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Chapter 10: Chemical Bonding II: Molecular Shapes. Valence Bond and Molecular Orbital Theories P a g e | 1

Unit 2:

Chapter 10: Chemical Bonding II: Molecular Shapes; VSEPR,

Valence Bond and Molecular Orbital TheoriesHomework:

Read Chapter 10: Work out sample/practice exercises.Complete Expt 10 in Chemistry 1A lab manual.Suggested Chapter 10 Problems: 33, 35, 41, 45, 47, 59, 63, 65, 67, 71,

77, 79, 81, 83, 87, 89, 93Check for the MasteringChemistry.com assignment and complete before due date

Limitations in Lewis Structures:

Lewis theory generally predicts trends in properties, but does not give goodnumerical predictions of bond strength and bond length

Lewis theory gives good first approximations of the bond angles in molecules, butusually cannot be used to get the actual angle

Lewis theory cannot write one correct structure for many molecules whereresonance is important

Lewis theory often does not predict the correct magnetic behavior of molecules.Oxygen, O 2, is paramagnetic, though the Lewis structure predicts it is diamagnetic

Valence Shell Electron Pair Repulsion (VSEPR) Theory:Three-dimensional

Electron groups (all negatively charged) around the central atom are most stablewhen they are as far apart as possible valence shell electron pair repulsion theory.

Use all the information that has been gained in the Lewis Dot Structure and convertit to a three dimensional model to predict electronic and molecular shapes, angles,and polarity of the molecule.VSEPR Guidelines:

Use all the information from a Lewis Dot StructureThree-DimensionalIdentify Electronic and Molecular ShapesBonds anglesPolarity of whole substance (ionic, ion, nonpolar, polar molecule)Lone pair (nonbonding) electrons take up more space

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Electronic and Molecular Geometry: Count the electron regions . Electron regions will give an electronic shapewhile the number of bonded versus nonbonded regions will give themolecular shape.

#Electronregions

2 3 4 5 6

Electronicgeometry

Linear180

trigonal planar120

tetrahedral109.5

trig.bipyramidal

90, 120, 180

octahedral90, 180

moleculargeometry

Linear Trig planar,bent

Tetrahedral,Trigonal

pyramidal,bent

trig.bipyramida,see saw,

T-shaped,linear

octahedral, squarepyramidal, square

planar

Samples

Cool website to try: ChemEdDL.org Click on molecules 360. This website shows the3D structure of many chemicals and allows you to rotate in three dimensions, showingbonding, bond length, dipole arrows, dipole moment, etc.

Valence Bond (VB) Theory: The Valence Bond theory is a quantum mechanical model that expands the previoustwo theories to describe the electronic nature of covalent bonds.

Linus Pauling and others applied the principles of quantum mechanics tomolecules

They reasoned that bonds between atoms would occur when the orbitals onthose atoms interacted to make a bond

The kind of interaction depends on whether the orbitals align along the axisbetween the nuclei ( bonds), or outside the axis ( bonds)

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VB Guidelines:Use all the information from a Lewis Dot StructureThree-DimensionalVisualize orbital picture using atomic (s, p) and hybridized (sp, sp 2, sp 3,sp3d, and sp 3d2) orbitalsDirect overlap orbitals, sigma ( ) bondsIndirect overlap orbitals, pi ( ) bondsAll bonds have a bond, while double bonds have 1 and 1 and triplebonds have 1 and 2 bonds(Bubble) Pictures draw the orbitals in balloon type picturesDelocalized bonding occurs in substances with resonance

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Double bond:

Triple bond:

Limitations in Valence Bond Theory:VB theory predicts many properties better than Lewis theory bonding schemes, bondstrengths, bond lengths, bond rigidity

However, there are still many properties of molecules i t doesnt predict perfectly magnetic behavior of O 2

VB theory presumes the electrons are localized in orbitals on the atoms in themolecule it doesnt account for delocalization

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Molecular Orbital (MO) Theory: The Molecular Orbital Theory is separate from the first three. This theory explainsthe paramagnetic behavior found in O 2 gas molecules.

In MO theory, Schrdingers wave equation is applied to the molecule tocalculate a set of molecular orbitals

Electrons and orbitals belong to the whole molecule Delocalization When the wave functions combine constructively, the resulting molecular

orbital has less energy than the original atomic orbitals it is called aBonding Molecular Orbital where most of the electron density is betweenthe nuclei. Lower energy-stabilizing

When the wave functions combine destructively, the resulting molecularorbital has more energy than the original atomic orbitals it is called anAntibonding* Molecular Orbital where most of the electron densityisoutside the nuclei creating nodes between nuclei. Higher energy-unstable

Sigma ( ) p x molecular orbitals

Pi ( ) p y or p z molecular orbitals

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Sigma ( ) s molecular orbitals

MO Guidelines:Use all electrons in atoms, not just the valence electronsFor this class, limit discussion and examples to diatomic species such as:H2, O 2, CN

-1, HF.Sometimes gives a more accurate electronic structure than VBCombination of 2 atomic orbitals make a molecular orbitalBonding orbitals can be sigma or pi orbitals. Sigma orbitals directlyoverlap and pi orbitals indirectly overlapAntibonding* sigma or pi orbitals create a node between the atoms with nooverlapTwo atomic s orbitals combine to form a lower energy bonding and ahigher energy * antibonding* orbitalsix atomic p orbitals combine to form lower energy bonding orbitals,

and 2 degenerate orbitals and higher energy antibonding*orbitals, and 2 degenerate orbitalsPredict paramagnetic or diamagnetic behaviorPredict bond orderCompare bond lengths and bond strengthsFor diatomic molecules with fewer than 15 total electrons like N 2, energyincreases as follows: s, 1s*, 2s, 2s*, 2p , 2p , 2p , 2p*, 2p*, 2p*For diatomic molecules with 15 or more total electrons like O 2, energyincreases as follows: s, 1s*, 2s, 2s*, 2p , 2p , 2p , 2p*, 2p*, 2p*

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Heteronuclear Diatomic Elements and Ions: The more electronegative an atom is, the

lower in energy are its orbitals Lower energy atomic orbitals contribute

more to the bonding MOs

Higher energy atomic orbitals contributemore to the antibonding MOs Nonbonding MOs remain localized on the

atom donating its atomic orbitals

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Polyatomic Molecular Orbitals: When many atoms are combined together, the atomic orbitals of all the atoms are

combined to make a set of molecular orbitals, which are delocalized over the entiremolecule

Gives results that better match real moleculeproperties than either Lewis or valence bond

theories

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Molecular Shapes, Handedness and Drugs: The shapes of molecules can dramatically change its characteristics. Mirror imageshave different biological properties due to the specific shapes of receptor sites in thebody. For a molecule to exhibit handedness it needs 4 different groups attached to acarbon.

Identify:

Electronic and molecular geometries, angles, and VB hybridization of center atom.

a) h)

b) i)

c)

j)

d) k)

e) l)

f)

g) m)

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Fill in the following tables: First page follows octet and duet rules, second page has extended octets.#of electronregions andVB hybrid

number of bondedatoms

electronicgeometryname

moleculargeometryname

bond angles rough3-Dsketch

an examplemolecule or ion

any 1 linear linear (180 ) OO

H2COHFN2

CN -1

CO 2

3 120

3sp2

bentor angular

4 109.5

trigonalpyramidal

H2O

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#of electronregions andVB hybrid

number of bondedatoms

electronicgeometryname

moleculargeometryname

bond angles rough3-Dsketch

an examplemolecule or ion

5 trigonalbipyramidal

see-saw

3

180

90

(120 )

5sp3d

2

6 octahedral

BrF 5

6

sp3d2squarePlanar

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Examples:1. The valence bond hybrid atomic orbitals sp 3 are used by both C in CH 4 and O in H 2O. Yet, the

bond angles between atoms in H 2O are less than in CH 4. Explain.

2. Describe completely the main features of each of the following and explain what useful informationwe gain from each.

a) Lewis Structuresb) Valence Shell Electron Pair Repulsion (VSEPR) theoryc) Valence Bond (VB) theoryd) Molecular Orbital (MO) theory

3. a) Draw all possible resonance Lewis structures for NO 3-1. Include formal charges and the correctangles.

b) Draw the "realistic" hybrid resonance structure with appropriate angles that takes andaverage of the Lewis structures in part a. Include formal charges (fractions) and bondorders (fractions). Include nonbonding electrons on central atom but not on terminalatoms.

c) Sketch the valence bond (bubble) probability picture of one of the NO 3-1 resonances.Identify and label the hybridized orbitals. Identify sigma and pi bonds.

4. Draw and identify the cis and trans isomers for 1,2-dichloroethene, C 2H2Cl2

5. For each of the following: B 2, Ne 2, O 2 a) Give the molecular orbital (MO) energy diagram for each.b) Write the MO configurations for O 2 1s)2 c) Give the bond order of each B 2, Ne 2, O 2 d) List the species in decreasing order of bond energy and stabilitye) Identify each as diamagnetic or paramagnetic?f) Using the bond order information, which is least expected to exist. Explain why.g) Which would have the shortest bond length? Explain.

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6. Complete the following table for the indicated substances.Electronegativities: Na = 0.9, N = 3.0, O = 3.5, F = 4.0, Cl = 3.0, Br = 2.8, I = 2.5

substance SO 2 C 2H 4O 2 ICl 5 NaBrO 3 a) Draw the bestLewisstructure(s),resonances, andstructural isomersif any with octetb) Include formalcharges if theyare not zeroc) Indicate polarbonds with dipolearrows towardthe more

electronegativename electronicgeometry aroundcentral atomgive hybridorbital for centername moleculargeometry aroundcentral atomshow 3-D sketchwith atoms &

bonds in itgive all bondangleshow many sigmabonds? howmany pi bonds?is it an ioniccompound, polaror nonpolarmolecule or anion?

Draw the VBhybrid resonance(bubble) picture

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7. Complete the following table for the indicated substances.substance SCN -1 I 3

-1 SF 6 K 2SO 3 a)Draw the bestLewis structure(s),resonances, andstructural isomersif any with octetb) Include formalcharges if they arenot zeroc) Indicate polarbonds with dipolearrows toward themoreelectronegative

Answer questionsbelow for SO 3

-2

name electronicgeometry aroundcentral atomgive hybrid orbitalfor centername moleculargeometry aroundcentral atomshow 3-D sketchwith atoms &bonds in it

give all bondangleshow many sigmabonds? how manypi bonds?is it an ioniccompound, polaror nonpolarmolecule or anion?Draw the VBhybrid resonance(bubble) picture

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