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Molecular Geometry

Molecular Geometry. VSEPR model. V alence S hell E lectron P air R epulsion Shape of molecule is determined by the repulsion of electron pairs around the central atom . The repulsion must be minimum , i.e. Electron pairs are farthest from each other ( potential energy minimum ).

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Molecular Geometry

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  1. Molecular Geometry VSEPR model • Valence Shell Electron Pair Repulsion • Shape of molecule is determined by the repulsion of electron pairs around the central atom. • The repulsion must be minimum, i.e. Electron pairs are farthest from each other (potential energy minimum).

  2. Rules • Draw the right Lewis Structure (formal charge!). • Determine the number of electron pairs around the central atom. • Multiple bonds are treated as if they were one electron pair because these electron pairs can not be separated from each other • 2, 3, 4, 5 or 6 EP.

  3. CA Two EP around Central Atom (AX2) BeCl2 Cl Be Cl 2 EP Cl Be Cl Shape:linear Bond angleCl-Be-Cl = 180º X-A-X

  4. F F B F CA Three EP around Central Atom (AX3) 3 EP AX3 BF3 F B F F Shape:planar triangular Bond angleF-B-F = 120º X-A-X

  5. O S O CA Three EP around Central Atom (AX3) 3 EP AX2E SO2 S O O < Shape:V-shaped (bent) Bond angleO-S-O = 120º X-A-X Lone EP occupies a larger space than bonding EP.

  6. H C H H H CA Four EP around Central Atom (AX4) 4 EP AX4 CH4 H C H H H Shape:tetrahedral Bond angleH-C-H = 109.5º X-A-X

  7. N H H H CA N H H H Four EP around Central Atom (AX4) AX3E 4 EP NH3 < Shape:trigonal pyramidal Bond angleH-N-H = 109.5º X-A-X Lone EP occupies a larger space than bonding EP.

  8. O H H CA O H H Four EP around Central Atom (AX4) AX2E2 4 EP H2O < Shape:V-shaped Bond angleH-O-H = 109.5º X-A-X Lone EP occupies a larger space than bonding EP.

  9. Cl Cl P Cl Cl Cl CA Five EP around Central Atom (AX5) AX5 5 EP PCl5 Cl Cl Cl P Cl Cl Shape:trigonal bipyramidal Bond angleCl-P-Cl = 120º, 90º X-A-X

  10. F S F F F F CA F S F F Five EP around Central Atom (AX5) AX4E 5 EP SF4 Lone EP occupies a larger space than bonding EP. Shape:disordered tetrahedral Bond angleCl-P-Cl = 120º, 90º X-A-X

  11. Cl I Cl Cl Cl CA Cl I Cl Five EP around Central Atom (AX5) ICl3 5 EP AX3E2 Lone EP occupies a larger space than bonding EP. Shape:T-shaped Bond angleCl-P-Cl = 120º, 90º X-A-X

  12. Cl I Cl Cl CA I Cl Five EP around Central Atom (AX5) ICl2- 5 EP AX2E3 Lone EP occupies a larger space than bonding EP. Shape:linear Bond angleCl-I-Cl = 180º X-A-X

  13. F F F S F F F CA Six EP around Central Atom (AX6) SF6 6 EP AX6 F F F S F F F Shape:octahedral Bond angleF-S-F = 90º X-A-X

  14. Cl Cl Cl I Cl Cl CA Six EP around Central Atom (AX6) ICl5 6 EP AX5E Cl Cl Cl I Cl Cl Shape:square pyramidal Bond angleCl-I-Cl = 90º X-A-X

  15. F F Xe F F CA Six EP around Central Atom (AX6) XeF4 6 EP AX4E2 F F Xe F F Shape:square planar Bond angleF-Xe-F= 90º X-A-X

  16. Polarity of molecules • Molecules polar if net dipole moment ≠ 0 • If molecule is “pure” AXn type • No lone EP • All surrounding atoms are the same element non-polar m1 m3 m2 m1 m2

  17. Usually, if there is a lone EP  polar NH3 AX3E H2O AX2E2

  18. Exceptions: Cl AX2E3 I Cl F F AX4E2 Xe F F

  19. Different ligand atoms Cl m3 BF2Cl m1+m2 > m3 m1 B F Net dipole moment ≠ 0 m2 m1+m2 F XeF2Cl2 F F Cl Cl Xe Xe Cl Cl F F nonpolar polar

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