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Chapter 10 Chemical Bonding II

Chapter 10 Chemical Bonding II. The taste of a food depends on the interaction between the food molecules and taste cells on your tongue The main factors that affect this interaction are the shape of the molecule and charge distribution within the molecule

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Chapter 10 Chemical Bonding II

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  1. Chapter 10Chemical Bonding II

  2. The taste of a food depends on the interaction between the food molecules and taste cells on your tongue • The main factors that affect this interaction are the shape of the molecule and charge distribution within the molecule • The food molecule must fit snugly into the active site of specialized proteins on the surface of taste cells • When this happens, changes in the protein structure cause a nerve signal to transmit Taste

  3. Sugar molecules fit into the active site of taste cell receptors called Tlr3 receptor proteins • When the sugar molecule (the key) enters the active site (the lock), the different subunits of the T1r3 protein split apart • This split causes ion channels in the cell membrane to open, resulting in nerve signal transmission • Artificial sweeteners also fit into the Tlr3 receptor, sometimes binding to it even stronger than sugar • making them “sweeter” than sugar Sugar & Artificial Sweeteners

  4. Properties of molecular substances depend on the structure of the molecule • The structure includes many factors, such as: • the skeletal arrangement of the atoms • the kind of bonding between the atoms • ionic, polar covalent, or covalent • the shape of the molecule • Bonding theory should allow you to predict the shapes of molecules Structure Determines Properties! Tro: Chemistry: A Molecular Approach, 2/e

  5. Molecules are 3-dimensional objects • We often describe the shape of a molecule with terms that relate to geometric figures • These geometric figures have characteristic “corners” that indicate the positions of the surrounding atoms around a central atom in the center of the geometric figure • The geometric figures also have characteristic angles that we call bond angles Molecular Geometry Tro: Chemistry: A Molecular Approach, 2/e

  6. Lewis theory predicts there are regions of electrons in an atom • Some regions result from placing shared pairs of valence electrons between bonding nuclei • Other regions result from placing unshared valence electrons on a single nuclei Lewis Theory PredictsElectron Groups Tro: Chemistry: A Molecular Approach, 2/e

  7. Lewis theory says that these regions of electron groups should repel each other • because they are regions of negative charge • This idea can then be extended to predict the shapes of molecules • the position of atoms surrounding a central atom will be determined by where the bonding electron groups are • the positions of the electron groups will be determined by trying to minimize repulsions between them Using Lewis Theory to PredictMolecular Shapes Tro: Chemistry: A Molecular Approach, 2/e

  8. Electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory • because electrons are negatively charged, they should be most stable when they are separated as much as possible • The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule VSEPR Theory Tro: Chemistry: A Molecular Approach, 2/e

  9. there are three electron groups on N three lone pair one single bond one double bond • • • • • • • • O N O • • • • Electron Groups • The Lewis structure predicts the number of valence electron pairs around the central atom(s) • Each lone pair of electrons constitutes one electron group on a central atom • Each bond constitutes one electron group on a central atom • regardless of whether it is single, double, or triple Tro: Chemistry: A Molecular Approach, 2/e

  10. There are five basic arrangements of electron groups around a central atom • based on a maximum of six bonding electron groups • though there may be more than six on very large atoms, it is very rare • Each of these five basic arrangements results in five different basic electron geometries • in order for the molecular shape and bond angles to be a “perfect” geometric figure, all the electron groups must be bonds and all the bonds must be equivalent • For molecules that exhibit resonance, it doesn’t matter which resonance form you use – the electron geometry will be the same Electron Group Geometry Tro: Chemistry: A Molecular Approach, 2/e

  11. Linear • When there are two electron groups around the central atom, they will occupy positions on opposite sides of the central atom • This results in the electron groups taking a linear geometry • The bond angle is 180° Tro: Chemistry: A Molecular Approach, 2/e

  12. TrigonalPlanar • When there are three electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom • This results in the electron groups taking a trigonal planar geometry • The bond angle is 120° Tro: Chemistry: A Molecular Approach, 2/e

  13. Tetrahedral • When there are four electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom • This results in the electron groups taking a tetrahedral geometry • The bond angle is 109.5° Tro: Chemistry: A Molecular Approach, 2/e

  14. TrigonalBipyramidal • 5 electron groups around a central atomoccupy positions in the shape of two tetrahedrabase-to-base with the central atom in the center of the shared bases • The positions above and below the central atom are called the axial positions • The positions in the same base plane as the central atom are called the equatorial positions • The bond angle between equatorial positions is 120° • The bond angle between axial and equatorial positions is 90° Tro: Chemistry: A Molecular Approach, 2/e

  15. Octahedral • When there are six electron groups around the central atom, they will occupy positions in the shape of two square-base pyramids that are base-to-base with the central atom in the center of the shared bases • This results in the electron groups taking an octahedral geometry • it is called octahedral because the geometric figure has eight sides • All positions are equivalent • The bond angle is 90° Tro: Chemistry: A Molecular Approach, 2/e

  16. The actual geometry of the molecule may be different from the electron geometry • When the electron groups are attached to atoms of different size, or when the bonding to one atom is different than the bonding to another, this will affect the molecular geometry around the central atom • Lone pairs also affect the molecular geometry • they occupy space on the central atom, but are not “seen” as points on the molecular geometry Molecular Geometry Tro: Chemistry: A Molecular Approach, 2/e

  17. Because the bonds and atom sizes are not identical in formaldehyde, the observed angles are slightly different from ideal Not Quite Perfect Geometry Tro: Chemistry: A Molecular Approach, 2/e

  18. Lone pair groups“occupy more space” on the central atom • because their electron density is exclusively on the central atom rather than shared like bonding electron groups • Relative sizes of repulsive force interactions is Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair • This affects the bond angles, making the bonding pair – bonding pair angles smaller than expected The Effect of Lone Pairs Tro: Chemistry: A Molecular Approach, 2/e

  19. The bonding electrons are shared by two atoms, so some of the negative charge is removed from the central atom The nonbonding electrons are localized on the central atom, so area of negative charge takes more space Effect of Lone Pairs Tro: Chemistry: A Molecular Approach, 2/e

  20. Bond Angle Distortion from Lone Pairs Tro: Chemistry: A Molecular Approach, 2/e

  21. Bond Angle Distortion from Lone Pairs Tro: Chemistry: A Molecular Approach, 2/e

  22. Bent Molecular Geometry:Derivative of Trigonal Planar Electron Geometry • When there are three electron groups around the central atom, and one of them is a lone pair, the resulting shape of the molecule is called a trigonal planar — bent shape • The bond angle is less than 120° • because the lone pair takes up more space Tro: Chemistry: A Molecular Approach, 2/e

  23. Pyramidal Geometry: A Derivative of Tetrahedral Electron Geometry • When there are four electron groups around the central atom, and one is a lone pair, the result is called a pyramidal shape • because it is a triangular-base pyramid with the central atom at the apex • Bond angle is less than 109.5°

  24. Bent Tetrahedral Molecular Geometry: A Derivative of Tetrahedral Electron Geometry • When there are four electron groups around the central atom, and two are lone pairs, the result is called a tetrahedral—bent shape • it is planar • it looks similar to the trigonal planar—bent shape, except the angles are smaller • the bond angle is less than 109.5° Tro: Chemistry: A Molecular Approach, 2/e

  25. Derivatives of theTrigonal Bipyramidal Electron Geometry • When there are five electron groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room • Seesaw Shape - five electron groups around the central atom, and one is a lone pair (aka distorted tetrahedron) • T-shaped – 5 electron groups around the central atom, and two are lone pairs • Linear shape– 5 electron groups around the central atom, and three are lone pairs • The bond angles between equatorial positions are less than 120° • The bond angles between axial and equatorial positions are less than 90° • linear = 180° axial–to–axial Tro: Chemistry: A Molecular Approach, 2/e

  26. Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System Tro: Chemistry: A Molecular Approach, 2/e

  27. Seesaw Shape Tro: Chemistry: A Molecular Approach, 2/e

  28. T–Shape Tro: Chemistry: A Molecular Approach, 2/e

  29. T–Shape Tro: Chemistry: A Molecular Approach, 2/e

  30. Linear Shape Tro: Chemistry: A Molecular Approach, 2/e

  31. Derivatives of the Octahedral Geometry • When there are six electron groups around the central atom, and some are lone pairs, each even number lone pair will take a position opposite the previous lone pair • Square Pyramid Shape – 6 electron groups around the central atom, and one is a lone pair, the result is called a • the bond angles between axial and equatorial positions is less than 90° • Square Planar Shape – 6 electron groups around the central atom, and two are lone pairs, the result is called a • the bond angles between equatorial positions is 90° Tro: Chemistry: A Molecular Approach, 2/e

  32. Square Pyramidal Shape Tro: Chemistry: A Molecular Approach, 2/e

  33. Square Planar Shape Tro: Chemistry: A Molecular Approach, 2/e

  34. Tro: Chemistry: A Molecular Approach, 2/e

  35. 1. Draw the Lewis structure 2. Determine the number of electron groups around the central atom 3. Classify each electron group as bonding or lone pair, and count each type • remember, multiple bonds count as one group 4. Use Table 10.1 to determine the shape and bond angles Predicting the Shapes Around Central Atoms Tro: Chemistry: A Molecular Approach, 2/e

  36. Example 10.2: Predict the geometry and bond angles of PCl3 1. Draw the Lewis structure a) 26 valence electrons 2. Determine the Number of electron groups around central atom a) four electron groups around P Tro: Chemistry: A Molecular Approach, 2/e

  37. Example 10.2: Predict the geometry and bond angles of PCl3 3. Classify the electron groups a) three bonding groups b) one lone pair 4. Use Table 10.1 to determine the shape and bond angles a) four electron groups around P = tetrahedral electron geometry b) three bonding + one lone pair = trigonal pyramidal molecular geometry c) trigonal pyramidal = bond angles less than 109.5° Tro: Chemistry: A Molecular Approach, 2/e

  38. Practice – Predict the molecular geometry and bond angles in SiF5− Tro: Chemistry: A Molecular Approach, 2/e

  39. Si least electronegative 5 electron groups on Si Si is central atom 5 bonding groups 0 lone pairs Si = 4e─ F5 = 5(7e─) = 35e─ (─) = 1e─ total = 40e─ Shape = trigonal bipyramid Bond angles Feq–Si–Feq = 120° Feq–Si–Fax = 90° Practice – Predict the molecular geometry and bond angles in SiF5─ Tro: Chemistry: A Molecular Approach, 2/e

  40. Practice – Predict the molecular geometry and bond angles in ClO2F Tro: Chemistry: A Molecular Approach, 2/e

  41. Cl least electronegative 4 electron groups on Cl Cl is central atom 3 bonding groups 1 lone pair Cl = 7e─ O2 = 2(6e─) = 12e─ F = 7e─ Total = 26e─ Shape = trigonal pyramidal Bond angles O–Cl–O < 109.5° O–Cl–F < 109.5° Practice – Predict the molecular geometry and bond angles in ClO2F Tro: Chemistry: A Molecular Approach, 2/e

  42. One of the problems with drawing molecules is trying to show their dimensionality • By convention, the central atom is put in the plane of the paper • Put as many other atoms as possible in the same plane and indicate with a straight line • For atoms in front of the plane, use a solid wedge • For atoms behind the plane, use a hashed wedge Representing 3-Dimensional Shapes on a 2-Dimensional Surface Tro: Chemistry: A Molecular Approach, 2/e

  43. Tro: Chemistry: A Molecular Approach, 2/e

  44. F F F S F F F SF6 Tro: Chemistry: A Molecular Approach, 2/e

  45. Many molecules have larger structures with many interior atoms • We can think of them as having multiple central atoms • When this occurs, we describe the shape around each central atom in sequence shape around left C is tetrahedral shape around center C is trigonal planar Multiple Central Atoms shape around right O is tetrahedral-bent Tro: Chemistry: A Molecular Approach, 2/e

  46. Describing the Geometryof Methanol Tro: Chemistry: A Molecular Approach, 2/e

  47. Describing the Geometryof Glycine Tro: Chemistry: A Molecular Approach, 2/e

  48. Practice – Predict the molecular geometries in H3BO3 Tro: Chemistry: A Molecular Approach, 2/e

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