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MOLECULAR SHAPES

Learn about molecular geometry and how it determines the reactivity of covalent compounds. Explore the concepts of bond lengths and angles, as well as electron domains and VSEPR theory.

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MOLECULAR SHAPES

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  1. MOLECULAR SHAPES OF COVALENT COMPOUNDS

  2. 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.

  3. 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.

  4. 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.

  5. H H O Molecular Geometry • Bond angle:the angle made by the “lines” joining the nuclei of the atoms in a molecule 109.5o

  6. 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

  7. VSepR tHEORY ALENCE HELL VSEPR LECTRON AIR EPULSION

  8. What Vsepr means Since electrons do not like each other, because of their negative charges, they orient themselves as far apart as possible, from each other. This leads to molecules having specific shapes.

  9. Things to remember • Atoms bond to form an Octet (8 outer electrons/full outer energy level) • Bonded electrons take up less space then un-bonded/unshared pairs of electrons.

  10. 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

  11. 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

  12. 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

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

  14. 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.”

  15. HERE ARE THE RESULTING MOLECULAR SHAPES

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

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

  18. Molecular Geometry 6electron domains Octahedral electron domain geometry

  19. Linear EXAMPLE: BeF2 • Number of Bonds = 2 • Number of Shared Pairs of Electrons = 2 • Bond Angle = 180°

  20. Trigonal Planar EXAMPLE: GaF3 • Number of Bonds = 3 • Number of Shared Pairs of Electrons = 3 • Number of Unshared Pairs of Electrons = 0 • Bond Angle = 120°

  21. Bent EXAMPLE: O3 • Number of Bonds = 2 • Number of Shared Pairs of Electrons = 2 • Number of Unshared Pairs of Electrons = 1 • Bond Angle = 120°

  22. Super Bent EXAMPLE: H2O • Number of Bonds = 2 • Number of Shared Pairs of Electrons = 2 • Number of Unshared Pairs of Electrons = 2 • Bond Angle = 109.5

  23. Tetrahedral EXAMPLE: CH4 • Number of Bonds = 4 • Number of Shared Pairs of Electrons = 4 • Number of Unshared Pairs of Electrons = 0 • Bond Angle = 109.5°

  24. Trigonal Pyramidal EXAMPLE: NH3 • Number of Bonds = 3 • Number of Shared Pairs of Electrons = 4 • Number of Unshared Pairs of Electrons = 1 • Bond Angle = <109.5°

  25. Trigonal bIPyramidal EXAMPLE: NbF5 • Number of Bonds = 5 • Number of Shared Pairs of Electrons = 5 • Number of Unshared Pairs of Electrons = 0 • Bond Angle = <120°

  26. OCTAHEDRAL EXAMPLE: SF6 • Number of Bonds = 6 • Number of Shared Pairs of Electrons = 6 • Number of Unshared Pairs of Electrons = 1 • Bond Angle = 90°

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