Other pages in this set

Page 2

Preview of page 2

Here's a taster:

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 chemistry; alcohols and halogenoalkanes.
Mechanisms
Mass and infrared spectrometry
Green chemistry.
Assessment
The Unit examination will be 1hour 15 minutes. It will carry 80 marks.
It will contain three sections, A, B and C.…read more

Page 3

Preview of page 3

Here's a taster:

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 will repel each other.
Each electron region takes up a position to minimise repulsion.
By considering the valence shell electron regions and their positions according to repulsive
effects we can explain the shape of molecules.…read more

Page 4

Preview of page 4

Here's a taster:

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 cross diagram. Ammonia has three bonding electron pairs
and a lone pair of electrons in the valence
shell.…read more

Page 5

Preview of page 5

Here's a taster:

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 valence shell.
The molecule is trigonal bipyramidal.
Cl Cl Cl
P 90
O Cl
Cl Cl P 120O
Cl
Two of the bonds are at 180o.…read more

Page 6

Preview of page 6

Here's a taster:

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.…read more

Page 7

Preview of page 7

Here's a taster:

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 pair (or one region of electron
density).
The table shows the shapes expected for different numbers of electron pairs.…read more

Page 8

Preview of page 8

Here's a taster:

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
H
H
H
Example
Electrons
-
IF4 Central atom = I 7
Bonding atoms = 5 x F 4 ( Forms 4 bonding pairs)
Negatively charged +1
Total = 12 6 pairs = Octahderal
The…read more

Page 9

Preview of page 9

Here's a taster:

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 by strong covalent bonds, is very difficult to break apart. So
diamond has very high melting and boiling points. It is the hardest natural substance.…read more

Page 10

Preview of page 10

Here's a taster:

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 atom has a free electron, graphite is
able to conduct electricity. The layers held together by weak intermolecular forces can slide
over each other, making a soft slippery substance.…read more

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all resources »