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Chapter 10 Bonding Theory and Molecular Structure. General Chemistry: An Integrated Approach Hill, Petrucci, 4 th Edition. Mark P. Heitz State University of New York at Brockport © 2005, Prentice Hall, Inc. Molecular Geometry.

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chapter 10 bonding theory and molecular structure

Chapter 10Bonding Theory and Molecular Structure

General Chemistry: An Integrated ApproachHill, Petrucci, 4th Edition

Mark P. Heitz

State University of New York at Brockport

© 2005, Prentice Hall, Inc.

molecular geometry
Molecular Geometry

The molecular geometry, or the shape of a molecule is described by the geometric figure formed when the atomic nuclei are imagined to be joined in straight lines.

EOS

Chapter 10: Bonding Theory...

valence shell electron pair repulsion vsepr

The mutual repulsions among electron groups lead to an orientation of the groups that is called electron-group geometry

EOS

Valence-Shell Electron-Pair Repulsion (VSEPR)

Ideal molecular geometry is based on the idea that pairs of valence electrons in bonded atoms repel one another

An electron group is any collection of valence electrons, localized in a region around a central atom, that repels other groups of valence electrons

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electron group geometries

2 electron groups – linear

3 electron groups – trigonal planar

4 electron groups – tetrahedral

5 electron groups – trigonal bipyramidal

6 electron groups – octahedral

EOS

Electron-Group Geometries

How can electrons on the central atom be arranged so they are as far apart as possible?

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vsepr notation

In the VSEPR notation used to describe molecular geometries, the central atom in a structure is denoted as A, terminal atoms as X, and the lone pairs of electrons as E

EOS

VSEPR Notation

Table 10.1 in the text summarizes various possibilities for molecular geometries in relation to electron-group geometries

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a vsepr summary

Geometry is unaffected by lone pairs of electrons

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A VSEPR Summary

The optimal repulsion arrangement is opposite ends of a line

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a vsepr summary7

The optimal repulsion arrangement is the corners of a triangle—note the planarity

With a lone pair, the geometry is essentially the same

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A VSEPR Summary

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a vsepr summary8

The optimal repulsion arrangement is the corners of a regular tetrahedron—four equal triangular faces

There can be one or two lone pairs on the central atom

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A VSEPR Summary

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a vsepr summary9

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A VSEPR Summary

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a vsepr summary10

The optimal repulsion arrangement is a triangle with a bisecting line through the triangle face

Up to three lone pairs can be included here

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A VSEPR Summary

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a vsepr summary11
A VSEPR Summary

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a vsepr summary12

One or two lone-pair geometries are most common

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A VSEPR Summary

The optimal repulsion arrangement is at the face centers of a cube

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a vsepr summary13
A VSEPR Summary

Illustration

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polar molecules and dipole moments

The dipole moment (m) of a molecule is the product of the magnitude of the charge (d) and the distance (d) that separates the centers of positive and negative charge m = dd

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Polar Moleculesand Dipole Moments

A molecule with separate centers of positive and negative charge is called a polar molecule

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polar molecules and dipole moments15

Dipolar molecules align with electric fields

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Polar Moleculesand Dipole Moments

Dipole moments are generally expressed in a quantity called a debye, D

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bond and molecular dipoles

All polar covalent bonds have a bond dipole; a separation of positive and negative charge centers in an individual bond

A charge separation in the molecule as a whole, considering all the bonds, is a molecular dipole

These concepts explain why CO2 is linear with no dipole moment (m = 0 D) and water is bent (bond angle = 104.5o) with a dipole moment of m = 1.84 D

EOS

Bond and Molecular Dipoles

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molecular shapes and dipole moments

From the molecular shape, determine whether bond dipoles cancel to give a nonpolar molecule or combine to produce a resultant dipole moment for the molecule

EOS

Molecular Shapesand Dipole Moments

Molecules can be predicted to be polar or nonpolar based on the following three-step approach

  • Use electronegativity values to predict bond dipoles
  • Use the VSEPR method to predict the molecular shape

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atomic orbital overlap

Valence Bond (VB) Theory states that a covalent bond is formed by the pairing of two electrons with opposing spins in the region of overlap of atomic orbitals between two atoms

The more extensive the overlap between two orbitals, the stronger is the bond between two atoms

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Atomic Orbital Overlap

This overlap region has a high electron charge density

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bonding in h 2 s
Bonding in H2S

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several important points

For orbitals with directional lobes, maximum overlap occurs when atomic orbitals overlap end to end; that is, a hypothetical line joining the nuclei of the bonded atoms passes through the region of maximum overlap

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Several Important Points

Most of the electrons in a molecule remain in the same orbital locations that they occupied in the separated atoms

Bonding electrons are localized in the region of atomic orbital overlap

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hybridization of atomic orbitals

Based on ground-state electron configuration, carbon should have only two bonds

If a 2s electron is promoted to an empty 2p orbital, then four unpaired electrons can give rise to four bonds

These four orbitals become mixed, or hybridized to form bonds

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Hybridization of Atomic Orbitals

HybridizationVideo

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sp 3 hybridization

The total number of hybrid orbitals is equal to the number of atomic orbitals combined

Hybrid orbitals may overlap with pure atomic orbitals or with other hybrid orbitals

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sp3 Hybridization

Occurs most often for central atom only

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sp 3 hybridization23
sp3 Hybridization

The carbon atom in methane (CH4) has bonds that are sp3 hybrids

Note that in this molecule carbon has all singlebonds

EOS

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bonding in ammonia
Bonding in Ammonia

Ammonia (NH3) is similar except the lone pair of electrons occupies the 4th hybrid orbital

EOS

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sp 2 hybridization

The empty 2p orbital remains unhybridized

EOS

sp2 Hybridization

This hybridization scheme is useful in describing double covalent bonds

Comprised of one 2s orbital and two 2p orbitals to produce a set of three sp2 hybrid orbitals

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determining empirical formulas

This distribution gives a trigonal planar molecular geometry, as predicted by VSEPR

EOS

Determining Empirical Formulas

The geometric distribution of the three sp2 hybrid orbitals is within a plane, directed at 120o angles

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sp hybridization

The geometric distribution of the two sp hybrid orbitals is on a line, directed at 180o angles

This distribution gives a linear molecular geometry

EOS

sp Hybridization

This hybridization scheme is useful in describing triple covalent bonds

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d subshells hybrid orbitals

This hybridization allows for expanded valence shell compounds – typical for group 5A elements, e.g., P

A 3s electron can be promoted to a 3d subshell, which gives rise to a set of five sp3d hybrid orbitals

EOS

d Subshells Hybrid Orbitals

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d subshells hybrid orbitals29

This hybridization allows for expanded valence shell compounds – typically group 6A elements, e.g., S

A 3s and a 3p electron can be promoted to the 3d subshell, which gives rise to a set of six sp3d2 hybrid orbitals

EOS

d Subshells Hybrid Orbitals

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predicting hybridization schemes

Describe the orbital overlap and molecular geometry

EOS

Predicting Hybridization Schemes

In hybridization schemes, one hybrid orbital is produced for every simple atomic orbital involved

Write a plausible Lewis structure for the molecule or ion

Use the VSEPR method to predict the electron-group geometry of the central atom

Select the hybridization scheme that corresponds to the VSEPR prediction

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hybrid orbitals and their geometric orientations
Hybrid Orbitals and TheirGeometric Orientations

EOS

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hybrid orbitals and multiple covalent bonds

A triple bond is made up of one sigma bond and two pi bonds

EOS

Hybrid Orbitals andMultiple Covalent Bonds

Covalent bonds formed by the end-to-end overlap of orbitals, regardless of orbital type, are called sigma (s) bonds. All single bonds are sigma bonds

A bond formed by parallel, or side-by-side, orbital overlap is called a pi (p) bond

A double bond is made up of one sigma bond and one pi bond

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carbon carbon double bonds
Carbon–Carbon Double Bonds

A double bond is made up of one sigma bond and one pi bond

EOS

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carbon carbon triple bonds

EOS

Carbon–Carbon Triple Bonds

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geometric isomerism

Each compound is distinctly different in both physical and chemical properties

EOS

Geometric Isomerism

Geometric isomers are isomers that differ only in the geometric arrangement of certain substituent groups

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molecular orbitals

Molecular orbitals (MOs) are built up (Aufbau principle) in the same way as atomic orbitals

EOS

Molecular Orbitals

Molecular orbitals (MOs) are mathematical equations that describe the regions in a molecule where there is a high probability of finding electrons

Molecular orbitals (MOs) are essentially combinations of atomic orbitals – two types exist, bonding and antibonding orbitals

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visualizing mos

Bonding MO = enhanced region of electron density

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Visualizing MOs

The hydrogen molecule

Antibonding MO = region of diminished electron density

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mos for the 2 p electrons
MOs for the 2p Electrons

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molecular orbital diagram

Antibonding MOs

= higher energy

s* and p* MOs

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Molecular Orbital Diagram

Bonding MOs

s AOs = s MOs

p AOs = p MOs

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second period homonuclear diatomic molecules
Second-Period Homonuclear Diatomic Molecules

EOS

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bonding in benzene

Kekulé discovered that benzene has a cyclic structure and he proposed that a hydrogen atom was attached to each carbon atom and that alternating single and double bonds joined the carbon atoms together

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Bonding in Benzene

The structure of benzene, C6H6, discovered by Michael Faraday in 1825, was not figured out until 1865 by F. A. Kekulé

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benzene

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Benzene

This kind of structure gives rise to two important resonance hybrids and leads to the idea that all three double bonds are delocalized across all six carbon atoms

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the s bonding framework
The s-Bonding Framework

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aromatic compounds

Aromatic compound simply refers to a substance with a ring structure and with bonding characteristics and properties related to those of benzene

EOS

Aromatic Compounds

Many of the first benzene-like compounds discovered had pleasant odors and hence acquired the name aromatic

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summary of concepts
Summary of Concepts
  • The VSEPR method is used to predict the shapes of molecules and polyatomic ions
  • If all electron groups are bonding groups, the molecular geometry is the same as the electron-group geometry
  • A polar covalent bond has separate centers of positive and negative charge, creating a bond dipole

EOS

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summary of concepts46

Unhybridized p orbitals overlap in a side-by-side fashion to form p bonds

EOS

Summary of Concepts
  • In the valence bond theory, a covalent bond is formed by the overlap of atomic orbitals of the bonded atoms in a region between the atomic nuclei
  • Hybridized orbitals include sp, sp2, sp3, sp3d, and sp3d2

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summary cont

Benzene-like compounds are called aromatic compounds

EOS

Summary (cont.)
  • Single bonds are all hybridized s bonds, double bonds have one s bond and one p bond, and triple bonds have one s bond and two p bonds
  • In molecular orbital theory, atomic orbitals of separated atoms are combined into molecular orbitals
  • The benzene molecule is usually represented by its resonance hybrid

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