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Molecular GeometriesUnit 3: Chemical Bonding
V S
E P
R
+Lesson Essential Question:
How can Lewis Structures be used to determine molecular geometry?
EQ: How can Lewis structures be used to predict molecular geometries in covalently bonded molecules?
The VSEPR theory is used. VSEPR stands for: ________________________________________________
______________________________which basically means that when a molecule shares electrons it tries to
minimize repulsions between them in the arrangement formed.
HOW can we figure this out? Follow a step wise procedure
1.____________________________________________________________________________________
2. ___________________________________________________________________________________
3. ___________________________________________________________________________________
4.____________________________________________________________________________________
Remember the most stable electron arrangement is when all atoms can achieve an octet.
Continue with remaining steps as needed . . . #5., #6. #7. etc from separte handout
then explain the geometry of the shape formed.
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How can Lewis Structures be used to determine molecular geometry? How can Lewis Structures be used to determine molecular geometry?
Molecular Geometry
# of Bonded Atoms
Number of Unshared Electron Pairs
ABE notation(AXE)
Bond Angles
PredictedMoleculePolarity
Linear
Bent
Trigonal Planar
Trigonal Pyramidal
Tetrahedral
+VSEPR stands for….
ValenceShell
ElectronPairRepulsion
The structure around a given atom is determined principally by minimizing electron pair repulsions.
+How to predict a VSEPR Structure?
follow steps on the handout . .
1. Count the total number of valence electrons available from all atoms involved in the molecule. (if the molecule acts as an ion, add or subtract electrons here)2. Attach each atom to the center molecule with a single bond (2 electrons (dots) for each line)3. Complete the octet for each of the attached
atoms by distributing remaining electrons in pairs
4. Complete the octet around the center atom.
+Electron Pair Count and Geometry
Linear – two pairs on the central atom.
Trigonal – three pairs on the central atom.
Tetrahedral – four pairs on the central atom.
+How do I figure out the geometry?
Shared Electron Pair #
GeometricalArrangement
Types of e- Pairs
Molecular Shapes
Ex’s Ball and Stick Model
2 Linear,180o
2 BP Linear
BeCl2
Be– 2 valence e-2 Cl – 14 valence e-Total = 16 valence e- C
lBe
Cl
Two Bonding Pair (BP) of electrons and no unbonded pair of electrons fits the Linear criteria.
AB2E0
+Shared Electron Pair #
GeometricalArrangement
Types of e- Pairs
Molecular Shapes
Ex’s Ball and Stick Model
2Bent
104.5o2 BP1 LP
BentSO2
S – 6 valence e-2 O– 12 valence e-Total = 18 valence e-
O S OThis structure has resonance so the bonds between the S and O represent 2 Bonding Pairs (BP) of electrons.
One Lone Pair (LP) of electrons.
AB2E1
+Electron Pair #
GeometricalArrangement
Types of e- Pairs
Molecular Shapes
Ex’s Ball and Stick Model
2 Bent104.5o
2 BP2 LP
BentH2O
2 H – 2 valence e-O– 6 valence e-Total = 8 valence e- H O H
3 Bonding Pairs of electrons and Two Lone Pair of electrons has this compound as a Trigonal Pyramidal.
AB2E2
+Electron Pair #
GeometricalArrangement
Types of e- Pairs
Molecular Shapes
Ex’s Ball and Stick Model
3Trigonal Planar,120o
3 BP Trigonal Planar
BF3
B– 3 valence e-3 F – 21 valence e-Total = 24 valence e-
F B FF
Three Bonding Pair (BP) of electrons and no unbonded pair of electrons fits the Trigonal Planar criteria.
AB3E0
+SharedElectron Pair #
GeometricalArrangement
Types of e- Pairs
Molecular Shapes
Ex’s Ball and Stick Model
3 Trigonal Pyramidal
107o
3 BP1 LP
Trigonal Pyramidal
NH3
N – 5 valence e-3 H– 3 valence e-Total = 8 valence e-
H N HH
3 Bonding Pairs of electrons and one Lone Pair of electrons has this compound as a Trigonal Pyramidal.
AX3E1
+Electron Pair #
GeometricalArrangement
Types of e- Pairs
Molecular Shapes
Ex’s Ball and Stick Model
4 Tetrahedral
109.5o
4 BPTetrahedr
al CH4
C – 4 valence e-4 H– 4 valence e-Total = 8 valence e-
H CH
HH
4 Bonding Pairs of electrons and no Lone Pair of electrons has this compound as a Tetrahedral.
AB4E0
+Dotted Line and Wedge Model
Linear AX2E0
Trigonal Planar AX3E0
Bent AX2E1
+Dotted Line and Wedge Model
Tetrahedral AX4E0
Trigonal Pyramidal
AX3E1
Bent AX2E2