Molecular Shape

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Molecular Shape
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Things to remember

• shape of molecule
• determined by location of nuclei
• nuclei go to certain locations because of its
  electron pairs

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Use the Lewis Structure

• Lewis structure is 2-D, but can help figure out
  3-D shape

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Bonding Capacity
..
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Molecular Shape

• Determined by overlap of orbitals
• Shape determined by two factors
• total atoms
• e- pairs in different locations on central
  atom
• Classify electron pairs as bonding or nonbonding

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Molecular Shape VSEPR

• Electron pairs repel each other
• want to be as far apart from each other as can be
• Nonbonding pairs take up a little more room than
  bonding pairs

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Orientation of Electron Pairs
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2-Atom Molecules

• Atoms located right next to each other
• linear molecules!

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3-Atom Molecules

• linear or bent

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CO2
Lewis structure O C O
4 bonding pairs But only 2 regions of electron
density 180? apart so CO2 is linear
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H2O

..

• 2 bonding pairs between O and the Hs
• 2 nonbonding pairs on the O
• 4 different regions of electron density

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H2O

• 4 electron pairs are 109? apart
• nonbonding pairs spread out a bit more
• squeeze the bonding pairs together
• bent shape
• H O
• H

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3-Atom Molecules

• Triangular?

Yes, ozone (O3) is triangular
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4-Atom Molecules

• Two possibilities
• Trigonal Planar in 1 plane
• Trigonal Pyramidal

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NH3
H

• 3 bonding pairs
• 1 nonbonding pair
• 4 regions
• 109? apart

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CH4
..

• 4 bonding pairs
• 4 regions of electron density
• electron pairs are 109? apart

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4-Atom Molecules Trigonal Planar
Bond angles 120? All 4 atoms lie
in the same plane
You arent likely to see this because these
molecules dont obey the octet rule!
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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 dont obey octet rule - so arent
likely to see them - 3 regions - FBF 120?
..
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• 4-atom molecule
• Shape trigonal pyramid
• A bit less than 109?

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4-Atom Molecules AX3
shape most likely to see
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5-Atom Molecules AX4
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5 Atoms Tetrahedrons
Tetrahedral means 4 faces - 1 atom in center -
4 atoms at the points
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CH4 a 5-atom molecule
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CH4 a 5-atom molecule
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Summary of Molecular Shapes

• Start with Lewis Structure!
• Look at regions of electron density on central
  atom
• Look at atoms bonded to central atom

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Molecular Polarity

• Look at type bonds in molecule
• Look at shape of molecule
• nonpolar molecular must be symmetrical

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Molecular Polarity

• molecule is symmetric if
• electrical charge on 1 side electrical charge
  at matching point on opposite side
• pull of one polar bond is offset by pull of
  another polar bond
• tug-of-war that can't be won!

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