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MolecularGeometries
and Bonding
Molecular Structures and Resonance
CHAPTER 9.3
MolecularGeometries
and Bonding
Structural Formulas
• Most useful of all; predict relative location of atoms by using letter symbols and bonds.
MolecularGeometries
and Bonding
Writing Lewis Structures for Ions
• Follow the same rules except forCations (+) = SUBTRACT the # on the chargeAnions (-) = ADD the # on the charge
• Place structure inside brackets and show the oxidation number outside.Ex: Phosphate ion, PO4
3-
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
Resonance
The organic molecule benzene has six bonds and a p orbital on each carbon atom.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
Draw Lewis structures for the following; include resonance
structures (if any) and formal charges
1. HCN
2. C2H2
3. HOF
4. BF3
5. HCFO
6. COCl27. N2F2
8. N2O3
9. ICl310. COS
11. PO43-
12. ClO4-
13. NO3-
14. NH4+
15. SO42-
MolecularGeometries
and Bonding
Molecular Geometriesand Bonding Theories
CHAPTER 9.4
MolecularGeometries
and Bonding
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 the molecule.
MolecularGeometries
and Bonding
What Determines the Shape of a Molecule?
• Electron pairs (both bonding & lone) repel each other.
• The shape of the molecule can be predicted by placing the electron pairs as far away from each other.
MolecularGeometries
and Bonding
Electron Domains
• We can refer to the electron pairs as electron domains.
• A double or triple bond, count as one electron domain.
• This molecule has four electron domains.
MolecularGeometries
and Bonding
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.”
MolecularGeometries
and Bonding
Electron-Domain Geometries
These are the electron-domain geometries for two through six electron domains around a central atom.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
Molecular Geometries
One electron domain may = 2 or more molecular geometries (shape), depending on lone pairs.
MolecularGeometries
and Bonding
Linear Electron Domain (2)
• 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.
MolecularGeometries
and Bonding
Trigonal Planar Electron Domain (3)
• There are two molecular geometries: Trigonal planar, if all the electron domains are
bonding Bent, if one of the domains is a nonbonding pair.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
Tetrahedral Electron Domain (4)
• There are three molecular geometries:Tetrahedral, if all are bonding pairsTrigonal pyramidal if one is a nonbonding pairBent if there are two nonbonding pairs
MolecularGeometries
and Bonding
Trigonal Bipyramidal Electron Domain (5)
• There are two distinct positions in this geometry:AxialEquatorial
MolecularGeometries
and Bonding
Trigonal Bipyramidal Electron Domain
Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.
Place lone pairs in equatorial positions ONLY!
MolecularGeometries
and Bonding
Trigonal Bipyramidal Electron Domain
• There are four distinct molecular geometries in this domain:Trigonal bipyramidalSeesawT-shapedLinear
MolecularGeometries
and Bonding
Octahedral Electron Domain (6)
• All positions are equivalent in the octahedral domain.
• There are three molecular geometries:OctahedralSquare pyramidalSquare planar
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
Larger Molecules
This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.
MolecularGeometries
and Bonding
Electronegativity and Polarity
CHAPTER 9.5
MolecularGeometries
and Bonding
Polarity• Occurs when electrons
are unequally shared.• Dipoles- regions where
atoms have large electronegativity differences creating two poles (+, -)
• But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.
MolecularGeometries
and Bonding
Polarity
By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.
MolecularGeometries
and Bonding
Polarity
MolecularGeometries
and Bonding
Hybrid Orbitals
But it’s hard to imagine tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize.
MolecularGeometries
and Bonding
Hybrid Orbitals
• Consider beryllium: In its ground electronic
state, it would not be able to form bonds because it has no singly-occupied orbitals.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
Hybrid Orbitals
• Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals called sp hybrid.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.
MolecularGeometries
and Bonding
Hybrid Orbitals
• These two degenerate orbitals would align themselves 180 from each other.
• This is consistent with the observed geometry of beryllium compounds: linear.
MolecularGeometries
and Bonding
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.
MolecularGeometries
and Bonding
Hybrid Orbitals
Using a similar model for boron leads to…sp2 hybrid orbital b/c it uses the one 2s orbital
and two orbitals from 2p.
MolecularGeometries
and Bonding
Hybrid Orbitals
The three degenerate sp2 orbitals.
MolecularGeometries
and Bonding
Hybrid Orbitals
With carbon we get…4 degenerate orbitals are sp3 hybrid Uses the one 2s orbital and all three 2p orbitals.
MolecularGeometries
and Bonding
Hybrid Orbitals
The four degenerate
sp3 orbitals.
MolecularGeometries
and Bonding
Hybrid Orbitals
For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.
MolecularGeometries
and Bonding
Hybrid Orbitals
This leads to five degenerate sp3d orbitals…
…or six degenerate sp3d2 orbitals.
MolecularGeometries
and Bonding
Hybrid Orbitals
Once you know the electron-domain geometry, you know the hybridization state of the atom.