Ch 10 - 10. 7 Valence Bond Theory: Hybridization
A double bond in the Lewis model always corresponds to one σ and one π bond in valence bond theory.
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A single bond is a sigma bond
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Carbon always hybridizes
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Even though we represent the two electrons in a π bond as two half arrows in the upper lobe, they are actually spread out over both the upper and lower lobes
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Formation of sp Hybrid Orbitals
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Two atoms can form only one sigma bond
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double bond consists of a sigma bond and a pi bond
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hybridization occurs only to the degree that the energy payback through bond formation is large
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rotation about a single bond is relatively unrestricted
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sp³d orbitals energy = 3p orbital energy
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the orbitals in a molecule are not necessarily the same as the orbitals in an atom
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triple bond consists of a sigma bond and two pi bonds
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Examples of sigma bonds
1. Two s orbitals could overlap (like in H₂) 2. hybridized orbital and an s orbital (like in C-H bond) 3. Two hybridized orbitals (C-C) bond
Sigma bonds (σ)
1. bonds in which orbital overlap occurs between the two bonding nuclei 2.
Pi bonds (π)
1. bonds where orbital overlap occurs above and below the nuclei, and not directly between them
Hybrid orbitals
still localized on individual atoms, but they have different shapes and energies from those of standard atomic orbitals
The more bonds that an atom forms...
the greater the tendency of its orbitals to hybridize Central or interior atoms have the greatest tendency to hybridize
the notation "sp³" orbitals means...
the hybrid orbitals are mixtures of 1 (s) orbital and 3 (p) orbitals
The number of standard atomic orbitals added together always equals
the number of hybrid orbitals formed. The total number of orbitals is conserved.
The particular type of hybridization that occurs is...
the one that yields the lowest overall energy for the molecule
The particular combinations of standard atomic orbitals added together determines
the shapes and energies of the hybrid orbitals formed
One type of overlap makes a pi bond
the side-by-side overlap between p orbitals With the side-by-side overlap of p orbitals, there is no overlap directly between the bonding nuclei, because the p orbitals have nodes in this region There is, however, overlap above and below the nuclei
The greater the overlap
the stronger the bond and the lower the energy
Why are 3d orbitals involved in hybridization?
their energies are close to the energies of the 3s and 3p orbitals
sp³ orbitals all have the same energy
they are degenerate
Hybridization of one s and two p orbitals results in
three sp² hybrids and one leftover unhybridized p orbital.
sp³d hybrid orbital shape
trigonal bipyramidal Results in 5 hybrid orbitals and 4 unhybridized d orbitals Example: AsF₅
Hybridization of one s and one p orbital results in..
two sp hybrid orbitals and 2 leftover unhybridized p orbitals
Which bond is easier to break?
π bond
Which bond is stronger?
σ bonds because the end-to-end overlap is more efficient than the side-to-side
How do we assign a hybridization scheme?
From the electron geometry
Why is the rotation about a double bond severely restricted?
Because of the side-by-side overlap of the p orbitals, the π bond must essentially break for rotation to occur
Formation of sp³ orbitals
Carbon has 4 half-filled orbitals and can form 4 bonds with 4 hydrogen atoms
Example of sp³ orbitals with lone pair
NH₃ (ammonia)
Tetrahedral geometry is accounted for by
The hybridization of: one 2s orbital and three 2p orbitals on the carbon atom NOTE: 4 new orbitals result called sp³ hybrids
What is a Chemical Bonds in Valence Bond Theory
a chemical bond is the overlap of 2 orbitals that together contain 2 electrons
Hybridization
a mathematical procedure in which the standard atomic orbitals are combined to form new atomic orbitals called hybrid orbitals
Why are Pi bonds less common than sigma bonds
because they are found only in double and triple bonds, not in single bonds
How do hybrid orbitals minimize the energy of the molecule?
by maximizing the orbital overlap in a bond
If the central atom of a molecule contains lone pairs...
hybrid orbitals can also accommodate them Note: The presence of the lone pair Lowers the tendency of an atom's orbitals to hybridize
Expanded octets =
hybridization involving d orbitals of 3rd period elements
sp³d²
octahedral geometry Results in 6 hybrid orbitals and 3 unhybridized d orbitals Example SF₆
trans
opposite side
cis
same side
