theories of bonding and structure chapter 10

Theories of Bonding and Structure

CHAPTER 10

Chemistry: The Molecular Nature of Matter, 6th editionBy Jesperson, Brady, & Hyslop

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CHAPTER 10: Bonding & Structure

Learning Objectives VESPR theory:

Determine molecular geometry based on molecular formula and/or lewis dot structures.

Effect of bonded atoms & non-bonded electrons on geometry Molecular polarity & overall dipole moment

Assess overall dipole moment of a molecule Identify polar and non-polar molecules

Valence Bond Theory Hybridized orbitals Multiple bonds Sigma vs pi orbitals

Molecular Orbital Theory Draw & label molecular orbital energy diagrams Bonding & antibonding orbitals Predict relative stability of molecules based on MO diagrams

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Molecular Geometry Basic Molecular Geometries

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E

Linear3 atoms

Trigonal Planaror

Planar Triangular

4 atoms

Tetrahedral:5 atoms

Trigonal Bipyramidal6 atoms

Octahedral:7 atoms

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VESPR Definition

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6E http://chemistry-desk.blogspot.com/2011/05/prediction-of-shape-of-molecules-by.html

Valence Shell Electron Pair Repulsion ModelElectron pairs (or groups of electron pairs) in the valence shell of an atom repel each other and will position themselves so that they are far apart as

possible, thereby minimizing the repulsions.

Electron pairs can either be lone pairs or bonding pairs.

Tetrahedral arrangement of electron pairsBent geometry

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VESPR Definition


Valence Shell Electron Pair Repulsion ModelElectron pairs (or groups of electron pairs) in the valence shell of an atom repel each other and will position themselves so that they are far apart as

possible, thereby minimizing the repulsions.

Text uses “electron domain” to describe electron pairs:

Bonding domain: contains electrons that are shared between two atoms. So electrons involved in single, double, or triple are

part of the same bonding domain.

Nonbonding Domain: Valence electrons associated with one atom, such as a lone pair, or a unpaired electron.

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VESPR Basic Examples


2 bonding domains

3 bonding domains

4 bonding domains

5 bonding domains

6 bonding bonding domains

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VESPR When Lone Pairs or Multiple Bonds Present


Including lone pairs: • Take up more space around central atom • Effect overall geometry • Counted as nonbonded electron domains

Including multiple bonds (double and triple) • For purposes of determining geometry focus on the number

of atoms bonded together rather then the number of bonds in between them: ie, treat like a single bond.

• Treat as single electron bonding domain

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VESPR Electrons that are Bonding & Not Bonding


Bonding Electrons – More oval in shape – Electron density focused

between two positive nuclei.

Nonbonding Electrons– More bell or balloon shaped– Take up more space – Electron density only has positive

nuclei at one end

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VESPR 3 atoms or lone pairs


Number of Bonding Domains

3

2

Number of Nonbonding Domains

0

1

Molecular Shape

Planar Triangular(e.g. BCl3)All bond angles 120

NonlinearBent or V-shaped(e.g. SnCl2)

Bond <120

Structure

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4

3

2


0

1

2

Molecular Shape

Tetrahedron(e.g. CH4)All bond angles 109.5

Trigonal pyramidal(e.g. NH3)Bond angle less than 109.5

Nonlinear, bent(e.g. H2O)Bond angle less than109.5

Structure

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90

120

Trigonal Bipyramidal• Two atoms in axial position

– 90 to atoms in equatorial plane

• Three atoms in equatorial position– 120 bond angle to atoms

in axial position– More room here– Substitute here first

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5

4


0

1

Molecular Shape

Trigonal bipyramid(e.g. PF5)Ax-eq bond angles 90Eq-eq 120

Distorted Tetrahedron, or Seesaw(e.g. SF4)Ax-eq bond angles < 90

Structure

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5 atoms or lone pairs


• Lone pair takes up more space• Goes in equatorial plane• Pushes bonding pairs out of way• Result: distorted tetrahedron

VESPR

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3

2


2

3

Molecular Shape

T-shape(e.g. ClF3)Bond angles 90

Linear(e.g. I3

–)Bond angles 180

Structure

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6

5

Molecular Shape

Octahedron(e.g. SF6)

Square Pyramid(e.g. BrF5)

StructureNumber of Nonbonding Domains

0

1

16




4


2

Molecular Shape

Square planar(e.g. XeF4)

Structure

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VESPR Determining 3-D Structures


1. Draw Lewis Structure of Molecule– Don't need to compute formal charge– If several resonance structures exist, pick only one

2. Count electron pair domains– Lone pairs and bond pairs around central atom– Multiple bonds count as one set (or one effective pair)

3. Arrange electron pair domains to minimize repulsions• Lone pairs

– Require more space than bonding pairs– May slightly distort bond angles from those predicted.– In trigonal bipyramid lone pairs are equatorial – In octahedron lone pairs are axial

4. Name molecular structure by position of atoms—only bonding electrons

Molecular Polarity Polar Molecules


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• Have net dipole moment– Negative end– Positive end

• Polar molecules attract each other.– Positive end of polar molecule attracted to

negative end of next molecule.– Strength of this attraction depends on

molecule's dipole moment– Dipole moment can be determined

experimentally• Polarity of molecule can be predicted by taking

vector sum of bond dipoles• Bond dipoles are usually shown as crossed

arrows, where arrowhead indicates negative end

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Molecular Polarity Molecular Shape & Polarity

Jesperson, Brady, Hyslop. Chemistry: The Molecular Nature of Matter, 6Ehttp://wps.prenhall.com/wps/media/objects/3081/3155729/blb0903.html

• Many physical properties (melting and boiling points) affected by molecular polarity

• For molecule to be polar:– Must have polar bonds

• Many molecules with polar bonds are nonpolar - Possible because certain

arrangements of bond dipoles cancel

- For molecules with more than two atoms, must consider the combined effects of all polar bonds

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Molecular Polarity Symmetrical Nonpolar Molecules


• Symmetrical molecules – Nonpolar because bond dipoles cancel

• All five shapes are symmetrical when all domains attached to them are composed of identical atoms

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Molecular Polarity Symmetrical Nonpolar Molecules


Cancellation of Bond Dipoles In Symmetrical Trigonal Bipyramidal and Octahedral Molecules

• All electron pairs around central atom are bonding pairs and • All terminal groups (atoms) are same• The individual bond dipoles cancel

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Molecule is usually polar if – All atoms attached to central atom are NOT same Or, – There are one or more lone pairs on central atom

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Water and ammonia both have non-bonding domains Bond dipoles do not cancel Molecules are polar

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Molecular Polarity Polar Molecules: Execption


Exception to these general rules for identifying polar molecules:

Nonbonding domains (lone pairs) are symmetrically placed around central atom

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ProblemSet A

1. For the following molecules: a. Draw a lewis dot structure.b. Determine the molecular geometry at each central atom.c. Identify the bond angles.d. Identify all polar bonds: δ+ / δ-e. Assess the polarity of the molecule & indicate the overall

dipole moment if one exists

AsF5 AsF3 SeO2

GaH3

ICl2- SiO4-4

TeF6

theories of bonding and structure chapter 10

Documents

bonding pairs

molecular nature of

groups of electron pairs

unpaired electron

space electron density

shape electron density

hyslop chapter

text uses electron domain