chapter 9 lecture- molecular geometry
DESCRIPTION
Chapter nine lecture for AP Chemistry on molecular geometries and bonding theoriesTRANSCRIPT
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Chapter 9Molecular Geometriesand Bonding Theories
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Molecular Shapes
• The shape of a molecule plays an important role in its reactivity.
• The shape of a molecule is determined by its bond angles and bond lengths.
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Molecular Shapes
By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.
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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.
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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.
• This molecule has four electron domains.
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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.”
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Electron-Domain Geometries
These are the electron-domain geometries for two through six electron domains around a central atom.
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QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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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 that number of electron domains.
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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, not the nonbonding pairs.
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Molecular Geometries
Within each electron domain, then, there might be more than one molecular geometry.
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Linear Electron Domain
• In this 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.
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Trigonal Planar Electron Domain
• There are two molecular geometries:Trigonal planar, if all the electron domains are
bondingBent, if one of the domains is a nonbonding pair.
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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.
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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.
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In which of the following molecules is the HXH bond angle smallest (X is non-H atom): CH4, H2O, and NH3?
1. CH4
2. H2O
3. NH3
4. They are all equal
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1. CH4
2. H2O
3. NH3
4. They are all equal
Correct Answer:
Lone pairs exert greater repulsive forces on adjacent electron domains.Bond angles are compressed with increasing numbers of lone pairs.
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Yes, resonance equalizes repulsions between bonds, making all angles 120°.
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Tetrahedral Electron Domain
• There are three molecular geometries:Tetrahedral, if all are bonding pairsTrigonal pyramidal if one is a nonbonding pairBent if there are two nonbonding pairs
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Trigonal Bipyramidal Electron Domain
• There are two distinct positions in this geometry:AxialEquatorial
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Trigonal Bipyramidal Electron Domain
Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.
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Trigonal Bipyramidal Electron Domain
• There are four distinct molecular geometries in this domain:Trigonal bipyramidalSeesawT-shapedLinear
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Octahedral Electron Domain
• All positions are equivalent in the octahedral domain.
• There are three molecular geometries:OctahedralSquare pyramidalSquare planar
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Using VSEPR, predict the molecular geometry of NH3.
1. Linear
2. Trigonal planar
3. Trigonal pyramidal
4. Tetrahedral
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Correct Answer:
1. Linear
2. Trigonal planar
3. Trigonal pyramidal
4. Tetrahedral
As indicated in the illustration, the molecular geometry of NH3 is trigonal pyramidal.
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Using VSEPR, predict the molecular geometry of SF4.
1. T-shaped
2. Seesaw-shaped
3. Trigonal bipyramidal
4. Linear
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Correct Answer:
1. T-shaped
2. Seesaw-shaped
3. Trigonal bipyramidal
4. Linear
The molecular geometry of SF4 is seesaw.
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Using VSEPR, predict the molecular geometry of O3.
1. Bent
2. Trigonal planar
3. Trigonal pyramidal
4. Linear
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Correct Answer:
1. Bent
2. Trigonal planar
3. Trigonal pyramidal
4. Linear
The molecular geometry of O3 is bent.
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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.
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Larger Molecules
This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.
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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.
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Polarity
By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.
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Polarity
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CO2: Polar or nonpolar?
1.Polar
2.Nonpolar
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Correct Answer:
The bond dipoles in CO2 cancel one another such that the molecule will exhibit no net overall dipole. Thus, it is a nonpolar molecule.
1.Polar
2.Nonpolar
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Yes
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SO2: Polar or nonpolar?
1.Polar
2.Nonpolar
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Correct Answer:
1.Polar
2.Nonpolar
Clearly from the bond dipoles the molecule SO2 will exhibit a net overall dipole, thus being a polar molecule.
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Sections 9.4 - 9.8
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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.
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Overlap and Bonding
• Increased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion.
• However, if atoms get too close, the internuclear repulsion greatly raises the energy.
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QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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Yes
Cl2 forms from the overlap of two 3p orbitals while F2 forms from the overlap of two 2p orbitals which are smaller than 3p orbitals.
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Hybrid Orbitals
But it’s hard to imagine tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize.
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Hybrid Orbitals
• Consider beryllium: In its ground electronic
state, it would not be able to form bonds because it has no singly-occupied orbitals.
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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.
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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.
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Hybrid Orbitals
• These two degenerate orbitals would align themselves 180 from each other.
• This is consistent with the observed geometry of beryllium compounds: linear.
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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.
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Hybrid Orbitals
Using a similar model for boron leads to…
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Hybrid Orbitals
…three degenerate sp2 orbitals.
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Hybrid Orbitals
With carbon we get…
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Hybrid Orbitals
…four degenerate
sp3 orbitals.
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MolecularGeometries
and Bonding
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MolecularGeometries
and Bonding
The unhybridized p-orbital is perpendicular to the plane of the sp2 orbitals.
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Hybrid Orbitals
For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.
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Hybrid Orbitals
This leads to five degenerate sp3d orbitals…
…or six degenerate sp3d2 orbitals.
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QuickTime™ and aSorenson Video decompressorare needed to see this picture.
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Hybrid Orbitals
Once you know the electron-domain geometry, you know the hybridization state of the atom.
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All you need to know about orbital hybridization
Valence shell atomic orbitals (including inner valence d orbitals) on two atoms overlap to produce covalent bonds.
Atomic orbitals hybridize to produce orbitals of equal energy with spatial orientations that can be predicted by VSEPR theory.
--based on ELECTRON DOMAIN Geometry
Linear tri plan tetra tri bypyr octa
2 = sp 3 = sp2 4 = sp3 5 = sp3d 6 = sp3d2
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MolecularGeometries
and Bonding
Which kind of hybridization is about the central atom in PCl5?
1. sp2
2. sp3
3. sp4
4. sp3d
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MolecularGeometries
and Bonding
Correct Answer:
1. sp2
2. sp3
3. sp4
4. sp3dThe hybridization for PCl5 is sp3d, as shown to the right.
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MolecularGeometries
and Bonding
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MolecularGeometries
and Bonding
Yes, they’d be equivalent and we’d expect the bond angles to be 90°.
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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.
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Sigma () Bonds
• Sigma bonds are characterized byHead-to-head overlap.Cylindrical symmetry of electron density about the
internuclear axis.
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Pi () Bonds
• Pi bonds are characterized bySide-to-side overlap.Electron density
above and below the internuclear axis.
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Single Bonds
Single bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.
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Multiple Bonds
In a multiple bond one of the bonds is a bond and the rest are bonds.
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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.
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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.
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MolecularGeometries
and Bonding
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MolecularGeometries
and Bonding
The molecule is not linear but is planar.
The 2 - 4 electron domains that come off two atoms in a double bond are always planar with the double bond
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MolecularGeometries
and Bonding
In the molecule HCN, how many sigma bonds () and how many pi bonds () are there?
1. One sigma, one pi
2. Two sigma, one pi
3. Two sigma, two pi
4. None of the above
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MolecularGeometries
and Bonding
Correct Answer:
1. One sigma, one pi
2. Two sigma, one pi
3. Two sigma, two pi
4. None of the aboveTotal = Two sigma + two pi
C NH
One sigma
One sigma Two pi
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MolecularGeometries
and Bonding
In the molecule C2H4, how many sigma bonds () and how many pi bonds () are there?
1. Five sigma, one pi
2. Four sigma, two pi
3. Six sigma, zero pi
4. None of the above
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MolecularGeometries
and Bonding
Correct Answer:
1. Five sigma, one pi
2. Four sigma, two pi
3. Six sigma, zero pi
4. None of the above
The sigma and pi bonds for C2H4 are indicated in the illustration.
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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.
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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.
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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.
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Resonance
The organic molecule benzene has six bonds and a p orbital on each carbon atom.
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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.
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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.
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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.
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Molecular Orbital (MO) Theory
If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.
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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.
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MO Theory
For hydrogen, with two electrons in the bonding MO and none in the antibonding MO, the bond order is
12
(2 - 0) = 1
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MO Theory
• In the case of He2, the bond order would be
12
(2 - 2) = 0
• Therefore, He2 does not exist.
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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.
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MO Theory
• The resulting MO diagram looks like this.
• There are both and bonding molecular orbitals and * and * antibonding molecular orbitals.
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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 s and p molecular orbitals in these elements.
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Second-Row MO Diagrams