Chapter 9 Molecular Geometries and Bonding Theories

Chapter 9 Molecular Geometries and Bonding Theories

Chemistry, The Central Science, 11th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten Chapter 9 Molecular Geometries and Bonding Theories John D. Bookstaver St. Charles Community College Cottleville, MO Molecular Geometries

and Bonding 2009, Prentice-Hall, Inc. Molecular Shapes The shape of a molecule plays an important role in its reactivity. By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of

Molecular the molecule. Geometries and Bonding 2009, Prentice-Hall, Inc. What Determines the Shape of a Molecule? Simply put, electron pairs, whether they be bonding or nonbonding,

repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Electron Domains

The central atom in this molecule, A, has four electron domains. We can refer to the electron pairs as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom;

therefore, they count as one electron domain. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Valence Shell Electron Pair Repulsion Theory (VSEPR) The best arrangement of a given number of electron domains is

the one that minimizes the repulsions among them. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Electron-Domain Geometries These are the electron-domain

geometries for two through six electron domains around a central atom. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Electron-Domain Geometries All one must do is count the number of electron domains in

the Lewis structure. The geometry will be that which corresponds to the number of electron domains. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Molecular Geometries

The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry is that defined by the positions of only the atoms in the molecules, Molecular not the nonbonding pairs. Geometries and Bonding 2009, Prentice-Hall, Inc. Molecular Geometries Within each electron

domain, then, there might be more than one molecular geometry. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Linear Electron Domain In the linear domain, there is only one

molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Trigonal Planar Electron Domain There are two molecular geometries:

Trigonal planar, if all the electron domains are bonding, Bent, if one of the domains is a nonbonding pair. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds.

Therefore, they also affect bond angles. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Tetrahedral Electron Domain There are three molecular geometries: Tetrahedral, if all are bonding pairs, Trigonal pyramidal if one is a nonbonding pair, Bent if there are two nonbonding pairs.

Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: Axial

Equatorial Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this

geometry. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain:

Trigonal bipyramidal Seesaw T-shaped Linear Molecular Geometries and Bonding 2009, Prentice-Hall, Inc.

Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: Octahedral Square pyramidal Square planar Molecular Geometries and Bonding

2009, Prentice-Hall, Inc. Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole. Molecular

Geometries and Bonding 2009, Prentice-Hall, Inc. Larger Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule. Molecular

Geometries and Bonding 2009, Prentice-Hall, Inc. Polarity In Chapter 8 we discussed bond dipoles. But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar. Molecular

Geometries and Bonding 2009, Prentice-Hall, Inc. Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule. Molecular

Geometries and Bonding 2009, Prentice-Hall, Inc. Polarity Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Overlap and Bonding We think of covalent bonds forming through

the sharing of electrons by adjacent atoms. In such an approach this can only occur when orbitals on the two atoms overlap. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Overlap and Bonding Increased overlap brings the electrons and nuclei closer together while

simultaneously decreasing electronelectron repulsion. However, if atoms get too close, the internuclear repulsion greatly raises the energy. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals

But its hard to imagine tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals Consider beryllium: In its ground electronic state, it would not be

able to form bonds because it has no singly-occupied orbitals. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals But if it absorbs the small amount of energy needed to

promote an electron from the 2s to the 2p orbital, it can form two bonds. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. These sp hybrid orbitals have two lobes like a p orbital.

One of the lobes is larger and more rounded as is the s orbital. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals These two degenerate orbitals would align themselves 180 from each other. This is consistent with the observed geometry of beryllium compounds: linear.

Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals With hybrid orbitals the orbital diagram for beryllium would look like this. The sp orbitals are higher in energy than the 1s orbital but lower than the 2p. Molecular

Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals Using a similar model for boron leads to Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals

three degenerate sp2 orbitals. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals With carbon we get Molecular Geometries and Bonding

2009, Prentice-Hall, Inc. Hybrid Orbitals four degenerate sp3 orbitals. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals For geometries involving expanded octets on

the central atom, we must use d orbitals in our hybrids. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals This leads to five degenerate sp3d orbitals or six degenerate sp3d2

orbitals. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Hybrid Orbitals Once you know the electron-domain geometry, you know the hybridization state of the atom.

Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Valence Bond Theory Hybridization is a major player in this approach to bonding. There are two ways orbitals can overlap to form bonds between atoms. Molecular Geometries

and Bonding 2009, Prentice-Hall, Inc. Sigma () Bonds Sigma bonds are characterized by Head-to-head overlap. Cylindrical symmetry of electron density about the internuclear axis. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc.

Pi () Bonds Pi bonds are characterized by Side-to-side overlap. Electron density above and below the internuclear axis. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc.

Single Bonds Single bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Multiple Bonds In a multiple bond one of the bonds is a bond

and the rest are bonds. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Multiple Bonds In a molecule like formaldehyde (shown at left) an sp2 orbital on carbon overlaps in fashion with the

corresponding orbital on the oxygen. The unhybridized p orbitals overlap in fashion. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Multiple Bonds In triple bonds, as in acetylene, two sp

orbitals form a bond between the carbons, and two pairs of p orbitals overlap in fashion to form the two bonds. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Delocalized Electrons: Resonance

When writing Lewis structures for species like the nitrate ion, we draw resonance structures to more accurately reflect the structure of the molecule or ion. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Delocalized Electrons: Resonance In reality, each of the four atoms in the nitrate ion has a

p orbital. The p orbitals on all three oxygens overlap with the p orbital on the central nitrogen. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Delocalized Electrons: Resonance This means the electrons are not localized between the

nitrogen and one of the oxygens, but rather are delocalized throughout the ion. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Resonance The organic molecule benzene has six bonds and a p orbital

on each carbon atom. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Resonance In reality the electrons in benzene are not localized, but delocalized. The even distribution of the electrons in benzene makes the molecule unusually stable.

Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Molecular Orbital (MO) Theory Though valence bond theory effectively conveys most observed properties of ions and molecules, there are some concepts better represented by molecular orbitals.

Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Molecular Orbital (MO) Theory In MO theory, we invoke the wave nature of electrons. If waves interact constructively, the resulting orbital is lower in energy: a bonding

molecular orbital. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. Molecular Orbital (MO) Theory If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.

Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. MO Theory In H2 the two electrons go into the bonding molecular orbital. The bond order is one half the difference between the number of

bonding and antibonding electrons. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. MO Theory For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is

1 (2 - 0) = 1 2 Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. MO Theory In the case of He2, the bond order would be 1

(2 - 2) = 0 2 Therefore, He2 does not exist. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. MO Theory For atoms with both s and p orbitals, there are two types of

interactions: The s and the p orbitals that face each other overlap in fashion. The other two sets of p orbitals overlap in fashion. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. MO Theory

The resulting MO diagram looks like this. There are both and bonding molecular orbitals and * and * antibonding molecular orbitals. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc. MO Theory

The smaller p-block elements in the second period have a sizeable interaction between the s and p orbitals. This flips the order of the and molecular orbitals in these elements. Molecular Geometries and Bonding 2009, Prentice-Hall, Inc.

Second-Row MO Diagrams Molecular Geometries and Bonding 2009, Prentice-Hall, Inc.

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