron cloud of valence electrons which
decreases the repulsive forces between positive metal ions in the crystal lattice.
2. Each metal has a crystal lattice shape in which the positive ions take certain
arrangement.
3. The electrons of the outermost energy levels are associated together to form a cloud
of electrons with free movement to bind the positive ions.
4. The free valence electrons are the main cause of the good electrical and thermal
conductivity.
5. The number of the valence electrons determines the strength of the metal bond.
6. . As the number of the valence electrons increase:
a. The metal bond becomes stronger.
b. The metal becomes harder.
c. The metal has higher boiling point and higher melting point.
Metal
Na
Mg
Al
13
Valence
1
2
3
Hardness
Soft
Mild
Hard
Melting point
98
650
660