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Molecular Shape The Geometry of molecules. Molecular Geometry nuclei The shape of a molecule is determined by where the nuclei are located. nuclei electron.

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Presentation on theme: "Molecular Shape The Geometry of molecules. Molecular Geometry nuclei The shape of a molecule is determined by where the nuclei are located. nuclei electron."— Presentation transcript:

1 Molecular Shape The Geometry of molecules

2 Molecular Geometry nuclei The shape of a molecule is determined by where the nuclei are located. nuclei electron But the nuclei go to certain locations because of the electron pairs. Goal: minimize electron-pair repulsions.

3 Molecular Shape Electron pairs repel each other. They want to be as far apart from each other as they can. Nonbonding pairs take up a little more room than bonding pairs They will repel with a greater force.

4 To determine molecular geometry start with the Lewis e - dot Structure Lewis dot structures are 2-D, but they can help you figure out the 3-D shape. To help us predict the shape of molecules we will use the VSEPR Theory.

5 VSEPR Theory Valence Shell Electron Pair Repulsion 1) Draw the Lewis structure. 2) Identify the regions of high electron density {Bonding and nonbonding Sites} on the central atom. a) Each single, double, &/or triple bond counts as 1 region of bonded electron density. b) Each nonbonding electron pair counts as 1 region of non-bonded electron density. c) An unpaired electron counts as 1 region of non-bonded electron density. 3) Assign a VSEPR formula according to the # of regions. Resonance structures will fluctuate their regions.

6 VSEPR Theory The shape is always referenced around the central atom. Determine the formula using A as the central atom X as the bonded atoms or shared pairs around the central atom(A). E as the unshared electron pairs. Example H 2 O would be AX 2 E 2  A = O ; X 2 = H 2 ; E 2 = 2 unshared pairs of e-.

7 Most Common VSEPR Shapes VSPER Formula # bonding sites Most stable arrangement AX 2 2Linear AX 3 3Trigonal Planar AX 4, AX 3 E, AX 2 E 2 4 Tetrahedral, Trigonal Pyramidal or Bent AX 5 5Trigonal bipyramidal AX 6 6Octahedral

8 TWO Share Pairs. LINEAR 2 bonding regions. 0 unshared pairs. Linear. Bond angle = 180 . Central atom & regions of electron density arranged in a straight line. :Cl  Be  Cl: : : : : AX 2

9 Three Shared pairs Trigonal Planor Bond angle = 120 . B :F:    :F: :F: : :: 3 bonding regions. 0 unshared pairs. 3 bonding regions. 0 unshared pairs. AX 3

10 3 regions of high electron density 3 regions of high electron density. NO 2 - Lewis Structure: O  N  O: : : : : : 1- 3 unequal regions of electron density Since only 2 are shared, the molecule will look bent. What is the bond angle? 1 unshared pair. 1 unshared pair. 2 Shared pairs ( one single or double bonds). 2 Shared pairs ( one single or double bonds). AX 2 E

11 Bent slightly < 120° AX 2 E

12 Four shared pairs 4 shared pairs 0 unshared pairs Lewis structure:H HCHHCHHCHHCH H   Tetrahedral Bond angles = . Bond angles = . AX 4

13 Trigonal Pyramidal 3 shared pair 1 unshared pair. HNHHNHHNHHNH:H  4 regions of electron density would be apart. But only 3 end in atoms, not all 4. The molecule will look like a squashed pyramid. Trigonal pyramidal. Bond angles? 107° AX 3 E

14 4 regions of electron density. 2 Shared pairs & 2 unshared pairs NH 2 - with 8 valence electrons: 4 regions of electron density but only 2 end in atoms. Molecule will look bent. AX 2 E 2 Bent °

15 5 regions of electron density # of bonding regions # of lone pairsShape 50Trigonal Bipyramid 41“See-Saw” 32T-Structure 23Linear

16 Trigonal Bipyramidal

17 See-Saw” “See-Saw”

18 T-shaped

19 Linear

20 6 regions of electron density # of bonding regions # of lone pairs shape 60Octahedral 51 Square Pyramid 42 Square Planar

21 Octahedral

22 Square Pyramid

23 Square Planar


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