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Molecular GeometryMolecular Geometry

Molecules have different shapes (geometries)depending on the type of atoms making it upand the number of electron pairs present.

Molecular shapes are described in terms of bond angles and bond lengths.

The length of a chemical bond is measuredbetween two chemically bonded atoms fromthe nucleus of one to the nucleus of the other.

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The bond angle is the angle between twobonds that include a common atom.

Molecular shapes determine the properties of

molecules such as polarity and solubility.

Lewis structures are explained in theCovalent Bonds and Ionic BondsPowerPoints.

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Lewis Structure of WaterLewis Structure of Water

The Lewis structure only shows the bonding of each atom, not the shape of the water molecule.

H O H

The actual shape is with a bond angle of

O 104.5°

H H

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The previous slide shows that oxygen is thecentral atom having four electron pairs.

The O shares two pairs of electrons with theH’s and two of the pairs are not shared.

The unshared pair repel each other and they

also strongly repel the shared pair.

Instead of water being a linear molecule, it is

said to be bent or angular.

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VSEPR TheoryVSEPR Theory

The valence shell electron pair repulsiontheory says that the shape of a moleculeresults from the repulsive interaction of electron pair in the valence shell of an atom.

The most important atom for determining thegeometry is the central atom.

The geometry depends on the atoms havingminimal interaction between the valence

shellelectron pairs.

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VSEPR TheoryVSEPR Theory

The minimal interaction between the pairs ofvalence electrons maximizes the distancebetween the electron pairs and between theatoms making up the molecule.

A multiple bond (double or triple) holds themulti-bonded atom in the same position as asingle bond.

A multiple bond is treated as a single bond for

determining the molecular geometry.

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VSEPR TheoryVSEPR Theory

The following VSEPR structures were madeby Dr. Mark R. Leach who granted

permissionfor their use. Check out all his graphics athttp://www.chemistry-drills.com/VSEPR.php.

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX2

Valence electron pairs: 2Bonding electron pairs: 2Nonbonding electron pairs: 0Examples: BeCl2, CO2

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX3

Valence electron pairs: 3Bonding electron pairs: 3Nonbonding electron pairs: 0Examples: BF3, In(CH3)3

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX2E

Valence electron pairs: 3Bonding electron pairs: 2Nonbonding electron pairs: 1Examples: SO2, GeF2

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX4

Valence electron pairs: 4Bonding electron pairs: 4Nonbonding electron pairs: 0Examples: CH4, CCl4

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX3E

Valence electron pairs: 4Bonding electron pairs: 3Nonbonding electron pairs: 1Examples: NH3, H3O+

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX2E2

Valence electron pairs: 4Bonding electron pairs: 2Nonbonding electron pairs: 2Examples: OF2, H2O

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX5

Valence electron pairs: 5Bonding electron pairs: 5Nonbonding electron pairs: 0Examples: PCl5

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX4E

Valence electron pairs: 5Bonding electron pairs: 4Nonbonding electron pairs: 1Examples: SF4

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX3E2

Valence electron pairs: 5Bonding electron pairs: 3Nonbonding electron pairs: 2Examples: ClF3

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX2E3

Valence electron pairs: 5Bonding electron pairs: 2Nonbonding electron pairs: 3Examples: ICl2-, XeF2

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX6

Valence electron pairs: 6Bonding electron pairs: 6Nonbonding electron pairs: 0Examples: SF6

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX5E

Valence electron pairs: 6Bonding electron pairs: 5Nonbonding electron pairs: 1Examples: BrF5

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VSEPR for Molecules and Ions VSEPR for Molecules and Ions

AX4E2

Valence electron pairs: 6Bonding electron pairs: 4Nonbonding electron pairs: 2Examples: XeF4, ICl4-

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Exceptions to the Octet RuleExceptions to the Octet Rule

Three major exceptions to the octet rule:

Molecules or ions with more than eight electrons around the central atom.

Species with fewer than eight electrons around the central atom.

Species with an odd number of valence electrons.

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Expanded OctetsExpanded Octets

Starting with period three, atoms have thecapability to accommodate d electrons (3d).

AX4E molecules such as SF4 are able toaccommodate 4 bonding pairs of electronsand one nonbonding pair of electrons. Thisresults in S being surrounded by 5 electronpairs.

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The favored bonding scenario includes largecentral atoms (starting in the third period)and small terminal atoms such as fluorine,chlorine, and oxygen.

As shown below, S also has the ability to accommodate six pairs of valence electrons as found in SF6.

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Less Than an OctetLess Than an Octet

Molecules having either boron or beryllium as their central atom result in the central atom having only 2 or 3 valence pairs of electrons.

These molecules are very reactive with a molecule having an unshared pair ofelectrons.

BeCl2 BF3

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Odd Number of Valence ElectronsOdd Number of Valence Electrons

Most molecules have an even number ofvalence electrons.

In rare cases, molecules such as NO andNO2, there is one unpaired electron which isvery reactive.

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Odd Number of Valence ElectronsOdd Number of Valence Electrons

nitrogen(II) oxide nitrogen(IV) oxide

N O O N O

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Polar Molecules and Polar BondsPolar Molecules and Polar Bonds

The molecular geometry of a molecule or ion

determines if polar bonds in a species result

in the species itself being polar.

If all the bonds in a molecule are nonpolar,then the molecule itself is nonpolarregardless of geometry.

A polar molecule has an asymmetricaldistribution of charge.

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The charge results from the atoms in themolecule having different

electronegativitiesand their spatial arrangement.

The polarity of the O-H bond contributes tothe resultant polarity ( ) of the watermolecule.

H H

O

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Each O-H bond is polar and because of itsbent or angular shape, the water moleculeitself is polar.

If the bond angle was 180° as it is in HCl,water would be a nonpolar molecule.

The HCl molecule is polar because of thedifference in electronegativities between Hand Cl.

H Clδ+ δ-

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Molecular Geometry in SummaryMolecular Geometry in Summary

Molecular geometry is determined by the position of the atoms, not by the position

of electron pairs.

Lone pairs of electrons repel other lone pairs more strongly.

The electron cloud surrounding a lone pair of electrons is much bigger than the cloud surrounding a bonding pair of electrons.

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Bonding pairs of electrons have the smallest force of repulsion.

The order of electron pair repulsion is:

lp-lp > lp-bp > bp-bp

Molecules or ions with lone pairs of electrons will have smaller bond angles than predicted.