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A Level Chemistry
UNIT 2

APPLICATION OF CORE
PRINCIPLES OF CHEMISTRY

NOTES

Written by Mr Sergeant and Dr Lawrence


-1-

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Introduction
This unit includes the following.
Shapes of molecules and ions. Intermediate bonding and bond polarity.
Consideration of intermolecular forces.
The study of the Periodic Table looking at Groups 2 and 7.
Redox reactions, particularly those concerning Group 7 elements and their
compounds.
Rate of reaction (kinetics)
Chemical equilbria
Organic…

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SHAPES OF MOLECULES AND IONS
The shape of a molecule depends upon its electronic structure. It is the outer shell or
valence shell electrons which are responsible for forming bonds and it is the arrangement of
these electrons which determine molecular shape.
The electrons are all negatively charged and so…

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Methane, CH4

Dot and cross diagram.

Methane has four bonding electron pairs in the
H valence shell. These position themselves as far
apart as possible to form a tetrahedral shape
with a bond angle of 109.5o.

H C H
H
H 109.5O
C
H
H
H
Ammonia, NH3

Dot and…

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Carbon dioxide, CO2

Dot and cross diagram.
Carbon dioxide has two electron regions in the valence
shell. These position themselves as far apart as possible.
The molecule is linear.
O C O
Molecule
O C O
180O
Phosphorus pentachloride, PCl5
Phosphorus pentachloride has five bonding
Cl electron pairs in the…

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Ammonium ion, NH4+

+
H
The ammonium ion has four bonding pairs of electrons
in the valence shell (one of theses being a dative
H N H bond). The molecule is tetrahedral with bond angles
of 109.5o.
+
H
H

N
H
H
H
Bond angles in CH4, NH3, H2O…

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More complicated molecules
The shapes of more complicated molecules and ions can also be "explained" by electron
pair repulsion theory, or EPR.
In applying this principle, we must include both bonding and non-bonding (or "lone") pairs,
and we must count a double or triple bond as if it were one…

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Example
Electrons
+
PH4 Central atom = P 5
Bonding atoms = 4 x H 4 (Forms 4 bonding pairs)
Positively charged -1

Total = 8 4 pairs = Tetrahedral

The number of Lone pairs = Total number of pairs ­ bonding pairs = 4-4 = 0

+
H

P…

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Structures of Carbon Allotropes
Diamond
Diamond is composed of carbon atoms. Each carbon atom is connected to four other carbon
atoms




Each of these atoms is also connected to three other carbon atoms. In this way a giant
structure is built up from these tetrahedral units.




This structure, held together…

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Graphite is made up of these layers held together by London forces (Van der Waals' forces).
Sometimes it is referred to as a layer structure.




The carbon atoms in graphite are held together by covalent bonds forming a giant structure,
so the melting and boiling points are high. Since each…

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