Structure Determines Properties!
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Transcript of Structure Determines Properties!
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Tro, Chemistry: A Molecular Approach 1
Structure Determines Properties!• properties of molecular substances depend on
the structure of the molecule• the structure includes many factors, including:
the skeletal arrangement of the atomsthe kind of bonding between the atoms
ionic, polar covalent, or covalentthe shape of the molecule
• bonding theory should allow you to predict the shapes of molecules
Chapter 10 Chemical Bonding II
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Tro, Chemistry: A Molecular Approach 2
Molecular Geometry• Molecules are 3-dimensional objects• We often describe the shape of a molecule
with terms that relate to geometric figures• These geometric figures have characteristic
“corners” that indicate the positions of the surrounding atoms around a central atom in the center of the geometric figure
• The geometric figures also have characteristic angles that we call bond angles
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Tro, Chemistry: A Molecular Approach 3
Using Lewis Theory to PredictMolecular Shapes
• Lewis theory predicts there are regions of electrons in an atom based on placing shared pairs of valence electrons between bonding nuclei and unshared valence electrons located on single nuclei
• this idea can then be extended to predict the shapes of molecules by realizing these regions are all negatively charged and should repel
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Tro, Chemistry: A Molecular Approach 4
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 theorysince 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
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Tro, Chemistry: A Molecular Approach 5
Electron Groups• the Lewis structure predicts the arrangement of valence
electrons around the central atom(s) • each lone pair of electrons constitutes one electron group
on a central atom• each bond constitutes one electron group on a central
atom regardless of whether it is single, double, or triple
O N O ••••••
••
•••• there are 3 electron groups on N1 lone pair1 single bond1 double bond
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Tro, Chemistry: A Molecular Approach 6
Molecular Geometries• there are 5 basic arrangements of electron groups
around a central atombased on a maximum of 6 bonding electron groups
though there may be more than 6 on very large atoms, it is very rare
• each of these 5 basic arrangements results in 5 different basic molecular shapes in order for the molecular shape and bond angles to be a
“perfect” geometric figure, all the electron groups must be bonds and all the bonds must be equivalent
• for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same
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Tro, Chemistry: A Molecular Approach 7
Linear Geometry• when there are 2 electron groups around the central
atom, they will occupy positions opposite each other around the central atom
• this results in the molecule taking a linear geometry• the bond angle is 180°
ClBeCl
O C O
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Tro, Chemistry: A Molecular Approach 8
Trigonal Geometry• when there are 3 electron groups around the central
atom, they will occupy positions in the shape of a triangle around the central atom
• this results in the molecule taking a trigonal planar geometry
• the bond angle is 120°
F
F B F
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Tro, Chemistry: A Molecular Approach 9
Not Quite Perfect Geometry
Because the bonds are not identical, the observed angles are slightly different from ideal.
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Tro, Chemistry: A Molecular Approach 10
Tetrahedral Geometry• when there are 4 electron groups around the central
atom, they will occupy positions in the shape of a tetrahedron around the central atom
• this results in the molecule taking a tetrahedral geometry
• the bond angle is 109.5°
F
F C F
F
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Tro, Chemistry: A Molecular Approach 11
Methane
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Tro, Chemistry: A Molecular Approach 12
Trigonal Bipyramidal Geometry• when there are 5 electron groups around the central atom, they
will occupy positions in the shape of a two tetrahedra that are base-to-base with the central atom in the center of the shared bases
• this results in the molecule taking a trigonal bipyramidal geometry
• the positions above and below the central atom are called the axial positions
• the positions in the same base plane as the central atom are called the equatorial positions
• the bond angle between equatorial positions is 120°• the bond angle between axial and equatorial positions is 90°
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Tro, Chemistry: A Molecular Approach 13
Trigonal Bipyramidal Geometry
P
ClCl
ClCl
Cl•••• ••••
••••
••••
••
••
••••
••
••
••
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Tro, Chemistry: A Molecular Approach 14
Octahedral Geometry• when there are 6 electron groups around the central
atom, they will occupy positions in the shape of two square-base pyramids that are base-to-base with the central atom in the center of the shared bases
• this results in the molecule taking an octahedral geometry it is called octahedral because the geometric figure has 8
sides• all positions are equivalent• the bond angle is 90°
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Tro, Chemistry: A Molecular Approach 15
Octahedral Geometry
S
F F
FF
F
F
•••• ••••
••••••••
••••••
••••
••••
••••
••
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Tro, Chemistry: A Molecular Approach 16
The Effect of Lone Pairs• lone pair groups “occupy more space” on the central
atombecause their electron density is exclusively on the
central atom rather than shared like bonding electron groups
• relative sizes of repulsive force interactions is:Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair • this effects the bond angles, making them smaller
than expected
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Tro, Chemistry: A Molecular Approach 17
Derivative of Trigonal Geometry• when there are 3 electron groups around the central
atom, and 1 of them is a lone pair, the resulting shape of the molecule is called a trigonal planar - bent shape
• the bond angle is < 120°
O S O
O S O
O S O
Molecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengths, but the shape will be a derivative of one of the basic shapes
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Tro, Chemistry: A Molecular Approach 18
Derivatives of Tetrahedral Geometry
• when there are 4 electron groups around the central atom, and 1 is a lone pair, the result is called a pyramidal shapebecause it is a triangular-base pyramid with the central
atom at the apex• when there are 4 electron groups around the central
atom, and 2 are lone pairs, the result is called a tetrahedral-bent shape it is planar it looks similar to the trigonal planar-bent shape, except the
angles are smaller• for both shapes, the bond angle is < 109.5°
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Tro, Chemistry: A Molecular Approach 19
Bond Angle Distortion from Lone Pairs
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Tro, Chemistry: A Molecular Approach 20
Tetrahedral-Bent Shape
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Tro, Chemistry: A Molecular Approach 21
Derivatives of theTrigonal Bipyramidal Geometry
• when there are 5 electron 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 electron 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 electron groups around the central atom, and 2 are lone pairs, the result is called T-shaped
• when there are 5 electron 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
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Tro, Chemistry: A Molecular Approach 22
See-Saw Shape
F S F
F
F
••••
••••
••
••••••
••
••
••
••
••
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Tro, Chemistry: A Molecular Approach 23
T-Shape
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Tro, Chemistry: A Molecular Approach 24
Linear Shape
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Tro, Chemistry: A Molecular Approach 25
Derivatives of theOctahedral Geometry
• when there are 6 electron groups around the central atom, and some are lone pairs, each even number lone pair will take a position opposite the previous lone pair
• when there are 6 electron 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°
• when there are 6 electron 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°
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Tro, Chemistry: A Molecular Approach 26
Square Pyramidal Shape
Br
FF
FF
F•••• ••••
••••
••••
••
••
••••
••
••
••••
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Tro, Chemistry: A Molecular Approach 27
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Tro, Chemistry: A Molecular Approach 28
Practice – Predict the Molecular Geometry and Bond Angles in SiF5
─
Si = 4e─
F5 = 5(7e─) = 35e─
(─) = 1e─
total = 40e─
5 Electron Groups on Si
5 Bonding Groups0 Lone Pairs
Shape = Trigonal Bipyramid
Si
FF
FF
F•••• ••••
••••
••••
••
••
••••
••
••
••
-1
Bond AnglesFeq-Si-Feq = 120°Feq-Si-Fax = 90°
Si Least Electronegative
Si Is Central Atom
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Tro, Chemistry: A Molecular Approach 29
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 AtomO Cl
O
F
•••••• ••••
••
••
••
••
••
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Tro, Chemistry: A Molecular Approach 30
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
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Tro, Chemistry: A Molecular Approach 31
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Tro, Chemistry: A Molecular Approach 32
SF6
S
F
FF
F F
FS
F F
FF
F
F
•••• ••••
••••••••
••••••
••••
••••
••••
••
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Tro, Chemistry: A Molecular Approach 33
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
H|
HOCCH|||OH
shape around left C is tetrahedral
shape around center C is trigonal planar
shape around right O is tetrahedral-bent
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Tro, Chemistry: A Molecular Approach 34
Describing the Geometry of Methanol
Describing the Geometry of Glycine
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35
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
H••••
••
••
••
•• 4 Electron Groups on O
O has2 Bonding Groups2 Lone Pairs
Shape on O = Tetrahedral Bent
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Tro, Chemistry: A Molecular Approach 36
Polarity of Molecules• in order for a molecule to be polar it must
1) have polar bonds electronegativity difference - theory bond dipole moments - measured
2) have an unsymmetrical shape vector addition
• polarity affects the intermolecular forces of attraction therefore boiling points and solubilities
like dissolves like
• nonbonding pairs affect molecular polarity, strong pull in its direction
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Tro, Chemistry: A Molecular Approach 37
Molecule Polarity
The H-Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule.
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Tro, Chemistry: A Molecular Approach 38
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Tro, Chemistry: A Molecular Approach 39
Molecule Polarity
The O-C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule.
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Tro, Chemistry: A Molecular Approach 40
Molecule Polarity
The H-O bond is polar. The both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule.
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Tro, Chemistry: A Molecular Approach 41
Molecule Polarity
The H-N bond is polar. All the sets of bonding electrons are pulled toward the N end of the molecule. The net result is a polar molecule.
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Tro, Chemistry: A Molecular Approach 42
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
• some molecules have both polar and nonpolar parts
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Tro, Chemistry: A Molecular Approach 43
A Soap MoleculeSodium Stearate
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Tro, Chemistry: A Molecular Approach 44
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 ••••••
••
••••
O S
O
O
•••••• ••
••••
••
••
TrigonalBent Trigonal
PlanarCl
NO
3.0
3.0
3.5O
O
OS
3.5
3.5 3.52.5
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Tro, Chemistry: A Molecular Approach 45
Problems with Lewis Theory• Lewis theory gives good first approximations of
the bond angles in molecules, but usually cannot be used to get the actual angle
• Lewis theory cannot write one correct structure for many molecules where resonance is important
• Lewis theory often does not predict the correct magnetic behavior of moleculese.g., O2 is paramagnetic, though the Lewis structure
predicts it is diamagnetic