Molecular Geometry (Shapes of Molecules)
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Transcript of Molecular Geometry (Shapes of Molecules)
Molecular Geometry(Shapes of Molecules)
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VSEPR Theory
CP Chemistry
VSEPR Theory
• Electron groups around the central atom will be most stable when they are as far apart as possible – we call this valence shell electron pair repulsion theory– because electrons are negatively
charged, they should be most stable when they are separated as much as possible
• The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule
Electrons vs. Molecular Geometry
• The geometry of electron pairs around a central atom is called the electron geometry.
• The arrangement of bonded nuclei around a central atom forms the molecular geometry.
• Lone pair electrons on a central atom will repel other pairs but will not be visible in the molecular geometry (no nuclei)
• If there are lone pairs on the central atom the electron geometry and the molecular geometry will differ.
Examples:
CS2, HCN, BeF2
Two electron pairs on central atom
3 electronpairs on central atom•All are in bonds
•Trigonal Planar Shape
Examples:
SO3, BF3, NO3-, CO3
2-
Other Examples:
•SO2, O3, PbCl2, SnBr2
• Un-bonded electron pair takes up more space and “repels” more.
• The bond angle will change to less than the original 120o to about 117o-115o
About 117o
3 Electron Pairs•2 Bonded •1 Un-Bonded
Bent Shape
Four electron pairs on central atom•All 4 in bonds
Examples:
CH4, SiCl4, SO4
2-, ClO4-
Examples: NH3, PF3, ClO3. H3O+
4 Electron Pairs•3 bonded•1 un-bonded (lone pair)Trigonal Pyramidal Shape
Bond angles are reduced from 109.5o to 107o due to extra repulsion by lone pair
Examples: H2O, OF2, SCl2
4 Electron Pairs2 Bonded2 Un-bonded (lone pairs)BENT SHAPE
Bond angles are reduced a little more due to repulsionTo 104.5o
The steps in determining a molecular shape
Molecular formula
Lewis structure
Electron-group arrangement
(electron geometry) Bond
angles
Molecular geometry
Count all e- pairs around central atom
Note lone pairs and double bonds
Consider bonding e- pairs only
Step 1
Step 2
Step 3
Step 4
Representing 3-Dimensional Shapes on a 2-Dimensional
Surface• One of the problems with drawing molecules is trying to show their dimensionality
• By convention, the central atom is put in the plane of the paper
• Put as many other atoms as possible in the same plane and indicate with a straight line
• For atoms in front of the plane, use a solid wedge
• For atoms behind the plane, use a hashed wedge
Predicting Molecular Shapes with Two, Three, or Four Electron Groups
PROBLEM: Draw the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) PF3
SOLUTION: (a) For PF3 - there are 26 valence electrons, 1 nonbonding pair
PF F
F
The shape is based upon the tetrahedral arrangement.
The F-P-F bond angles should be <109.50 due to the repulsion of the nonbonding electron pair.
The final shape is trigonal pyramidal.
PF F
F
< 109.50
Predicting Molecular Shapes with More Than One Central Atom
SOLUTION:
PROBLEM: Determine the shape around each of the central atoms in acetone, (CH3)2C=O.
Find the shape of one atom at a time after writing the Lewis structure.
C C C
OH
H
H
HH
H
tetrahedral tetrahedral
trigonal planar
C
O
HC
HHH
CH
H>1200
<1200