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Molecular Geometry

Molecular Geometry

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Molecular Geometry

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  1. Molecular Geometry • Lewis structures show the number and type of bonds between atoms in a molecule. • All atoms are drawn in the same plane (the paper). • Do not show the shape of the molecule.

  2. Molecular Shapes • The shape of a molecule plays an important role in its reactivity. • The shape of a molecule is determined by the bond angles and the bond lengths. • By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.

  3. Molecular Geometry • Bond length:the distance between two atoms held together by a chemical bond • Bond length decreases as the number of bonds between two atoms increases. • Single bond is the longest. • Triple bond is the shortest.

  4. H H O Molecular Geometry • Bond angle:the angle made by the “lines” joining the nuclei of the atoms in a molecule 104.5o

  5. Molecular Geometry • Many of the molecules we have discussed have central atoms surrounded by 2 or more identical atoms: ABn where A = central atom B = outer atoms n = # of “B” atoms Examples: CO2, H2O, BF3, NH3, CCl4

  6. Molecular Geometry • The shapes that ABn molecules can have depend, in part, on the value of n. • For a specific value of n, only a few general shapes are observed. • AB2 molecules • linear • bent

  7. H O H Molecular Geometry • AB2 molecules can either be linear or bent. CO2 linear O O C H2O bent

  8. F F B F Molecular Geometry • AB3 molecules can either be trigonal planar, trigonal pyramidal, or T-shaped. • Trigonal planar: “A” atom in the center and “B” atoms at each corner of an equilateral triangle. All atoms in the same plane.

  9. H H H Molecular Geometry • Trigonal pyramidal: “A” atom in the center with “B” atoms in the corners of an equilateral triangle. • “A” is above the plane of the triangle formed by “B” atoms N

  10. Molecular Geometry • Why are some AB2 molecules linear while others are bent? • Why are some AB3 molecules trigonal planar while others are trigonal pyramidal or T-shaped? • How can we accurately predict the shape of various ABn molecules?

  11. Molecular Geometry • If “A” is a main group element, the valence-shell electron-pair repulsion model (VSEPR) can be used to predict the shape of an ABn molecule. • VSEPR counts the number of regions around the central atom where electrons are likely to be found and uses this number to predict the shape.

  12. Molecular Geometry • Electron domains:regions around the central atom where electrons are likely to be found. • Two types of electron domains are considered: • bonding pairs of electrons • nonbonding (lone) pairs of electrons

  13. Molecular Geometry • Bonding pairs of electrons: electrons that are shared between two atoms Cl Cl C Cl Cl Bonding pairs CCl4 has 4 bonding pairs, C has 4 electron domains Bonding pairs

  14. Molecular Geometry • Nonbonding (lone) pairs of electrons: electrons that are found principally on one atom, not in between atoms • = unshared electrons • H N H • H Nonbondingpair

  15. Molecular Geometry • N in Ammonia (NH3) has 4 electron domains: H N H H 1 nonbonding pair 3 bonding pairs

  16. Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

  17. Molecular Geometry • By considering the arrangement that minimizes repulsions between electron domains, we can determine the electron domain geometry • The arrangement of electron domains around thecentral atom

  18. Molecular Geometry 3electron domains 2electron domains Linear electron domain geometry Trigonal planar e- domain geometry

  19. Molecular Geometry 4electron domains Tetrahedral electron domain geometry Trigonal bipyramidal e- domain geometry 5electron domains

  20. Molecular Geometry 6electron domains Octahedral electron domain geometry

  21. Electron-Domain Geometries • All one must do is • draw the Lewis structure • count the total number of electron domains around the central atom • double and triple bonds count as 1 electron domain • The geometry will be that which corresponds to the number of electron domains.

  22. O C O Molecular Geometry Determine the electron domain geometry of CO2. Valence electrons:16 Lewis structure: # of electron domains of C: 2 linear Electron domain geometry:

  23. Cl Cl P Cl Molecular Geometry Determine the electron domain geometry of PCl3. Valence electrons:26 Lewis structure: # of electron domains of P: 4 tetrahedral Electron domain geometry:

  24. Molecular Geometries • The electron-domain geometry is often not the shape of the molecule, however. • The molecular geometry is that defined by the positions of onlythe atoms in the molecules, not the nonbonding pairs. • Molecular geometry is a consequence of electron-domain geometry.

  25. O Molecular Geometry • H2O has 4 electron domains-- • electron-domain geometry = tetrahedral If you ignore the lone pairs of electrons, however, the atoms are arranged in abent shape. H H

  26. Molecular Geometry • The molecular geometry is a consequence of electron domain geometry because the lone pairs of electrons take up space around the central atom. • This forces the atoms in the molecule to occupy positions around the central atom in a way that minimizes repulsion between the electron domains.

  27. Molecular Geometry • Electron domain geometry and molecular geometry are the same only if there are no non-bonding electron domains. • See tables 9.2 and 9.3 for the relationship between electron domain geometries and molecular geometries.

  28. Linear Electron Domain • In the linear domain, there is only one molecular geometry: linear. • NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

  29. Trigonal Planar Electron Domain • There are two molecular geometries: • Trigonal planar, if all the electron domains are bonding, • Bent, if one of the domains is a nonbonding pair.

  30. Tetrahedral Electron Domain • There are three molecular geometries: • Tetrahedral, if all are bonding pairs, • Trigonal pyramidal if one is a nonbonding pair, • Bent if there are two nonbonding pairs.

  31. Trigonal Bipyramidal Electron Domain • There are four distinct molecular geometries in this domain: • Trigonal bipyramidal • Seesaw • T-shaped • Linear

  32. Octahedral Electron Domain • All positions are equivalent in the octahedral domain. • There are three molecular geometries: • Octahedral • Square pyramidal • Square planar

  33. Molecular Geometry Trigonal planar Tetrahedral

  34. Molecular Geometry Trigonal bipyramidal octahedral

  35. Molecular Geometry • In order to determine the actual molecular geometry: • draw the Lewis structure • count the total # of electron domains • multiple bonds = 1 electron domain • determine the electron-domain geometry • describe the molecular geometry in terms of the arrangement of the bonded atoms

  36. Molecular Geometry What is the molecular geometry of NH3? Lewis Structure: # of electron domains = 4

  37. Molecular Geometry • Electron domain geometry: tetrahedral • Molecular geometry: trigonal pyramidal

  38. Molecular Geometry Example: Predict the molecular geometry of IF5. Lewis structure: # electron domains:

  39. Molecular Geometry • Electron domain geometry = octahedral • Molecular geometry = square pyramidal

  40. Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

  41. Polarity of Molecules • Consider the carbon dioxide molecule: • contains two polar covalent bonds • nonpolar molecule • Just because a molecule contains polar covalent bonds does not mean the molecule as a whole will be polar.

  42. d- d+ d- d+ d+ d- d+ d+ Polarity of Molecules • Polar Molecules • contain polar covalent bonds which are asymmetrically distributed within the molecule • contain a “positive” end and a “negative”end • Examples: • HCl • H2O • CH3OH

  43. Polarity of Molecules • Polar molecules have large dipole moments • A measure of the separation between the positive and negative charges in polar molecules. d+ d- H – F d+ d-

  44. Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. • The overall polarity of a molecule is determined by doing a vector addition of the individual bond dipoles: • add both the magnitude and direction of the dipole moments • must consider the molecular geometry!

  45. Polarity of Molecules • Examples: