Chapter 8 Molecular ShapeObjectives8-1 The Shape of Small Molecules· Describe the VSEPR theory· Identify the common shapes of small molecules8-2 Polarity· Explain what determines the polarity of a molecule· Explain why water is a polar molecule8-1 The Shape of Small MoleculesWhat are some household uses for water?What is some way that water has been a memorable part of an experience in your life?What properties of water made this possible?Molecular Geometry· How bonds are arranged in a molecule determines the shape of a molecule· Structural formula does not indicate shape of a molecule. Draw Lewis dot structure for ammonia, NH3Ø Structural formula does not show how bonds are arranged in space (3D)· Ball & Stick models- are used to indicate molecular shape. Balls are nuclei & sticks are bonds. See figures 8.4, 8.5 & 8.7· There are many shapes of small molecules but all are symmetrical- meaning the bonds and atoms are arranged in a regular pattern with equal distances separating the atoms that are not bonded together. WHY? Ø Valence shell electrons repel each other because of similar electrical charge· VSEPR (valence shell electron pair repulsion)Theory- in a small molecule, the pairs of valence electrons are arranged as far apart from each other as possible.Ø This is due to the repulsion of like charges on e-Ø Each pair of valence electrons we are talking about is a chemical bondØ There are some exceptions with transition metals but this is generally trueØ This makes the molecular shape of small molecules predictable· 5 most common shapes1) Lineara. Molecules w/ 2 atoms, IE> O2, HClb. Many molecules w/ 3 atoms, IE> CO2c. Bond angle- geometric angle between 2 adjacent bonds. For CO2 (linear shape) bond angle is 180°, so that the valence electrons can be as far apart from each other as possible.Build CO22) Trigonal Planar- 4 atomsa. Meaning triangular & flatb. BCl3 or boron trichloride-i. the 3 Cl atoms are arranged in the shape of an equilateral triangle (all sides are equal)ii. boron is in the center and has no unshared electron pairs (3 valence e-). This is typical of trigonal planar shaped molecules. iii. bond angle is 120°Build BCl33) Tetrahedal- 5 atomsa. Tetrahedal = four surfacesb. = methane, an important fuel & principle component of natural gasc. Bond angle = 109.5° if only 2 dimensional greatest angle would be 90°Build CH4ConfigurationBonding PartnersBond AnglesExampleTetrahedral4109.5ºTrigonal3120ºLinear2180º4) Pyramidal-(trigonal pyramidal) 4 atoms w/ extra pair of electronsa. NH3 = ammoniab. 3 single bonds & 1 unshared pair of electronsc. if there were equal repulsion between the electrons, you would have a tetrahedal shaped. unshared pair is held by only one atom’s nucleus & thus takes up more room = more repulsive forcee. bond angle 107°5) Bent- 3 atoms w/ 2 extra pair of electronsa. H2O = waterb. With 3 atoms you might expect it to be linear, but count the number of electron pairs c. 4 pairs with 2 of them unshared, unshared pairs have greater repulsive force, so the bond angle between H’s is slightly less than pyramidald. bond angle = 105°Small molecules can take on a variety of other shapes (fig 8-10 p.261 but these are the most common.Sample problems p.262. Practice problems 1 & 2. Alt practice 1 & 2.Total Number of electron pairsArrangement of electron pairsNumber of bonding pairs of electronsNumber of lone pairs of electronsShape of MoleculeName of ShapeBond AngleExamplesnot applicablelinear1not applicablelinear180oH2, HCl2linear20linear180oCO2, HCN3trigonal planar30trigonal planar120oBCl3, AlCl34tetrahedral40tetrahedral109.5o CH4, SiF431trigonal pyramidal<109.5o (bond angles in ammonia, NH3, are 107o)NH3, PCl322bent<109.5o (bond angles in water, H2O, are 105o)H2O, SCl25trigonal bipyramidal50trigonal bipyramidal120o in the trigonal planar part of the molecule, 90o for the othersPCl56octahedral60octahedral90oSF6ExamplesI. Hydrogen chloride, HClHCl is composed of only 2 atoms, 1 atom of hydrogen and 1 atom of chlorine covalently bonded.HCl is, therefore, diatomic.All diatomic molecules are linear in shape.H-Cl is linear in shapeII. Hydrogen cyanide, HCNHCN is composed of 3 atoms, 1 atom of hydrogen, 1 atom of carbon and 1 atom of nitrogen covalently bonded.Carbon, C, is the central atom in the molecule.Carbon has 4 valence electrons (electrons that can be used in bonding).1 of carbon's valence electrons will be used to form a covalent bond with hydrogen.3 of carbon's valence electrons will be used to form 3 covalent bonds with nitrogen (a triple bond).The central carbon atom therefore has no lone pairs of electrons.The bonding p。