Chapter Chapter 1010Chemical Chemical Bonding IIBonding II

Valence Bond TheoryValence Bond Theory Linus Pauling and others applied the principles of quantum

mechanics to molecules they reasoned that bonds between atoms would arise when

the orbitals on those atoms interacted to make a bond the kind of interaction depends on whether the orbitals

align along the axis between the nuclei, or outside the axis Valence Bond Theory: A quantum mechanical model

which shows how electron pairs are shared in a covalent bond. ◦ Bond forms between two atoms when the following conditions are met:

◦ Covalent bonds are formed by overlap of atomic orbitals, each of which contains one electron of opposite spin.

◦ Each of the bonded atoms maintains its own atomic orbitals, but the electron pair in the overlapping orbitals is shared by both atoms.

◦ The greater the amount of overlap, the stronger the bond.

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Orbital InteractionOrbital Interaction In some cases, atoms use “simple” atomic orbital (e.g.,

1s, 2s, 2p, etc.) to form bonds. In other case, they use a “mixture” of simple atomic

orbitals known as “hybrid” atomic orbitals. as two atoms approached, the partially filled or empty

valence atomic orbitals on the atoms would interact to form molecular orbitals

the molecular orbitals would be more stable than the separate atomic orbitals because they would contain paired electrons shared by both atoms

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Valence Bond Theory - Valence Bond Theory - HybridizationHybridization

one of the issues that arose was that the number of partially filled or empty atomic orbital did not predict the number of bonds or orientation of bonds ◦ C = 2s22px

12py12pz

0 would predict 2 or 3 bonds that are 90° apart, rather than 4 bonds that are 109.5° apart

to adjust for these inconsistencies, it was postulated that the valence atomic orbitals could hybridize before bonding took place◦ one hybridization of C is to mix all the 2s and 2p

orbitals to get 4 orbitals that point at the corners of a tetrahedron

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Valence Bond Theory Main Valence Bond Theory Main ConceptsConcepts

1. the valence electrons in an atom reside in the quantum mechanical atomic orbitals or hybrid orbitals

2. a chemical bond results when these atomic orbitals overlap and there is a total of 2 electrons in the new molecular orbital

a) the electrons must be spin paired3. the shape of the molecule is determined by the

geometry of the overlapping orbitals

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Types of BondsTypes of Bonds a sigma () bond

results when the bonding atomic orbitals point along the axis connecting the two bonding nuclei◦ either standard

atomic orbitals or hybrids s-to-s, p-to-p,

hybrid-to-hybrid, s-to-hybrid, etc.

a pi () bond results when the bonding atomic orbitals are parallel to each other and perpendicular to the axis connecting the two bonding nuclei◦ between unhybridized

parallel p orbitals the interaction between

parallel orbitals is not as strong as between orbitals that point at each other; therefore bonds are stronger than bonds

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s s s p

Overlapping between s orbital to s orbital and s to p orbital

Side way overlapping of the 2 p orbitals

Tro, Chemistry: A Molecular Approach 7

HybridizationHybridization some atoms hybridize their orbitals to maximize

bonding◦ hybridizing is mixing different types of orbitals to

make a new set of degenerate orbitals◦ sp, sp2, sp3, sp3d, sp3d2

◦ more bonds = more full orbitals = more stability better explain observed shapes of molecules same type of atom can have different hybridization

depending on the compound◦ C = sp, sp2, sp3

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Hybrid OrbitalsHybrid Orbitals H cannot hybridize!! the number of standard atomic orbitals combined =

the number of hybrid orbitals formed the number and type of standard atomic orbitals

combined determines the shape of the hybrid orbitals the particular kind of hybridization that occurs is the

one that yields the lowest overall energy for the molecule◦ in other words, you have to know the structure of

the molecule beforehand in order to predict the hybridization

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spsp33 Hybridization of C Hybridization of C

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spsp3 3 HybridizationHybridization atom with 4 areas of electrons

◦ tetrahedral geometry◦ 109.5° angles between hybrid orbitals

atom uses hybrid orbitals for all bonds and lone pairs

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H C N H

H

H H

s

•• sp3

s

sp3

Methane Formation Methane Formation with with spsp33 C C

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Ammonia Formation with Ammonia Formation with spsp33 NN

spsp22

atom with 3 areas of electrons ◦ trigonal planar system

C = trigonal planar N = trigonal bent O = “linear”

◦ 120° bond angles◦ flat

atom uses hybrid orbitals for bonds and lone pairs, uses nonhybridized p orbital for bond

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3-D representation of ethane (C2H4)

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Bond RotationBond Rotation because orbitals that form the bond point along the

internuclear axis, rotation around that bond does not require breaking the interaction between the orbitals

but the orbitals that form the bond interact above and below the internuclear axis, so rotation around the axis requires the breaking of the interaction between the orbitals

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spsp atom with 2 areas of

electrons◦ linear shape◦ 180° bond angle

atom uses hybrid orbitals for bonds or lone pairs, uses nonhybridized p orbitals for bonds

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spsp33dd atom with 5 areas of electrons

around it◦ trigonal bipyramid shape◦ See-Saw, T-Shape, Linear◦ 120° & 90° bond angles

use empty d orbitals from valence shell

d orbitals can be used to make bonds

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spsp33dd22

atom with 6 areas of electrons around it◦ octahedral shape◦ Square Pyramid, Square Planar◦ 90° bond angles

use empty d orbitals from valence shell d orbitals can be used to make bonds

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Five types of hybrid are shown below

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# of e- groups around central

atom

Hybrid orbitals used

Orientation of Hybrid Orbitals

2 sp  3 sp2  4 sp3  5 sp3d  6 sp3d2  

Predicting Hybridization and Predicting Hybridization and Bonding SchemeBonding Scheme1) Start by drawing the Lewis Structure2) Use VSEPR Theory to predict the

electron group geometry around each central atom

3) Use Table 10.3 to select the hybridization scheme that matches the electron group geometry

4) Sketch the atomic and hybrid orbitals on the atoms in the molecule, showing overlap of the appropriate orbitals

5) Label the bonds as or 20

Examples:Examples: Predict the Hybridization and Bonding Scheme of All

the Atoms in Then sketch a σ framework and a π framework

CH3CHO

CH2NH

H3BO3

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O N Cl ••

••

••

••

••••

Problems with Valence Bond Problems with Valence Bond TheoryTheoryVB theory predicts many

properties better than Lewis Theory◦bonding schemes, bond strengths,

bond lengths, bond rigidityhowever, there are still many

properties of molecules it doesn’t predict perfectly◦magnetic behavior of O2

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