Presentation on theme: "Section 3.3 – Molecular Shapes and Dipoles It is time for these molecules to get in shape! Nelson: pages 91-104."— Presentation transcript:
1 Section 3.3 – Molecular Shapes and Dipoles It is time for these molecules to get in shape! Nelson: pages
2 Molecular ShapeLewis structures tell us nothing about how atoms in a molecule are arranged in 3-dimensional spaceCould you have predicted the arrangement of atoms on the right from just seeing it’s Lewis structure?
3 VSEPR “vesper” THEORY Shapes of molecules: Molecules take on particular shapes depending on the amount of lone pairs and bonding electrons the atom in the middle of the molecule hasStereochemistry – study of the 3-D shape of molecules and how it affects their physical and chemical propertiesVSEPR = “valence- shell-electron-pair-repulsion” theory is based on:Electron pairs try to stay as far as possible from other e due to repulsion of negative chargesThe number, type and direction of bonds to the central atom (CA) determine the shape of the molecule
4 VSEPR Continued…VSEPR Theory is a powerful tool that helps us to guess the shape of a molecule. Such shapes are important as they determine the structure and function of the compound. For example, the arrangement of carbon atoms in a diamond help establish its unique hardness and usefulness.
5 According to VSEPR theory: Read and summarize the points on page 91 in the space below:
6 Using VSEPR to predict molecular shapes Bond angles – the angle formed by 2 bonds intersecting at an atomA few rules to predicting the shape of atomsget electron pairs as far away as possible (like charges repel)multiple bonds are treated as 1 bondlone pairs take slightly more room than bonding pairs
7 VSEPR Continued…There are 6 different arrangements that you will need to memorize!!!Let’s look at specific examples of molecules to help us come up with general formulas for each of the arrangementsLet A = Central atomLet X = bonding electrons (bond pair)Let E = one pairRemember: the key concept is that all pairs of valence e repel each other and try to get as far from each other as possibleSteps to determining shape:draw a L.D.D and consider the arrangement of valence e’s:Determine bond pairs and lone pairs around central atom (CA)
8 Consider beryllium dihydride Total pairs = 2 Bond pairs around Be (CA) = 2Lone pairs around CA = 0Bond pairs repel each other and try to get as far away as possible = opposite sides of BeGives a linear orientation with the two bonds at an angle of 180 ºSummary:General FormulaBond pairsLone pairsTotal pairsElectron pair arrangementStereochemical formulaAX2linear
15 nitrogen trichloride Lewis dot diagram: Total pairs = 4 Lone pairs = 1 Bonding pairs = 3Repulsion of electron pairs causes a tetrahedral shapeIf we ignore the lone pair, the shape becomes like a 3 sided (triangular) pyramid = trigonal pyramidalWe would predict that the bond angles would be 109.5º like the tetrahedral arrangement. However, lone pairs have a greater repulsion that bond pairs and therefore pushes that bond pair angles to be 107.3ºGeneral FormulaBond pairsLone pairsTotal pairsElectron pair arrangementStereochemical formulaAX3Etetrahedraltrigonal pyramidal
16 Water Lewis dot diagram: Total pairs = 4 Lone pairs = 2 Bonding pairs = 2Repulsion of bonding pairs causes a slightly altered (due to lone pairs) tetrahedral bonding pattern with the bond pairs having a bond angle of 104.5ºSummary:General FormulaBond pairsLone pairsTotal pairsElectron pair arrangementStereochemical formulaAX2E2tetrahedralAngular (V-shaped)
17 bonding-pair vs. bonding pair repulsionlone-pair vs. lone pairrepulsionlone-pair vs. bonding>
18 Hydrogen fluoride Lewis dot diagram: Total pairs = 4 Lone pairs = 3 Bonding pairs = 1Repulsion of bonding pairs causes a tetrahedral bonding pattern. Since there is only two atoms held together by one covalent bond, the shape is linear (like all diatomic molecules)Summary:General FormulaBond pairsLone pairsTotal pairsElectron pair arrangementStereochemical formulaAXE3tetrahedralLinear
19 Table 7: Using VSEPR to predict molecular shape – Nelson Page 95 General FormulaBond PairsLone PairsTotal PairsGeometryStereochemical formulaExamples:
20 General FormulaBond PairsLone PairsTotal PairsGeometryStereochemical formulaExamples:
21 Learning Activities: Read Pages 91-97 in your textbook Read the Learning Tips on pages 92, 94, and 96Try Practice Problems on Page 96 # 2-4= _____________________?= _____________________?= _____________________?
24 Everyone wants to be the center of the universe If you have more then one CA you follow three simple stepsstart with a structural or lewis diagramidentity all the CA and treat each one individuallydraw the molecule going from CA to CAExample – C2H6
25 More than one CA Continued… Example: What are the bond pairs and lone pairs around C in the following molecule? Around N? Draw a three dimensional structure for the compound.C: 4 bond pairs tetrahedralN: 3 bond pairs, 1 lone pair trigonal pyramidal
26 VSEPRFor the previous compound, two realistic 3-D structures would be:
27 Multiple bonds and VSEPR Can we predict stereochemistry for molecules with multiple bonds?Consider ethylene (used in welding torches): C2H4 (g)Crystallography indicates that the orientation around the C atoms is trigonal planerJust as before with multiple CA’s:Step 1: draw L.D.DStep 2: count bond and lone pairs around CA (Recall: double/ triple bonds count as one)Step 3: Determine general formula for each CA: AX3Step 4: draw a structural diagram if necessary
28 In acetic acid, CH3COOH, there are three central atoms, and one double bond. 3-D molecule applet
29 VSEPRExample: Draw a three dimensional structure for the following compound.First, determine the bond and lone pairs around each CATrigonal planartetrahedral
30 More common way to draw structure. VSEPRTwo possible 3-D structures:More common way to draw structure.
31 Learning Activities: Read Pages 91- 97 from Nelson Finish Practice Problems #2-4 on page 96Try Practice Problems 6-7 on page 98Complete Section 3.3 Questions 1-3 on page 104