Chapter 8 Molecular Shape：8章分子形状.docVIP

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, NH3 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? 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. This is due to the repulsion of like charges on e- Each pair of valence electrons we are talking about is a chemical bond There are some exceptions with transition metals but this is generally true 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 120Build 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 90Build CH4ConfigurationBonding PartnersBond AnglesExampleTetrahedral4109.5Trigonal3120Linear21804) 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 atoms nucleus & thus takes up more room = more repulsive forcee. bond angle 1075) 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 Hs is slightly less than pyramidald. bond angle = 105Small 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 pyramidal109.5o (bond angles in ammonia, NH3, are 107o)NH3, PCl322bent109.5o (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 carbons valence electrons will be used to form a covalent bond with hydrogen.3 of carbons 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 pairs will repel each other as much as possible, so the molecule will be linear.HCN is linear in shape.III. Aluminum chloride, AlCl3AlCl3 is composed of 4 atoms, 1 atom of aluminum and 3 atoms of chlorine.Aluminum, Al, is the central atom in the molecule.Aluminum has 3 valence electrons (electrons that can be used in bonding).Each of the aluminums valence electrons will be used to form a covalent bond with each chlorine atom.The central aluminum atom will therefore have no lone pairs of electrons and 3 bonding pairs of electrons.The bonding pairs of electrons will repel each other as much as possible, so the molecule will be trigonal planar.AlCl3 is trigonal planar in shape.IV. methane, CH4CH4 is composed of 5 atoms, 1 atom of carbon and 4 atoms of hydrogen covalently bonded.Carbon, C, is the central atom in the molecule.Carbon has 4 valence electrons (electrons that can be used in bonding).Each of carbons 4 valence electrons will form a bonding pair with 1 of hydrogens electrons.The central carbon atom will therefore have no lone pairs of electrons and 4 bonding pairs of electrons,The bonding pairs of electrons will repel each other as much as possible, so the molecule will be tetrahedral.CH4 is tetrahedral in shape.V. Ammonia, NH3NH3 is composed of 4 atoms, 1 nitrogen atom and 3 hydrogen atoms covalently bonded.Nitrogen, N, is the central atom in the molecule.Nitrogen has 5 valence electrons.3 of nitrogens valence electrons will be used to form bonding pairs of electrons with hydrogen (3 covalent bonds).These bonding pairs repel each other equally and will try to get as far away from each other as possible.2 of nitrogens valence electrons will be unused for bonding, these are a lone pair of electrons.lone pair-lone pair repulsion is greater than bonding pair-bonding pair repulsion, so the lone pair pushes the bonding pairs closer together than in a tetrahedral arrangement of the electron clouds.This distorted tetrahedral arrangement is called trigonal pyramidal.NH3 is trigonal pyramidal in shape.In this representation, the solid triangles represent bonds coming out of the plane of the screen; the broken lines represented a bond going behind the plane of the screen. VI. Water, H2OWater is composed of 3 atoms, 1 atom of oxygen and 2 atoms of hydrogen covalently bonded.Oxygen, O, is the central atom.Oxygen has 6 valence electrons.2 of oxygens electrons will be used to form bonding pairs of electrons with hydrogen (2 covalent bonds).These bonding pairs repel each other equally and will try to get as far away from each other as possible.4 of oxygens valence electrons will not be used for bonding, these will remain as 2 lone pairs of electrons.These lone pairs of electrons repel each other equally and will try to get as far away from each as possible.lone pair-lone pair repulsion is greater than lone pair-bonding pair or bonding pair-bonding pair repulsion, so the lone pairs of electrons push the bonding pairs of electrons closer together than in a tetrahedral arrangement of the electron clouds.This distorted tetrahedral arrangement is call bent.H2O is bent in shape.In this representation, solid lines represent bonds that are in the same plane as the screen. Hybrid Orbitals When atoms form bonds, orbitals mix creating hybrid orbitalsmoleculeorbitalbondsshape3 atomssp2linear4 atomssp23trigonal planar5 atomssp34tetrahedral* 3 or 4 atoms sp32 or 3bent/pyramidal* All of these molecules have 4 pairs of valence electrons surrounding the central atom due to unshared pairsBond Length (Fig 8-12 p.264) Very small- H2 = 0.075nm (shorter than wavelength of light) Larger the atom = larger the bond length Multiple bonds = shorter bond lengthHomework 8-1 Review, 8-1 Practice8-2 Polarity Already know how to determine if a bond is polar .4 is nonpolar covalent .4 formaldehyde & carbon dioxide Formaldehyde- oxygens electronegativity = 3.5 making it more electronegative than either C or H. (see Figure 8-16 p.267) As a result the electrons in the C-O bond are more attracted toward the O. The O gains a partial charge, signified by green arrow in Fig 8-16. The C-H bonds are not nearly as polar because the difference in electronegativity is much smaller so very little charge shift takes place, signified by smaller green arrows in Fig. 8-16. So one end of the mo