Hybrid Orbitals
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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.
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MolecularGeometries
and Bonding
Hybrid Orbitals
Using a similar model for boron leads to…
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MolecularGeometries
and Bonding
Hybrid Orbitals
…three degenerate sp2 orbitals.
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MolecularGeometries
and Bonding
Hybrid Orbitals
With carbon we get…
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MolecularGeometries
and Bonding
Hybrid Orbitals
…four degenerate
sp3 orbitals.
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MolecularGeometries
and Bonding
Hybrid Orbitals
For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids.
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MolecularGeometries
and Bonding
Hybrid Orbitals
This leads to five degenerate sp3d orbitals…
…or six degenerate sp3d2 orbitals.
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MolecularGeometries
and Bonding
Hybrid Orbitals
Once you know the electron-domain geometry, you know the hybridization state of the atom.
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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
Sigma () Bonds
• Sigma bonds are characterized byHead-to-head overlap.Cylindrical symmetry of electron density about the
internuclear axis.
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MolecularGeometries
and Bonding
Pi () Bonds
• Pi bonds are characterized bySide-to-side overlap.Electron density
above and below the internuclear axis.
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MolecularGeometries
and Bonding
Single Bonds
Single bonds are always bonds, because overlap is greater, resulting in a stronger bond and more energy lowering.
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MolecularGeometries
and Bonding
Multiple Bonds
In a multiple bond one of the bonds is a bond and the rest are bonds.
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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
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
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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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.
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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.
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MolecularGeometries
and Bonding
Resonance
The organic molecule benzene has six bonds and a p orbital on each carbon atom.
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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.
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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
Molecular Orbital (MO) Theory
If waves interact destructively, the resulting orbital is higher in energy: an antibonding molecular orbital.
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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
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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MolecularGeometries
and Bonding
Second-Row MO Diagrams