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Chapter 10Chemical Bonding II
Chemistry II
Chemical Bonding IIMolecular Shapes
Chemical Bonding IIVSEPR Theory
• e- groups (lone pairs and bonds) are most stable when they are as far apart as possible –
v________ s____ e_______ p_____ r_________ theory
• Maximum separation
• 3-D representation allows us to predict the shapes and bond angles in the molecule
Chemical Bonding IIVSEPR Theory
e.g. draw the 2 possible Lewis dot structures for NO2- and
discuss the behavior of the associated e- groups
there are _____ e- groups on N____ lone pair____ single bond____ double bond (counted as 1 group)
Chemical Bonding II2 e- Groups: Linear Geometry
• 5 basic shapes of molecules:
linear,
trigonal planar,
tetrahedral,
trigonal bipyramidal,
octahedral
Chemical Bonding II2 e- Groups: Linear Geometry
• Draw both 2-dimensional and 3-dimensional pictures of the molecules in the following slides
Chemical Bonding II2 e- Groups: Linear Geometry
• occupy positions opposite, around the central atomlinear geometry - bond angle is ________
e.g. CO2
Chemical Bonding II3 e- Groups: Trigonal Geometry
• occupy triangular positionstrigonal planar geometry - bond angle is __________
e.g. BF3
Chemical Bonding II3 e- Groups: Trigonal Geometry
3 e– groups around central atom – why not 120° ?
e.g. Formaldehyde, CH2O
Chemical Bonding II4 e- Groups: Tetrahedral Geometry
• occupy tetrahedron positions around the central atomtetrahedral geometry - bond angle is ________
e.g. CH4
Chemical Bonding II5 e- Groups: Trigonal Bipyramidal Geometry
• occupy positions in the shape of a two tetrahedra that are base-to-base
trigonal bipyramidal geometry
e.g. PCl5
Chemical Bonding II 6 e- Groups: Octahedral Geometry
• occupy positions in the shape of two square-base pyramids that are base-to-baseoctahedral geometry
e.g. SF6
Chemical Bonding II 3 e- Groups with Lone Pairs: Derivative of Trigonal Geometry
• when there are 3 e- groups around central atom, and 1 of them is a lone pairtrigonal planar - bent shape - bond angle < 120°
e.g. SO2
O S O
O S O
O S O
Chemical Bonding II 4 e- Groups with Lone Pairs : Derivatives of Tetrahedral Geometry
• when there are 4 e- groups around the central atom, and 1 is a lone pair
trigonal pyramidal shape – bond angle is 107 °
e.g. NH3
Chemical Bonding II 4 e- Groups with Lone Pairs: Derivatives of Tetrahedral Geometry
• when there are 4 e- groups around the central atom, and 2 are lone pairstetrahedral-bent shape – bond angle is 104.5 °
e.g. H2O
Chemical Bonding II Tetrahedral-Bent Shape
Chemical Bonding II 5 e- Groups with Lone Pairs
Derivatives of Trigonal Bipyramidal Geometry
• when there are 5 e- groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room
• when there are 5 e- groups around the central atom, and 1 is a lone pair, the result is called see-saw shape aka distorted tetrahedron
• when there are 5 e- groups around the central atom, and 2 are lone pairs, the result is called T-shaped
• when there are 5 e- groups around the central atom, and 3 are lone pairs, the result is called a linear shape
• the bond angles between equatorial positions is < 120°
• the bond angles between axial and equatorial positions is < 90° linear = 180° axial-to-axial
Chemical Bonding II Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System
Chemical Bonding II See-Saw Shape
F S F
F
F
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Chemical Bonding II T-Shape
Tro, Chemistry: A Molecular Approach 25
Chemical Bonding II Linear Shape
Br
FF
FF
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• when there are 6 e- groups around the central atom, and 1 is a lone pair, the result is called a square pyramid shape the bond angles between axial and equatorial positions is < 90°
Chemical Bonding II 6 e- Groups with Lone Pairs: Derivatives of Octahedral Geometry
F Xe F
F
F
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• when there are 6 e- groups around the central atom, and 2 are lone pairs, the result is called a square planar shape the bond angles between equatorial positions is 90°
6 e- Groups with Lone Pairs Derivatives of Octahedral Geometry
Chemical Bonding II Predicting the Shapes Around Central Atoms
1. Draw the Lewis Structure2. Determine the Number of Electron Groups around the Central Atom3. Classify Each Electron Group as Bonding or Lone pair, and Count
each typeremember, multiple bonds count as 1 group
4. Use Table 10.1 to Determine the Shape and Bond Angles
Practice – Predict the Molecular Geometry and Bond Angles in ClO2F (Chloryl Fluoride)
Practice – Predict the Molecular Geometry and Bond Angles in ClO2F
Cl = 7e─
O2 = 2(6e─) = 12e─
F = 7e─
Total = 26e─
4 Electron Groups on Cl
3 Bonding Groups1 Lone Pair
Shape = Trigonal Pyramidal
Bond AnglesO-Cl-O < 109.5°O-Cl-F < 109.5°
Cl Least Electronegative
Cl Is Central Atom
O Cl
O
F
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Chemical Bonding II 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
SF6
S
F
F
F
F F
F
S
F F
FF
F
F
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Multiple Central Atoms
• many molecules have larger structures with many interior atoms
• we can think of them as having multiple central atoms
• when this occurs, we describe the shape around each central atom in sequence
e.g. acetic acid
H|
HOCCH|||
OH
shape around left C is tetrahedral
shape around center C is trigonal planar
shape around right O is tetrahedral-bent
Describing the Geometryof Methanol
Describing the Geometryof Glycine
Tro, Chemistry: A Molecular Approach 41
Practice – Predict the Molecular Geometries in H3BO3
42
Practice – Predict the Molecular Geometries in H3BO3
B = 3e─
O3 = 3(6e─) = 18e─
H3 = 3(1e─) = 3e─
Total = 24e─
3 Electron Groups on B
B has3 Bonding Groups0 Lone Pairs
Shape on B = Trigonal Planar
B Least Electronegative
B Is Central Atom
oxyacid, so H attached to O
O B
O
OH H
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•• 4 Electron Groups on O
O has2 Bonding Groups2 Lone Pairs
Shape on O = Bent
Tro, Chemistry: A Molecular Approach 43
Practice – Predict the Molecular Geometries in C2H4
44
Practice – Predict the Molecular Geometries in C2H4
C = 2(4e─) = 8e ─
H = 4(1e─) = 4e─
Total = 12e─
3 Electron Groups on C
Shape on each C = Trigonal Planar
0 Lone Pairs
Practice – Predict the Molecular Geometries in CH3OCH3
Practice – Predict the Molecular Geometries in Dimethyl Ether (CH3OCH3)
C = 2(4e─) = 8e ─
H = 6(1e─) = 6e─
O = 6(1e─) = 6e─
Total = 20e─
4 Electron Groups on C
Shape on each C = Tetrahedral
2 Lone Pairs on O
Shape on O = Bent
Reminder about Eletronegativity!
• Electronegativity, is a chemical property that describes the tendency of an atom to e- towards itself
Polarity of Molecules
• in order for a molecule to be polar it must
1) have polar bonds electronegativity difference dipole moments (charge x distance)
2) have an unsymmetrical shape vector addition
• polarity affects the intermolecular forces of attraction therefore boiling points and solubilities
like dissolves like
• nonbonding pairs strongly affect molecular polarity
Molecule Polarity
The H-Cl bond is polarBonding e- are pulled toward the Cl end of the molecule
Net result is a polar molecule.
Vector Addition
Molecule Polarity
The O-C bond is polarThe bonding e- are pulled equally toward both O’sSymmetrical molecule
Net result is a nonpolar molecule
Molecule Polarity
The H-O bond is polarBoth sets of bonding e- are pulled toward the O
Net result is a polar molecule
Molecule Polarity
Molecule Polarity
The H-N bond is polarAll the sets of bonding electrons are pulled toward the NNot symmetrical
Net result is a polar molecule
Molecule Polarity
The C-H bond is polarFour equal dipoles cancel each other out due to symmetry
Net result is a non-polar molecule
Molecular Polarity Affects Solubility in Water
• polar molecules are attracted to other polar molecules
• since water is a polar molecule, other polar molecules dissolve well in waterand ionic compounds as well
Molecular Polarity Affects Solubility in Water
• Oil and water do not mix!
Mutual attraction causes polar molecules to clump together
• Water shrinks on melting (ice floats on water)
• Unusually high melting point
• Unusually high boiling point
• Unusually high surface tension
• Unusually high viscosity
• Unusually high heat of vaporization
• Unusually high specific heat capacity
• And more…
Unique Properties
Molecular Polarity Affects Solubility in Water
• some molecules have both polar and nonpolar partse.g. soap
Practice - Decide Whether the Following Are Polar
O N Cl ••
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O S
O
O
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••ENO = 3.5N = 3.0Cl = 3.0S = 2.5
Practice - Decide Whether the Following Are Polar
polarnonpolar
1) polar bonds, N-O2) asymmetrical shape 1) polar bonds, all S-O
2) symmetrical shape
O N Cl ••
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O S
O
O
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TrigonalBent Trigonal
PlanarCl
N
O
3.0
3.0
3.5
O
O
OS
3.5
3.5 3.52.5
