Lesson Plan
Lesson: Shape of Molecules
Aim: To investigate the shapes of molecules and ions
Learning Outcomes :
At the end of the lesson, students will be able to :
1. explain the Valence Shell Electron Pair Repulsion Theory that governs the
shape of molecules and ions.
2. predict and describe the shapes and bond angles of some covalent molecules and ions.
Assumed prior knowledge :
Students should already :
1. be able to draw Lewis structures of covalent molecules and ions.
2. be familiar with the concept of lone pairs and bonding pairs of electrons.
Underlying Principles
1.
2.
Making the invisible, visible.
Enabling students to know what to look for.
Time taken to complete the activities : 80 minutes
Differentiation
Questions in the student notes are designed to enable all students to complete the activity. The pop-up
answers are provided for the students to view when they have considered their responses. Worksheet
questions include questions that require recall, understanding and application of the new concepts
learned.
© 2003 Ministry Of Education Malaysia. All Rights Reserved.
1
Development of Lesson :
No.
1
2
Steps
Set Induction.
(Ascertaining prior
knowledge and
introducing lesson
topic for the day).
Strategy
•
Teacher to quiz students on the Lewis
symbols of some elements. Teacher to
get students to suggest shapes of some
simple molecules.
•
Teacher to point out the role of electron
pairs in determining molecular shapes
and to introduce lesson objectives.
Student Activity
Teacher to go through Activities 1 - 3
with the students.
•
Activity 1 : Valence Shell Electron Pair
Repulsion
Students learn how the repulsion of
electron pairs results in the shapes of
CH4, NH3 and H2O.
•
Activity 2 : More molecular shapes
Students get to investigate the shapes of
molecules of BeH2, BF3, PCl5 and SF6.
•
Activity 3 : Shapes of ions
Students get to investigate the shapes of
the following ions : NH4+ and H3O+.
Resources
•
Courseware
3
Evaluation
•
Students to answer questions in the
student worksheet on their own.
•
Worksheet
4
Extension activity
•
Students to go through the extension
activities on their own.
•
Website
and References.
© 2003 Ministry Of Education Malaysia. All Rights Reserved.
2
Worksheet Answers
1.
Valence Shell Electron Pair Repulsion Model
1.1 Bonding pair are two electrons of opposite spin localised between two atoms, and non-bonding pair are
two electrons of opposite spin found in an orbital of an atom not bonded to another atom.
1.2 a.
The central atoms of both haloalkanes have 4 bonding pairs of electrons.
There are no non-bonding pairs of electrons on their central atoms.
b.
Both molecules are tetrahedral with a bond angle of 109.5o. This is because the four bonding
pairs of electrons will repel each other, and the position of minimum repulsion is when they
occupy the four corners of a tetrahedron. Thus, the bond angle corresponds to the angle of a
tetrahedron, that is 109.5o.
c.
CH3Cl , CH2Cl2 , etc.
1.3 The Lewis structures of CCl4
The central atom of CCl4 has four bonding pairs of electrons. Thus, its molecular shape is tetrahedral.
The Lewis structures of NCl3 is
The central atom of NCl3 has three bonding pairs and one non-bonding pair of electron. Thus, its
molecular shape is trigonal pyramid.
The Lewis structures of OCl2 is
The central atom of OCl2 has two bonding pairs and two non-bonding pairs of electrons. Thus, its
molecular shape is ‘V’.
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1.4 PCl3 has similar molecular shape as ammonia.
The central atom of NH3 has three bonding pairs and one non-bonding pair of electrons. Thus, its
molecular shape is trigonal pyramid.
The central atoms of BCl3 and AlCl3 have three bonding pairs of electrons. Their molecular shape is
trigonal planar.
The central atom of PCl3 has tree bonding pairs and one non-bonding pair of electrons. Thus, its
molecular shape is the same as that of ammonia.
The central atom of ICl3 has three bonding pairs and two non-bonding pairs of electrons. Its molecular
shape is known as distorted T-shape.
2.
More Molecular Shapes
2.1
Bonding pair
2
2
Non-bonding pair
0
1
Molecular shape
linear
V-shape
2
2
V-shape
3
3
0
1
Trigonal planar
Trigonal pyramid
3
2
T-shape
Bond angle
180o
Slightly less than
120o
Slightly less than
109o
120o
Slightly less than
109o
~180o and ~90o
2.2 The Lewis structures of PCl3 and PCl5 are
The central atom of PCl3 has three bonding pairs and one non-bonding pair of electrons. Thus, its
molecular shape is trigonal pyramid with a bond angle of slightly less than 109.5o.
The central atom of PCl5 has five bonding pairs and no non-bonding pair of electrons. Thus, its
molecular shape is trigonal bipyramid with a bond angle of 120o and 90o.
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2.3 The Lewis structures of aluminium chloride and its dimer are
The central atom of AlCl3 has three bonding pairs and no lone pair of electrons. Thus, its molecular
shape is trigonal planar with a bond angle of 120o.
The central atoms of Al2Cl6 have four bonding pairs and no lone pair of electrons. Thus, the molecular
shape at each centre is tetrahedral with a bond angle of 109.5o.
2.4 The molecular shapes of carbon dioxide and sulphur dioxide are different.
The Lewis structures of carbon dioxide and sulphur dioxide are
The central atom of CO2 has two ( bonding pairs and no lone pair of electrons. Thus, its molecular
shape is linear with a bond angle of 180o.
The central atom of SO2 has two (
3.
bonding pairs and one lone pair. Thus, its molecule is V-shaped.
Shapes of Ions
3.1 a.
The central atom of NH4+ is N. The Lewis structure is
Shape : tetrahedral
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b.
The central atom of NO3- is N. The Lewis structure is
Shape : trigonal planar
c.
The central atom of CO32- is C.The Lewis structure is
Shape : trigonal planar
d.
The central atom of I3- is I. The Lewis structure is
Shape : linear
e.
The central atom of SO42- is S. The Lewis structure is
Shape : tetrahedral
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3.2 The Lewis structure of NO3- ion is
The central atom has three ( ) bonding pairs and no non-bonding pair. Thus, the molecular shape is
trigonal planar.
3.3 The Lewis structure of (C6H5)3C+ is
The central atom has three bonding pairs and no non-bonding pairs of electrons. Thus, the molecular
shape is expected to be trigonal planar with a bond angle of 120o.
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