molecular geometry and polarity
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Bond Angles in Carbon Compoundselectron configuration = 1s22s22p2
If they can, the bond angles should be 90o.
But…the bond angles are 109.5o!
2p orbitals with oneelectron in each.
Orbitals with oneelectron in each willoverlap to form singlebonds.
Can p orbitals with oneelectron in each findthe place where the3rd p orbital shouldbe?
It’s All in the Shape…
• So what’s going on?• Think back to the lab…• What is the primary reason
molecules form the geometry we find?
• Electron Pair Repulsion
VSEPR - Valence Shell Electron Pair Repulsion Theory
Each group of valence electrons around a central atom is located as faraway as possible from the others in order to minimize repulsions.
These repulsions maximize the space that each object attached to thecentral atom occupies.
The result is five electron-group arrangements of minimum energy seenin a large majority of molecules and polyatomic ions.
The electron-groups are defining the object arrangement, but themolecular shape is defined by the relative positions of the atomic nuclei.
Because valence electrons can be bonding or nonbonding, the sameelectron-group arrangement can give rise to different molecular shapes.
AXmEn
A - central atom X -surrounding atomE -nonbonding valence electron-group
integers
Silberberg, Principles of Chemistry
Figure 10.3
Electron-group repulsions and the five basic molecular shapes.
linear trigonal planar tetrahedral
trigonal bipyramidal octahedral
Silberberg, Principles of Chemistry
Figure 10.4 The single molecular shape of the linear electron-group
arrangement.
Examples:
CS2, HCN, BeF2
Silberberg, Principles of Chemistry
Figure 10.5 The two molecular shapes of the trigonal planar electron-group arrangement.
Class
Shape
Examples:
SO3, BF3, NO3-, CO3
2-
Examples:
SO2, O3, PbCl2, SnBr2
Silberberg, Principles of Chemistry
Figure 10.6The three molecular shapes of the tetrahedral electron-
group arrangement.
Examples:
CH4, SiCl4, SO4
2-, ClO4-
NH3
PF3
ClO3
H3O+
H2O
OF2
SCl2
Silberberg, Principles of Chemistry
Figure 10.8
The four molecular shapes of the trigonal bipyramidal electron-group arrangement.
SF4
XeO2F2
IF4+
IO2F2-
ClF3
BrF3
XeF2
I3-
IF2-
PF5
AsF5
SOF4
Silberberg, Principles of Chemistry
Figure 10.9
The three molecular shapes of the octahedral electron-group arrangement.
SF6
IOF5
BrF5
TeF5-
XeOF4
XeF4
ICl4-
Silberberg, Principles of Chemistry
Factors Affecting Actual Bond Angles
Bond angles are consistent with theoretical angles when the atoms attached to the central atom are the same and when all electrons are bonding electrons of the same order.
C O
H
Hideal
1200
1200
larger EN
greater electron density
C O
H
H
1220
1160
real
Lone pairs repel bonding pairs more strongly than bonding pairs repel each other.
Sn
Cl Cl
950
Effect of Double Bonds
Effect of Nonbonding(Lone) Pairs
Silberberg, Principles of Chemistry
Figure 10.10 The steps in determining a molecular shape.
Molecular formula
Lewis structure
Electron-group arrangement
Bond angles
Molecular shape
(AXmEn)
Count all e- groups around central atom (A)
Note lone pairs and double bonds
Count bonding and nonbonding
e- groups separately.
Step 1
Step 2
Step 3
Step 4
Silberberg, Principles of Chemistry
SAMPLE PROBLEM 10.6
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 and (b)
COCl2.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
The type of shape is
AX3E
Silberberg, Principles of Chemistry
SAMPLE PROBLEM 10.6
Predicting Molecular Shapes with Two, Three, or Four Electron Groups
continued
(b) For COCl2, C has the lowest EN and will be the center atom.
There are 24 valence e-, 3 atoms attached to the center atom.
CCl O
Cl
C does not have an octet; a pair of nonbonding electrons will move in from the O to make a double bond.
The shape for an atom with three atom attachments and no nonbonding pairs on the central atom is trigonal planar.C
Cl
O
Cl The Cl-C-Cl bond angle will be less than 1200 due to the electron density of the C=O.
CCl
O
Cl
124.50
1110
Type AX3
Silberberg, Principles of Chemistry
SAMPLE PROBLEM 10.7
Predicting Molecular Shapes with Five or Six Electron Groups
PROBLEM: Determine the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) SbF5 and (b) BrF5.SOLUTION: (a) SbF5 - 40 valence e-; all electrons around central atom will be in bonding pairs; shape is AX5 - trigonal bipyramidal.
F
SbF
F F
FF Sb
F
F
F
F
(b) BrF5 - 42 valence e-; 5 bonding pairs and 1 nonbonding pair on central atom. Shape is AX5E, square pyramidal.
BrF
F F
F
F
Silberberg, Principles of Chemistry
SAMPLE PROBLEM 10.8
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.
PLAN: 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
CHH
>1200
<1200Silberberg, Principles of Chemistry
Molecular Polarity
• Just like bonds can be polar because of even electron distribution, molecules can be polar because of net electrical imbalances.
• These imbalances are not the same as ion formation.
• How do we know when a molecule is polar?
Figure 10.12 The orientation of polar molecules in an electric field.
Electric field OFF
Electric field ON
SAMPLE PROBLEM 10.9
Predicting the Polarity of Molecules
(a) Ammonia, NH3 (b) Boron trifluoride, BF3
(c) Carbonyl sulfide, COS (atom sequence SCO)
PROBLEM: From electronegativity (EN) values (button) and their periodic trends, predict whether each of the following molecules is polar and show the direction of bond dipoles and the overall molecular dipole when applicable:
PLAN: Draw the shape, find the EN values and combine the concepts to determine the polarity.
SOLUTION: (a) NH3
NH
HH
ENN = 3.0
ENH = 2.1N
HHH
NH
HH
bond dipoles
molecular dipole
The dipoles reinforce each other, so the overall molecule is definitely polar.
SAMPLE PROBLEM 10.9
Predicting the Polarity of Molecules
continued
(b) BF3 has 24 valence e- and all electrons around the B will be involved in bonds. The shape is AX3, trigonal planar.
F
B
F
F
F (EN 4.0) is more electronegative than B (EN 2.0) and all of the dipoles will be directed from B to F. Because all are at the same angle and of the same magnitude, the molecule is nonpolar.
1200
(c) COS is linear. C and S have the same EN (2.0) but the C=O bond is quite polar(EN) so the molecule is polar overall.
S C O
Silberberg, Principles of Chemistry
More Molecular Polarity…
• http://academic.pgcc.edu/~ssinex/polarity/polarity.htm
• Work through the site listed above.
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