Chapter 10 Chemical Bonding II

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Chapter 10Chemical Bonding II
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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 atoms
• the kind of bonding between the atoms
• ionic, polar covalent, or covalent
• the shape of the molecule
• bonding theory should allow you to predict the
  shapes of molecules

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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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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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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
• since 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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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

there are 3 electron groups on N 1 lone pair 1
single bond 1 double bond
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Molecular Geometries

• there are 5 basic arrangements of electron groups
  around a central atom
• based 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 doesnt
  matter which resonance form you use the
  molecular geometry will be the same

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Parent electronic structure
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Examples

• How many electron groups (charge clouds) are
  around the central atom in the following?
• SO2 NH4 PCl5

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Trigonal Bipyramidal Geometry

• when there are 5 electron groups around the
  central atom, they will occupy positions in the
  shape of a two tetrahedral 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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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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The Effect of Lone Pairs

• lone pair groups occupy more space on the
  central atom
• because 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 gt Lone Pair Bonding Pair
  gt Bonding Pair Bonding Pair
• this effects the bond angles, making them smaller
  than expected

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Effect of Lone Pairs
The bonding electrons are shared by two atoms, so
some of the negative charge is removed from the
central atom.
The nonbonding electrons are localized on the
central atom, so area of negative charge takes
more space.
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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 lt 120

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Tetrahedral-Bent Shape
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Bond Angle Distortion from Lone Pairs
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Replacing Atoms with Lone Pairsin the Trigonal
Bipyramid System
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T-Shape
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Linear Shape
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Predicting the Shapes Around Central Atoms
Total of e- groups on central atom Parent electronic geometry Bonded atoms Lone pairs Idealized molecular shape Idealized bond angles
2 Linear 2 0 Linear 180o
3 Trigonal Planar 3 0 Trigonal Planar 120 o
3 Trigonal Planar 2 1 Bent 120 o
4 Tetrahedral 4 0 Tetrahedral 109.5 o
4 Tetrahedral 3 1 Trigonal Pyramidal 109.5 o
4 Tetrahedral 2 2 Bent 109.5 o
5 Trigonal Bipyramidal 5 0 Trigonal Bipyramidal 90 o, 120 o, 180 o
5 Trigonal Bipyramidal 4 1 Seesaw 90 o, 120 o, 180 o
5 Trigonal Bipyramidal 3 2 T-shaped 90 o, 180 o
5 Trigonal Bipyramidal 2 3 Linear 180 o
6 Octahedral 6 0 Octahedral 90 o, 180 o
6 Octahedral 5 1 Square Pyramidal 90 o, 180 o
6 Octahedral 4 2 Square Planar 90 o, 180 o
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Real bond angles vs. Idealized bond angles 

• VSEPR predicts the idealized bond angle(s) by
  assuming that all electron groups take up the
  same amount of space. Since lone pairs are
  attracted to only one nucleus, they expand into
  space further than bonding pairs, which are
  attracted to two nuclei. As a result, real
  molecules that has lone pairs on the central atom
  often have bond angles that are slightly
  different than the idealized prediction

Central atom without lone pairs has the same real
bond angle as the idealized angle.   The
exceptions to this are square planar shapes and
linear (derived from trigonal bipyramidal
electronic structure) shapes where the lone pairs
offset one another, thus causing no deviation
from ideality.
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Example
Lewis structure Shape Idealized bond angle Real bond angle
         
               
           
     
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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

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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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Polarity of Molecules

• in order for a molecule to be polar it must
• have polar bonds
• electronegativity difference - theory
• bond dipole moments - measured
• 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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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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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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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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Factors Affecting Dipole Moments

• Lone-pair electrons on oxygen and nitrogen
  project out into space away from positively
  charged nuclei giving rise to a considerable
  charge separation and contributing to the dipole
  moment

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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 water
• and ionic compounds as well
• some molecules have both polar and nonpolar parts

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A Soap MoleculeSodium Stearate
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Example - Decide Whether the Following Are Polar
EN O 3.5 N 3.0 Cl 3.0 S 2.5
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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 molecules
• e.g., O2 is paramagnetic, though the Lewis
  structure predicts it is diamagnetic