10 - 1 molecular shape and theory of chemical bonding shapes of molecules and polyatomic ions polar...

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Molecular Shape and Molecular Shape and Theory of Chemical BondingTheory of Chemical Bonding

Shapes of Molecules and Polyatomic IonsShapes of Molecules and Polyatomic Ions

Polar and Nonpolar MoleculesPolar and Nonpolar Molecules

Bonding TheoryBonding Theory

Molecular Orbital MethodMolecular Orbital Method

Delocalized ElectronsDelocalized Electrons

Band Theory of Bonding in SolidsBand Theory of Bonding in Solids

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Shapes of moleculesShapes of moleculesand polyatomic ionsand polyatomic ions

Molecules and polyatomic ions are not all ‘flat’ structures.

Many have a three dimensional arrangement that helps account for their various chemical and physical properties.

Several models are used to help predict and describe the geometries for these species.

One model is called the Valence Shell Valence Shell Electron Pair Repulsion model (VSEPR)Electron Pair Repulsion model (VSEPR)

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VSEPR modelVSEPR model

According to this model, for main group elements, electron pairs will be as far apart from each other as possible.

This occurs in three dimensional space.

Both bonded and unshared pairs will occupy space with unshared pairs taking up more space.

The geometry is based on the total number of electron pairs - total coordination number.

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VSEPR shapesVSEPR shapes

Coordination Electron pairs General Number Bonding Unshared Formula Shape

2 2 0 AB2 Linear

3 3 0 AB3 Trigonal planar 2 1 AB2 Bent

4 4 0 AB4 Tetrahedral 3 1 AB3 Trigonal

pyramidal

2 2 AB2 Bent 1 3 AB Linear

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Molecular geometryMolecular geometry

Molecules have specific shapes.Molecules have specific shapes.

•Determined by the number of electron pairs around the central species

•Bonded and unshared pairs count.

•Multiple bonds are treated as a single bond for geometry.

Geometry affects factors like polarity and solubility.

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Some common geometriesSome common geometries

ee-- pairs around pairs aroundShapeShape central atomcentral atom Example Example

Linear 2 BeH2

Trigonal planar 3 BF3

Tetrahedral 4 CH4

Trigonal pyramidal 4 NH3

Bent 4 H2O

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Linear - COLinear - CO22

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Trigonal planar, BClTrigonal planar, BCl33

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Bent, HBent, H22OO

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Pyramidal, NHPyramidal, NH33

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Tetrahedral, CHTetrahedral, CH44

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Molecular geometries Molecular geometries based on tetrahedralbased on tetrahedral

Bent and pyramidal areactually tetrahedral but

some of the electronpairs are not bonded.H

NH H

Pyramidal

H

CH H

HTetrahedral

Bent H

OH

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Other geometries.Other geometries.

Other shapes are also observed.

Five bonds or lone electron pairsFive bonds or lone electron pairsTrigonal bipyramidalSeesawT-shapedLinear

Six bonds or lone electron pairsSix bonds or lone electron pairsOctahedralSquare pyramidalSquare planar

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VSEPR shapesVSEPR shapes

Coordination Electron pairs General Number Bonding Unshared Formula Shape

5 5 0 AB5 Trigonal

bipyramidal

4 1 AB4 Seesaw 3 2 AB3 T-shaped 2 3 AB2 Linear

6 6 0 AB6 Octahedral 5 1 AB5 Square

pyramidal

4 2 AB4 Square Planar

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Trigonal bipyramidalTrigonal bipyramidal

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Square planarSquare planar

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OctahedralOctahedral

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Molecular geometryMolecular geometry

As molecules get larger, the rules regardingmolecular geometry still hold.

H

C H

H

H

C

HH

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EthaneEthane

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Geometry and polar moleculesGeometry and polar molecules

For a molecule to be polarFor a molecule to be polar- must have polar bonds- must have the proper geometry

CH4

non-polar

CH3Cl polar

CH2Cl2 polar

CHCl3 polar

CCl4non-polar

WHY?WHY?

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Polar and nonpolar moleculesPolar and nonpolar molecules

Polarity is an important property of molecules.

• It affects physical properties such as melting point, boiling point and solubility.

• Chemical properties also depend on polarity.

• Dipole momentDipole moment, , is a quantitative measure of the polarity of a molecule.

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Dipole momentDipole moment

This property can be measured by placing moleculesin an electrical field. Polar molecules will align whenThe field is on. Nonpolar molecules will not.

+ - + -

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Most bonds between atoms of dissimilar elements in a molecule are polar. That does not mean that the molecule will be polar.

O = C = O

Electronegativities: Oxygen = 3.5 Carbon = 2.5Difference 1.0 (polar bond)

Electronegativities: Oxygen = 3.5 Carbon = 2.5Difference 1.0 (polar bond)

The electronegativity valuesShow that the C-O bond would be polar with electronsBeing pulled towards the oxygens. However, due toThe geometry, the pull happens in equal and oppositedirections.

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For a molecule to be polar, the effects of bond polaritymust not cancel out.

One way is to have a geometry that is not symmetrical like in water.

Electronegativitydifference = 1.3

Here, the effects of the polar bonds do not canceled so the molecule is polar.

H HO....

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A molecule is nonpolar if the central atom is symmetrically substituted by identical atoms.

COCO22, CH, CH4 4 , CCl, CCl44

A molecule will be polar if the geometry is not symmetrical.

HH22O, NHO, NH33, CH, CH22ClCl22

The degree of polarity is a function of the number and type of polar bonds as well as the geometry.

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Bonding theoryBonding theory

Two methods of approximation are used to describe bonding between atoms.

Valence bond methodValence bond methodBonds are assumed to be formed by overlap of atomic orbitals

Molecular orbital methodMolecular orbital methodWhen atoms form compounds, their orbitals combine to form new orbitals - molecular orbitalsmolecular orbitals.

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Valence bond methodValence bond method

According to this model, the H-H bond forms as a result of the overlap of the 1s orbitals from each atom.

74 pm

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Valence bond methodValence bond method

Hybrid orbitals are need to account for the geometry that we observe for many molecules.

Example - CarbonExample - CarbonOuter electron configuration of 2Outer electron configuration of 2ss22 2p2pxx

11 2p2pyy11

We know that carbon will form2 four equivalent bonds - CH4, CH2Cl2 , CCl4.

The electron configuration appears to indicate that only two bonds would form and they would be at right angles -- not tetrahedral angles.

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HybridizationHybridization

To explain why carbon forms four identical single bonds, we assume the the original orbitals will blend together.

Unhybridized Hybridized

energ

y

2s

2p

2sp3

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HybridizationHybridization

In the case of a carbon that has 4 single bonds, all of the orbitals are hybrids.

sp3

25% s and 75% p character

+ 3

s p sp3

1 4

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Ethane, CHEthane, CH33CHCH33H

CC

1s orbital of H

bond

sp3

hybrids

bondbond - formed by an endwise (head-on) overlap.

Molecules are able to rotatearound singlebonds.

bondbond - formed by an endwise (head-on) overlap.

Molecules are able to rotatearound singlebonds.

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Ethane , CHEthane , CH33CHCH33

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Rotation of single bondRotation of single bond

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Rotation of single bondRotation of single bond

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spsp22 hybrid orbitals hybrid orbitals

To account for double bonds, a second type of hybrid orbital must be pictured. An sp2 hybrid is produced by combining one s and 2 p orbitals. One p orbital remains.


energ

y

2s

2p

2sp2

2p

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spsp22 hybrid orbitals hybrid orbitals

The unhybridized p orbitals are able to overlap, resulting in the formation of a second bond - bond.

A bond is asideways overlapthat occurs bothabove and below theplane of the molecule

Parts of the moleculeare no longer able to rotate about the bond.

A bond is asideways overlapthat occurs bothabove and below theplane of the molecule

Parts of the moleculeare no longer able to rotate about the bond.

C C

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EtheneEthene

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HH

H

CC

H

Bonding in etheneBonding in ethene

1s orbital overlap

bond

bond overlap

sp2

hybrids

sp2

hybrids

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Bonding in etheneBonding in ethene

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spsp hybrid orbital hybrid orbital

Forming a triple bond is also possible. This requires that two p orbitals remain unhybridized.


energ

y

2s

2p

2sp

2p

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spsp hybrid orbital hybrid orbital

C C

Now two p orbitals are available to form bonds.

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EthyneEthyne

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Bonding in ethyneBonding in ethyne

sp hybrid

p overlaps

CCH

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Bonding in ethyneBonding in ethyne

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Other hybrid orbitalsOther hybrid orbitals

d orbitals can also be involved in the formation of hybrid orbitals.

Hybrid Shape sp Linear sp2 Trigonal planar sp3 Tetrahedral sp3d Trigonal

bipyramidal sp3d2 Octahedral

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Molecular Orbital MethodMolecular Orbital Method

When atomic orbitals combine to form molecular orbitals, the number of molecular orbitals formed must equal the number of atomic orbitals mathematically combined.

Example - HExample - H22

Two 1s orbitals will combine forming two molecular orbitals. The overall energy of the new orbitals is the same as the original two 1s. However, they will be at different energies.

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HH2 2 molecular orbital diagrammolecular orbital diagram

H H

1s 1s

1s

H2

1s

Orbital shapes

energ

y

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Molecular orbitalsMolecular orbitals

When two atomic orbitals combine, three types of molecular orbitals are produced.

Bonding orbital - Bonding orbital - ororThe energy is lower than the atomic orbitals and the electron density overlaps.

Antibonding orbital - Antibonding orbital - * or * or

The energy is higher than the atomic orbitals and the electron density does not overlap.

Nonbonding - nNonbonding - nElectron pairs not involved in bonding.

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Homonuclear diatomic moleculesHomonuclear diatomic molecules

These molecules are simple diatomics where both atoms are of the same element.

Energy diagrams for these types of molecules are similar to the one for H2.

We can develop energy diagrams for a range of molecules or possible molecules to see if they bond and how.

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MO diagram of heliumMO diagram of helium

He He

1s 1s

1s

He2

1s

en

erg

y

If we develop adiagram for heliumwe see that botha bonding andantibonding orbital will be filled.

The result is thatit is no more stablethan the unbondedform -- it will notbond

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Molecular orbital bondingMolecular orbital bonding

For a molecule to be stable, you must have more electrons in bonding orbitals than in antibonding orbitals.

The bonded form will be at a lower energy so will be more stable.

Bonding and antibonding orbitals for both and bonds must be considered.

Lets look at the MO diagram for O2.

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MO diagram for OMO diagram for O22

1s

2s

2px 2py 2pz 2px 2py 2pz

2s

1s

*2pz

2pz

2s

2s

1s

1s

2px 2py

2px 2py

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Each oxygen atom has 8 electrons for a total of 16.

We can now place 16 electrons into the MO diagram and see what happens.

Remember, don’t pair electrons unless you need to and fill a lower energy orbital before proceeding to the next higher one.

O2 will form if we have more bonding than antibonding electrons.

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1s

2s

2px 2py 2pz 2px 2py 2pz

2s

1s

*2pz

2pz

2s

2s

1s

1s

2px 2py

2px 2py

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Heteronuclear diatomic moleculesHeteronuclear diatomic molecules

Molecular orbital diagrams become more complex when bonding between two nonidentical atoms is considered.

The atomic energy levels are not the same and there are differing numbers of electrons.

A simple example is NO where the orbitals are similar but not identical.

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MO diagram for NOMO diagram for NO

N O

1s

2s

2px 2py 2pz

2px 2py 2pz

2s

1s

*2px

2px

2s

2s

1s

1s

2py 2pz

2py 2pz

NO

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Delocalized electronsDelocalized electrons

MO diagrams for polyatomic species are often simplified by assuming that all and some orbitals are localized -- shared between two specific atoms.

Resonance structures require that electrons in some orbitals be pictured as delocalized.

DelocalizedDelocalized - free to move around three or more atoms.

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Delocalized electronsDelocalized electrons

Benzene is a good example of delocalized electrons.

We know that the bonding between carbons has an order of 1.5 and that all of the bonds are equal.

=

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Aromatic hydrocarbonsAromatic hydrocarbons

pp orbitals overlap orbitals overlapsidewise all aroundsidewise all aroundthe ring. No localizedthe ring. No localizeddouble bonds.double bonds.

H HH H

H H

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Band theory of bonding in solidsBand theory of bonding in solids

This is an extension of delocalized orbitals.

Each atom interacts with all of the others in the crystal, resulting in an enormous number of ‘molecular orbitals.’

3s9 Na

3s9 Na

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BandBandA group of very closely spaced energy levels.

Energy gapEnergy gapThe difference in energy between the bonding and antibonding orbitals.

Forbidden bandsForbidden bandsA ‘space’ that separates bands.

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Band theory of bonding in solidsBand theory of bonding in solidsEn

erg

y

Internuclear distance

s

p

The s and p bands ofGroup II (2)

metals overlap.

The s and p bands ofGroup II (2)

metals overlap.

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ConductorConductorA material with a partially filled energy band.

InsulatorInsulatorThe highest occupied band is filled or almost completely filled. The forbidden band just above the highest filled is wide.

SemiconductorSemiconductorThe gap between the highest filled band and the next higher permitted band is relatively narrow.

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insulator

semiconductor

conductor

Energ

y

Energ

y

Energ

y

Empty

Forbidden, wide

Filled

Empty

Forbidden, narrow

Filled

NoForbidden

10 - 1 molecular shape and theory of chemical bonding shapes of molecules and polyatomic ions polar...

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