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  • 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 atoms 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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    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.

    http://intro.chem.okstate.edu/1314F97/Chapter9/2BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/4BP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/3BP1LP.htmlhttp://intro.chem.okstate.edu/1314F97/Chapter9/2BP2LP.html

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    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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    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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    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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    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.