Center for Pre-College Programs
NJIT
6-21-12
Cloud Computing Workshop for Teachers
Center for Pre-College Programs
New Jersey Institute of Technology
LESSON PLAN TEMPLATE
Abey.K.Tharian
Leonia High School
Leonia, NJ
TOPIC: Molecular Geometry
STANDARD(S) & INDICATOR(S):
NJ Core Curriculum Standards for Physical Science (2009)
5.2.12. A1: Use atomic models to predict the behaviors of atoms in interactions
5.2.12. B1: Model how the outermost electrons determine the reactivity of elements and the
nature of the chemical bonds they tend to form.
OBJECTIVE(S):
1. Students will be able to draw Lewis dot diagrams of atoms and molecules
2. Students will explain VSEPR theory
3. Students will be able to predict the shapes and polarity (dipole moment) of molecules
using VSEPR theory.
MATERIALS: Notes, Text books, Projector, Computer, Ball and stick models.
LIST OF HANDOUTS:
CHAPTER NOTES
THE SHAPE OF MOLECULES
Summary of the lesson Molecular geometry is one of the challenging topics in chemistry. Many properties of substances can be well understood with the proper knowledge of this topic. To understand this topic, students should have the basic knowledge of electronic structure of atoms and different types of bonding between atoms. Outermost electrons in an atom are called valence electrons and they are important because during chemical bonding these electrons are being transferred or shared.
Center for Pre-College Programs
NJIT
6-21-12
Lewis dot diagrams are drawn for atoms and molecules using the valence electrons. Valence shell electron repulsion (VSEPR) theory is used to predict the shapes of molecules after drawing the Lewis dot diagrams. The electron pairs around the central atom in molecule or ion is identified. These electron pairs can be bond pairs or lone pairs. Depending on the number of electron pairs, molecular geometry can be determined. There are two factors affecting polarity of a molecule. They are electro negativity difference between the atoms in a molecule and shape of the molecule. Since electronegativities can be easily predicted from the position of elements in the periodic table, shape determination is more difficult for students. In the traditional setting, teachers draw two dimensional pictures of molecules on the boards. This can be confusing to many students. Textbooks try to explain the three dimensional pictures using projections and dots. Molecular model kits can solve the problem to a certain extent. Modern computer technology can easily help students and teachers alike in this topic. A variety of programs are available to visualize molecular models. They can be rotated at different angles and students can view them at different angles. Through cloud computing, students can easily share and exchange their thoughts and master the topic faster compared to the traditional approach.
Lewis Dot Diagrams : are used to show an atom’s valence electrons(outermost electrons).
Example : Dot diagrams for period 2 elements are as follows.
Why is the Shape of a Molecule Important?
We have represented molecules as two-dimensional structures, were as in fact most are
three-dimensional structures. The shape of a molecule can determine the reactivity, its
effectiveness as an enzyme or any number of other properties.
Valence Shell Electron Pair Repulsion (VSEPR Theory)
VSPER Theory is based on the concept that valence electrons in a molecule will repel
each other. There are bond pairs and lone pairs around the central atom of a molecule. The
order of repulsion is as follows: Lone pair – Lone pair > Lone pair – bond pair >
bond pair – bond pair.
According to this theory, the valence electron pairs around the central atom in a molecule
will be arranged in space so that the repulsion between them will be minimum.
So if there are two pairs, the molecule will be linear.
Three pairs - Trigonal planar
Four pairs - Tetrahedral
Bond angle: The angle made by atoms joined to the central atom.
Eg: The bond angle in Beryllium fluoride, BeF2 is 180o
The bond angle in Boron trifluoride, BF3 is 120o
The bond angle in methane, CH4 is 109.5o
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6-21-12
The steps involved in determining the shape of a molecule are as follows:
1. Draw the correct electron dot structure.
2. Count the number of electron groups (bonding groups plus lone pairs) around the central
atom.
3. Determine the electron arrangement around the central atom. (This is the shape of the
molecule if there are no lone pairs.)
4. If lone pairs are present, describe the shape of the molecule, ignoring the position(s)
occupied by any lone pairs.
The table below summarizes the molecular and electron-pair geometries for different
combinations of bonding groups and nonbonding pairs of electrons on the central atom.
# of lone pair
electrons on
'central' atom
#of bond pairs
electrons on
'central' atom
Electron-pair
Geometry
Molecular
Geometry
Bond
Angle
0 2 linear linear 180
0 3 trigonal
planar
trigonal
planar 120
1 2 trigonal
planar bent
less
than
120
0 4 tetrahedral tetrahedral
109.5
1 3 tetrahedral trigonal
pyramidal 107
2 2 tetrahedral bent 105
Polarity of Molecules
Many properties of compounds, such as boiling point and solubility, are determined by
the intermolecular attractive forces between the molecules. One of the most common types of
intermolecular attractive forces (or intermolecular forces) is the dipole-dipole interaction.
Dipole-dipole interactions occur when a compound is composed of polar molecules. In polar
molecule there is a partially positive end and a partially negative end for the molecule.
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6-21-12
Dipole Moment
Dipole moment is a measure of polarity of a molecule, usually represented by an arrow.
Two factors must be considered when determining whether a molecule is polar or
nonpolar:
1. Whether any polar bonds are present in the molecule, due to differences in
electronegativity. If there are no polar bonds in the molecule, the molecule must be
nonpolar.
2. Whether the individual bond dipoles cancel each other, based on the shape of the
molecule. (We use vectors to determine whether the individual bond dipoles cancel, as
described in section 6.3 of the textbook). If all of the bond dipoles cancel, the molecule
will be nonpolar.
Intermolecular Forces – Dipolar Interactions
Polar molecules attract each other because of the attraction of the unlike partial charges.
This attraction is what causes the molecules to stick to one another to form a liquid or solid under
normal conditions. The intermolecular attractive forces between polar molecules are called
dipole-dipole forces (or polar forces). Dipole-dipole forces are some of the strongest
intermolecular forces known, but they are considerably weaker than covalent and ionic bonds
(referred to as intramolecular forces).
In order to determine whether a molecule is polar – and thus has dipole – dipole
intermolecular attractive forces – you must do the following:
1. Draw the correct Lewis dot diagram for the molecule.
2. Apply the VSEPR theory to determine the shape of the molecule.
3. Draw the three dimensional shapes of the molecule, showing all bond dipole moments.
4. Determine whether the individual bond dipoles cancel (making the molecule nonpolar or
whether there is a net dipole moment in the molecule (making the molecule polar).
EXAMPLES FOR POLAR AND NONPOLAR MOLECULES
MOLECULE
FORMULA
SHAPE
STRUCTURE
POLAR /
NON
POLAR
HYDROGEN
H2
LINEAR
H----H
NON
POLAR
HYDROGEN FLUORIDE
HF
LINEAR
H----F
POLAR
HYDROGEN CHLORIDE
HCl
LINEAR
H----Cl
POLAR
WATER
H2O
BENT
O
H H
POLAR
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6-21-12
DICHLOROACETYLENE
C2Cl2 LINEAR Cl – C ≡ C -- Cl NON
POLAR
CARBON
TETRACHLORIDE
CCl4
TETRAHEDRAL
Cl
|
C
Cl
Cl Cl
NON
POLAR
PHOSGENE
Cl2CO
TRIGONAL
PLANAR
O
||
C
Cl Cl
POLAR
Homework
Answer the chapter review questions
BACKGROUND INFORMATION:
One of the important types of chemical bonding is covalent bonding. Outermost electrons
of an atom (valence electrons) are shared during this bonding. Lewis dot diagrams are
drawn to represent valence electrons in an atom. VSEPR theory can be used to predict the
shape of a molecule based on its Lewis dot diagram. With the knowledge of shape and
electronegativity of different elements involved, we can predict the polarity and dipole
moment of a molecule.
EDUCATION TECHNOLOGY INTEGRATION:
Power point presentation, Computer assisted simulations of molecular geometry, Online
discussion groups, internet searches for molecular shapes.
CLASSROOM ACTIVITY DESCRIPTION
(LABORATORY/EXERCISES/PROBLEMS) including detailed procedures:
Classroom lecture and discussion was conducted using power point presentation.
Worksheets are used to practice problems. Molecular model kits are used to conduct lab
activity. Students drew molecular shapes and predicted their polarity.
LAB ACTIVITY
Models of Molecular Shapes Report Sheet
NAME_________________________
Molecular
formula
Bond
angle(s)
Total
#
No. of
bond
No. of
lone
VSEPR
class
Molecular
geometry
Dipole
moment
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6-21-12
bonds
& lone
pairs
pairs pairs (yes/no)
BeCl2
BF3
CH4
NH3
H2O
HBr
CH2Cl2
HOCl
H2O2
NH2OH
CH3NH2
XeF4
Molecular
formula
Bond
angle(s)
Total
#
bonds
& lone
pairs
No. of
bond
pairs
No. of
lone
pairs
VSEPR
class
Molecular
geometry
Dipole
moment
(yes/no)
O2
C2H4
HONO
HCOOH
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6-21-12
C2HCl3
N2
C2H2
HOCN
CO2
C3H4
C2H2O
SAMPLE QUESTIONS TO ELICIT CLASS DISCUSSION:
1. How many valence electrons for sulfur atom?
2. Draw the Lewis dot diagram for chlorine atom
3. How many dots are needed to draw the Lewis diagram of sulfur hexafluoride?
4. Which molecule is polar; HCl or Cl2. Why?
5. Which molecule is more polar; HBr or HF. Why?
6. Draw the dot diagram of water
7. Explain the shape and polarity of water using VSEPR theory.
HOMEWORK ACTIVITY/EXERCISES/PROBLEMS:
1. Answer Section review and chapter review questions
2. Answer the following post lab questions:
Draw the Lewis dot diagrams for the following molecules/ions and predict their shape,
bond angle, and polarity. Also identify the molecules/ions with resonance forms.
1. Sulfur dioxide (SO2)
2. Bromine monoflouride (BrF)
3. Ozone (O3)
4. Phosphine (PH3)
5. Silane (SiH4)
6. Chlorate (ClO3-)
7. Carbonate (CO32-
)
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6-21-12
8. Thiocynate (SCN-)
9. Xenon hexafluoride (XeF6)
10. Xenon oxide tetrafluoride (XeOF4)
11. Antimony pentachloride (SbCl5)
12. Triodide (I3-)ion
PARAMETERS TO EVALUATE STUDENT WORK PRODUCTS:
1. Pre lab questions -10%
2. Lab activity: making models – 40 %
3. Answering post lab questions – 40 %
4. Home work -10%
REFERENCES:
Chang, Raymond (2002). Chemistry, 7th Edition, Boston, Mass., WCB McGraw-Hill
Watkins, Kenneth W. ((1998) Student Study Guide to Accompany Chemistry 6th
Edition,
Boston, Mass., WCB McGraw-Hill
Internet resources
This material is based on work supported by the National Science Foundation under Grant No. 1054754. Any
opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do
not necessarily reflect the views of the National Science Foundation.
Copyright © 2012 by the Center for Pre-College Programs, ofthe New Jersey Institute of Technology.All Rights
Reserved.
Supporting Program: Center for Pre-College Programs, at the New Jersey Institute of Technology
Contributors
Abey Tharian (Leonia High School, Leonia, NJ), Primary Author
Howard Kimmel, Levelle Burr-Alexander, John Carpinelli - Center for pre-College Programs, NJIT.