notes: 8.3 vsepr theory · bonding and non-bonding electron pairs of electrons on a molecule...
TRANSCRIPT
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NOTES: 8.3 –
VSEPR Theory
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Molecular Shape
● Lewis structures (electron dot structures)
show the structure of molecules…but only
in 2 dimensions (flat)…
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Molecular Shape
BUT, molecules are 3 dimensional!
● in 3D it is:
a tetrahedron!
= coming out of page
= going into page
= flat on page
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Why do molecules take on 3D
shapes instead of being flat?
● Valence Shell Electron Pair
Repulsion theory
● “because electron pairs repel one another,
molecules adjust their shapes so that the
valence electron pairs are as far apart
from another as possible.”
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Why do molecules take on 3D
shapes instead of being flat?
● Valence Shell Electron Pair Repulsion
theory
● Remember: both shared and unshared electron
pairs will repel one another.
H—N — H
H
Non-BondingPairs
BondingPairs
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VSEPR Theory
● Predicts molecular geometry by examining bonding and non-bonding electron pairs of electrons on a molecule
Bonding pair of electrons – electron pair used in a bond
Non-bonding pair of electrons – lone pair of electrons not used in bonding
● The assumption is electron pairs will be spaced out as far apart as possible (negatives charges repel) giving rise to particular molecular geometries
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How to determine molecule
shape:
● Draw electron dot or structural formula
● Count the number of bonding and non-bonding pairs of electrons around the central atom (number of places electrons are found)
● Multiple (double, triple) bonds count as one “location” or “region”
● Apply the correct geometry predicted by VSEPR Theory based on the number of bonding and non-bonding electron pairs
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5 Basic Molecule Shapes
1) Linear
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5 Basic Molecule Shapes
1) Linear
● Example: CO2
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5 Basic Molecule Shapes
2) Bent
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5 Basic Molecule Shapes
2) Bent
● Example: H2O
**Notice electron pair repulsion!!!
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5 Basic Molecule Shapes
3) tetrahedral
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5 Basic Molecule Shapes
3) tetrahedral
● example: CH4
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5 Basic Molecule Shapes
4) Pyramidal
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5 Basic Molecule Shapes
4) Pyramidal
● Example: NH3
(note: unshared pair of electrons repels, but
is not considered part of overall shape; no
atom there to contribute to the shape)
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5 Basic Molecule Shapes
5) Trigonal planar
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5 Basic Molecule Shapes
5) Trigonal planar
● Example: NO3-
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