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Molecular Shape

Molecular Shape. Things to remember. shape of molecule influenced by location of nuclei nuclei go to certain locations because of its electron pairs. Use the Lewis Structure. Lewis structure is 2-D, but can help figure out 3-D shape. Atom. Lewis Structure. # Unpaired Electrons.

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Molecular Shape

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  1. Molecular Shape

  2. Things to remember shape of molecule influenced by location of nuclei • nuclei go to certain locations because of its electron pairs

  3. Use the Lewis Structure • Lewis structure is 2-D, but can help figure out 3-D shape

  4. Atom Lewis Structure # Unpaired Electrons Bonding Capacity . H H 1 1 .. F, Cl, Br, I ·F: 1 1 .. . C, Si ·C· 4 4 . .. N, P ·N· 3 3 . O, S ·O: 2 2 · ·· Ne, Ar, Kr :Ne: 0 0 ·· Bonding Capacity ..

  5. Molecular Shape • Determined by overlap of orbitals • Shape determined by two factors: • total # atoms & • # e- pairsin different locationson central atom • classify e- pairs as bonding or non-bonding

  6. Molecular Shape & VSEPR • electron pairs repel each other: • want to be as far apart from each other as can be • non-bonding pairs take up a little more room than bonding pairs

  7. Total # of electron pairs Orientation 1 or 2 linear: 2 electron pairs 180 apart 3 planar Triangular: 3 electron pairs 120 apart 4 tetrahedral: 3-D, 4 e- pairs 109 apart Orientation of Electron Pairs

  8. 2-Atom Molecules • atoms located next to each other • linear molecules!

  9. Linear Bent 3-Atom Molecules • linear or bent

  10. .. .. .. .. CO2 Lewis structure: O :: C :: O 4 bonding pairs but only2 regionsof electron density: 180 apart so CO2 is linear

  11. Lewis Structure of H2O: H:O: H .. Count up the electron pairs: H2O .. • 2 bonding pairs between each O and H • 2 nonbonding pairs on O • 4 different regions of electron density

  12. H2O • 4 electron pairs are 109 apart • nonbonding pairs spread out a bit more • squeeze bonding pairs together so bent shape H – O H

  13. 3-Atom Molecules • Triangular? Yes, ozone (O3) is triangular

  14. 4-Atom Molecules • Two possibilities: • Trigonal Planar – in 1 plane • Trigonal Pyramidal

  15. 4-Atom Molecules: Trigonal Planar Bond angles =120 All 4 atoms lie in the same plane aren’t likely to see this because thesemolecules don’t obey octet rule!

  16. .. .. F .. .. .. .. .. F .. .. BF3: Trigonal Planar • B has 3 valence electrons • very small atom e • each F has 7 valence electrons • total = 3 X 7 + 3 = 24 e- : F : B These molecules don’t obey octet rule - so aren’t likely to see them - 3 regions - FBF = 120 ..

  17. 4-Atom Molecules: AX3 shape most likely to see

  18. .. Lewis Structure: H : N : H .. NH3 H • 3 bonding pairs • 1 non-bonding pair • 4 regions • little less than 109 apart • shape: trigonal pyramidal

  19. 5-Atom Molecules: AX4

  20. 5 Atoms & Tetrahedrons Tetrahedral means 4 faces: - 1 atom in center - 4 atoms at the points

  21. CH4 = a 5-atom molecule

  22. H Lewis structure = H : C : H .. H CH4 .. • 4 bonding pairs • 4 regions of electron density • electron pairs are 109 apart

  23. CH4 = a 5-atom molecule

  24. Summary of Molecular Shapes • start with Lewis Structure! • look at # regions of e-density on central atom • look at # atoms bonded to central atom

  25. Molecular Polarity • Look at shape of molecule: • non-polar molecular is symmetrical • polar molecular is asymmetrical

  26. Molecular Polarity • molecule is symmetric if: • electrical charge on 1 side is same as electrical charge at matching point on opposite side • “pull” of one polar bond is offset by “pull” of another polar bond

  27.  Nonpolar Symmetrical Molecules are Non-polar

  28. Asymmetric Molecules are Polar

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