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NAME: PERIOD: COVALENT COMPOUNDS MODELS LAB (67 points) PURPOSE: The purpose of this lab is to give the student experience building 3-D models of some simple covalent molecules. Students will then predict the shapes of the molecules and use their prior knowledge of bond polarity to predict the polarity of the molecule. INTRODUCTION Covalent Bonds A group of atoms held together by covalent bonds is called a molecule. The properties of a molecule, including its role in nature, depends primarily on its molecular structure, or shape. Molecular shape contributes toward determining a compound’s boiling point, freezing point, viscosity, solubility, types of reactions it can participate in, and a host of other physical and chemical properties. The covalent bonds holding the atoms of a molecule together can be classified as three different types. Covalent bonds are classified by comparing the differences in electronegativities of the two bonded atoms. Electronegativity is a scale used to determine an atom’s attraction for an electron in the bonding process. Bond Polarity Differences in electronegativities are used to predict whether the bond is nonpolar covalent, polar covalent, or ionic. The range of electronegativity differences and bond classification is: 0.0 – 0.4 0.4 – 2.1 2.1 – 4.0 nonpolar covalent polar covalent ionic In a polar covalent bond, the electrons are more attracted to the atom with the greater electronegativity. This results in a partial negative charge on that atom. The atom with the smaller electronegativity value acquires a partial positive charge. Molecular Polarity Molecules composed of covalently bonded atoms may also exhibit polar or nonpolar properties. For the molecule to be polar, it must, of course have polar bonds. But the key factor in determining the polarity of a molecule is its shape, or geometry. If the polar bonds are arranged symmetrically around the central atom they will offset each other and the resulting molecule is nonpolar. However, if the arrangement of the polar bonds is not symmetrical around the central atom, the electrons will be pulled to one end of the molecule and the resulting molecule is polar. Lewis Structures The structures used to show bonding in covalent molecules are called Lewis structures. When bonding, atoms tend to achieve a noble gas configuration. By sharing electrons, individual atoms can complete the outer energy level. OVER Predicting Shapes The model used to determine the molecular shape is referred to as the Valence Shell Electron pair Repulsion (VSEPR) model. The model is based on an arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom. By examining the central atom and identifying the number of atoms bonded to it and the number of unshared electron pairs surrounding it, one can determine the geometry, shape, of a molecule. Molecular Shapes TOTAL PAIRS SHARED PAIRS UNSHARED PAIRS SHAPE/ GEOMETRY EXAMPLE 2 2 0 linear N2 or CO2 3 3 0 trigonal planar BF3 4 4 0 tetrahedral CH4 4 3 1 trigonal pyramidal NH3 4 2 2 bent H2O 5 5 0 trigonal bipyramidal NbBr5 6 6 0 octahedral SF6 Models To study covalent molecules, chemists find the use of models helpful. Colored wooden, or plastic, balls are used to represent atoms. These balls have holes drilled into them according to the number of covalent bonds they will form. The holes are bored at angles that approximate the accepted bond angles. EQUIPMENT Molecular model building set. COLOR KEY (amount) Black (6) White (10) Red (4) Green (4) Blue (2) Yellow (2) Light brown (2) Gray (2) Medium gray links (20) Long gray links (6) NUMBER OF HOLES 4 1 2 1 4 4 5 6 ELEMENT Carbon Hydrogen Oxygen Halogen Nitrogen Sulfur Trigonal bipyramidal Octahedral SAFETY PRECAUTION Do not place model pieces in your mouth – they present a choking hazard. PROCEDURE 1. Obtain a molecular model set. 2. For each of the molecules in the data table: a. Build a model of the molecule. b. Draw the Lewis structure for each molecule. c. Determine the number of bonding pairs and lone pairs around each central atom. d. Draw a 3-D sketch of the molecule. e. Determine the shape of the molecule at each central atom. f. Determine whether the bonds between the central atom and the other atoms are nonpolar, polar, or ionic. g. Determine whether the molecule is polar or nonpolar. OVER DATA TABLE (44 points) Formula Cl2 HBr NH3 Electron Dot Diagram Bond Pairs Lone Pairs N/A N/A N/A N/A 3 1 N/A N/A SO3 N2 SbI5 CCl4 3D Drawing Name of Molecular Shape Bonds Ionic, Polar or Nonpolar Molecule Polar or Nonpolar Formula Electron Dot Diagram Bond Pairs Lone Pairs 3D Drawing CH2Cl2 CH4 TeBr6 C2H4 CH3OH CO2 H2O OVER Name of Molecular Shape Bonds Polar or Nonpolar Molecule Polar or Nonpolar POST-LAB DISCUSSION: (23 points) Read the entire lab and the relevant parts of your textbook (Chapter 9; pp. 259 – 267). Then answer the questions that follow. (1 point) 1. Define a covalent bond. (1 point) 2. Define a dipole. (2 points) 3. List the two factors that determine whether a molecule is polar. (5 points) 4. List the seven different molecular geometries (shapes) that were studied in this lab. (4 points) 5. Calculate the electronegativity difference and predict the type of bond for the following examples: a. Na―Cl b. C―H c. S―O d. N—N (5 points) 6. List five molecules in this lab which are nonpolar because all bonds are nonpolar. (3 points) 7. List three molecules in this lab that have polar covalent bonds but are nonpolar molecules because of symmetry. (2 points) 8. Which two shapes appear to produce polar molecules?