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

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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.”

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Electron-Domain Geometries

These are the electron-domain geometries for two through six electron domains around a central atom.

MolecularGeometries

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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.

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

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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

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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.

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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.

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

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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.

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Polarity

By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

MolecularGeometries

and Bonding

Polarity

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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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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

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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.

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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.

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.

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Hybrid Orbitals

The three degenerate sp2 orbitals.

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Hybrid Orbitals

With carbon we get…4 degenerate orbitals are sp3 hybrid Uses the one 2s orbital and all three 2p orbitals.

MolecularGeometries

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Hybrid Orbitals

The four degenerate

sp3 orbitals.

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Hybrid Orbitals

For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.

MolecularGeometries

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Hybrid Orbitals

This leads to five degenerate sp3d orbitals…

…or six degenerate sp3d2 orbitals.

MolecularGeometries

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Hybrid Orbitals

Once you know the electron-domain geometry, you know the hybridization state of the atom.