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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 Center for Pre-College Programs NJIT 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. for Pre-College Programs NJIT 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 Center for Pre-College Programs NJIT 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 Center for Pre-College Programs NJIT 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 Center for Pre-College Programs NJIT 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-) Center for Pre-College Programs NJIT 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.


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