CH 9 Molecular Geometries

8/3/2019 CH 9 Molecular Geometries

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MolecularGeometries

and Bonding

Chapter 9Molecular Geometries

and Bonding Theories


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MolecularGeometries

and Bonding

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Fifth level

What Determines the Shape of aMolecule?

Simply put, electron pairs,whether they be bonding ornonbonding, repel eachother.

By assuming the electronpairs are placed as far aspossible from each other,we can predict the shape ofthe molecule.


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MolecularGeometries

and Bonding

Electron Domains

We can refer to the electronpairs as electron domains.

In a double or triple bond,

all electrons sharedbetween those two atomsare on the same side of thecentral atom; therefore,they count as one electrondomain.



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This molecule hasfour electrondomains.


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MolecularGeometries

and Bonding

Valence Shell Electron PairRepulsion Theory (VSEPR)

The bestarrangement of a

given number ofelectron domains isthe one that minimizesthe repulsions amongthem.



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MolecularGeometries

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Electron-Domain Geometries

All one must do iscount the number ofelectron domains inthe Lewis structure.

The geometry will bethat which

corresponds to thatnumber of electrondomains.



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MolecularGeometries

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Molecular Geometries

The electron-domain geometry is often notthe

shape of the molecule, however.The molecular geometry is that defined by thepositions ofonlythe atoms in the molecules, notthe nonbonding pairs.


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MolecularGeometries

and Bonding

Trigonal Planar Electron Domain

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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MolecularGeometries

and Bonding

Nonbonding Pairs and Bond Angle

Nonbonding pairs are physicallylarger than bonding pairs.

Therefore, their repulsions aregreater; this tends to decreasebond angles in a molecule.



Fifth level


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MolecularGeometries

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Multiple Bonds and Bond Angles

Double and triplebonds place greaterelectron density onone side of the centralatom than do singlebonds.

Therefore, they alsoaffect bond angles.



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MolecularGeometries

and Bonding

Tetrahedral Electron Domain

There are three molecular geometries:

Tetrahedral, if all are bonding pairs

Trigonal pyramidal if one is a nonbonding pair

Bent if there are two nonbonding pairs



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MolecularGeometries

and Bonding

Trigonal Bipyramidal ElectronDomain

There are two distinctpositions in thisgeometry:

Axial

Equatorial



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MolecularGeometries

and Bonding


Lower-energy conformations result from

having nonbonding electron pairs in equatorial,rather than axial, positions in this geometry.



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MolecularGeometries

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There are four distinctmolecular geometriesin this domain:

Trigonal bipyramidal

Seesaw

T-shaped

Linear



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MolecularGeometries

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Octahedral Electron Domain

All positions areequivalent in theoctahedral domain.

There are threemoleculargeometries:

Octahedral

Square pyramidal

Square planar


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MolecularGeometries

and Bonding

Larger Molecules

In larger molecules,it makes more senseto talk about the

geometry about aparticular atom ratherthan the geometry ofthe molecule as a

whole.



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MolecularGeometries

and Bonding

Polarity

In Chapter 8 wediscussed bond dipoles.

But just because amolecule possesses polarbonds does not mean themolecule as a whole will

be polar.


Third level



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MolecularGeometries

and Bonding

Polarity

By adding theindividual bonddipoles, one candetermine the overalldipole moment for themolecule.

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MolecularGeometries

and Bonding

Polarity



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MolecularGeometries

and Bonding

Overlap and Bonding

We think of covalent bonds forming through thesharing of electrons by adjacent atoms.

In such an approach this can only occur whenorbitals on the two atoms overlap.



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MolecularGeometries

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Third level


Overlap and Bonding

Increased overlap bringsthe electrons and nucleicloser together whilesimultaneously decreasing

electron-electron repulsion.

However, if atoms get tooclose, the internuclearrepulsion greatly raises theenergy.


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MolecularGeometries

and Bonding

Hybrid Orbitals

But its hard to imagine tetrahedral, trigonal

bipyramidal, and other geometries arising fromthe atomic orbitals we recognize.



Fifth level


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MolecularGeometries

and Bonding

Hybrid Orbitals

Consider beryllium:

In its ground electronic state,

it would not be able to formbonds because it has nosingly-occupied orbitals.



Fifth level


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MolecularGeometries

and Bonding

Hybrid Orbitals

But if it absorbs thesmall amount of energyneeded to promote anelectron from the 2s tothe 2p orbital, it can formtwo bonds.



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MolecularGeometries

and Bonding

Hybrid Orbitals

Mixing the s andp orbitals yields two degenerateorbitals that are hybrids of the two orbitals.

These sp hybrid orbitals have two lobes like ap orbital.

One of the lobes is larger and more rounded as is the sorbital.



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MolecularGeometries

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Hybrid Orbitals

These two degenerate orbitals would alignthemselves 180 from each other.

This is consistent with the observed geometry of

beryllium compounds: linear.



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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 1sorbital but lower than the 2p.


Third level



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MolecularGeometries

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Hybrid Orbitals

Using a similar model for boron leads to

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MolecularGeometries

and Bonding

Hybrid Orbitals



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three degenerate sp2orbitals.


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MolecularGeometries

and Bonding

Hybrid Orbitals

With carbon we get

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MolecularGeometries

and Bonding

Hybrid Orbitals



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four degenerate

sp3 orbitals.


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MolecularGeometries

and Bonding

Hybrid Orbitals

For geometries involving expanded octets onthe central atom, we must use dorbitals in ourhybrids.



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MolecularGeometries

and Bonding

Hybrid Orbitals

This leads to five degeneratesp3dorbitals

or six degenerate sp3d2orbitals.



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MolecularGeometries

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Hybrid Orbitals

Once you know theelectron-domaingeometry, you knowthe hybridizationstate of the atom.


Third level



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MolecularGeometries

and Bonding

Valence Bond Theory

Hybridization is a major player in thisapproach to bonding.

There are two ways orbitals can overlapto form bonds between atoms.


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MolecularGeometries

and Bonding

Sigma () Bonds

Sigma bonds are characterized by

Head-to-head overlap.

Cylindrical symmetry of electron density about theinternuclear axis.



Fifth level


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MolecularGeometries

and Bonding

Pi () Bonds

Pi bonds arecharacterized by

Side-to-side overlap.Electron density aboveand below the internuclearaxis.



Fifth level


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MolecularGeometries

and Bonding

Single Bonds

Single bonds are always ,

,

.



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MolecularGeometries

and Bonding

Multiple Bonds

In a multiple bond one of the bonds is a

.



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MolecularGeometries

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Multiple Bonds

In a molecule likeformaldehyde (shown atleft) an sp2orbital on

carbon overlaps in

.

.



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ick to edit Master text stylescond levelThird level Fourth level

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MolecularGeometries

and Bonding

Delocalized Electrons: Resonance

When writing Lewis structures for species likethe nitrate ion, we draw resonance structures tomore accurately reflect the structure of themolecule or ion.


Second level Third level Fourth level

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MolecularGeometries

and Bonding

Delocalized Electrons: Resonance

In reality, each of the four atomsin the nitrate ion has ap orbital.

Thep orbitals on all threeoxygens overlap with theporbital on the central nitrogen.


Third level



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R


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MolecularGeometries

and Bonding

Resonance

The organic molecule

benzene has six

.



Fifth levelClick to edit Master text stylesSecond level


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R


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MolecularGeometries

and Bonding

Resonance

In reality the ,

.

.



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M l l O bit l (MO) Th


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MolecularGeometries

and Bonding

Molecular Orbital (MO) Theory

Though valence bondtheory effectively conveysmost observed properties

of ions and molecules,there are some conceptsbetter represented bymolecular orbitals.



Fifth level



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MolecularGeometries

and Bonding


In MO theory, we invokethe wave nature ofelectrons.

If waves interactconstructively, theresulting orbital is lower

in energy: a bondingmolecular orbital.



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MolecularGeometries

and Bonding


If waves interactdestructively, theresulting orbital is

higher in energy: anantibonding molecularorbital.



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MO Th


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MolecularGeometries

and Bonding

MO Theory

For hydrogen, with twoelectrons in the bondingMO and none in the

antibonding MO, the bondorder is



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1

2 (2 - 0) = 1

MO Th


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MolecularGeometries

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MO Theory

In the case of He2, thebond order would be Click to edit Master text styles

Second level

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12

(2 - 2) = 0

Therefore, He2

does not exist.

MO Th


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MolecularGeometries

and Bonding

MO Theory

For atoms with both s andp orbitals, there are twotypes of interactions:

The s and thep orbitals that

face each other overlap in

.

.



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MO Th


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MO Theory

The resulting MOdiagram looks like this.

There are both

.



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MO Th


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MolecularGeometries

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Second level Third level


MO Theory

The smallerp-block elements inthe second period have a sizeableinteraction between the s andp

orbitals.

This flips the order of the s and pmolecular orbitals in theseelements.



Fifth level

S d R MO Di


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MolecularGeometries

and Bonding

Second-Row MO Diagrams



Fifth level

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CH 9 Molecular Geometries