1 Molecular Shapes lCO 2 is linear, SO 2 is bent. Why are they different? lThe shape of a molecule is determined by its bond angles, the angles made by

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Molecular ShapesMolecular Shapes

COCO22 is linear, SO is linear, SO22 is bent. Why are they is bent. Why are they

different?different?

The shape of a molecule is determined by its The shape of a molecule is determined by its bond angles, the angles made by lines bond angles, the angles made by lines connecting the nuclei of the atoms in a molecule.connecting the nuclei of the atoms in a molecule.

The bond angle in COThe bond angle in CO22 is 180 is 180oo; the bond angle in ; the bond angle in

SOSO22 is 120 is 120oo..

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How can we predict the shape of a How can we predict the shape of a molecule? molecule?

Examine the shapes of collections of objects Examine the shapes of collections of objects with a common center: balloons, plastic with a common center: balloons, plastic eggs or rubber balls or acorns. Why do eggs or rubber balls or acorns. Why do they have these shapes?they have these shapes?

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The VSEPR ModelThe VSEPR Model

Valence-Shell Electron-Pair RepulsionValence-Shell Electron-Pair RepulsionElectron pairs (or groups of pairs) try to Electron pairs (or groups of pairs) try to

avoid one another because of repulsions avoid one another because of repulsions between like-charged particlesbetween like-charged particles

Regions where electrons are likely to be Regions where electrons are likely to be found will be called electron domains.found will be called electron domains.

A double or triple bond comprises a single A double or triple bond comprises a single electron domain.electron domain.

Electron domains occur as far apart as Electron domains occur as far apart as possiblepossible

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Repulsions of AtomsRepulsions of Atoms

Pairs of electrons, being negatively charged, Pairs of electrons, being negatively charged, repel one another just like the objects just repel one another just like the objects just examined.examined.

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VSEPR TheoryVSEPR Theory

Can predict the angles between electron Can predict the angles between electron domains, whether bonding or nonbonding domains, whether bonding or nonbonding (unshared):(unshared):

2 domains - linear (1802 domains - linear (180oo))3 domains - trigonal planar (1203 domains - trigonal planar (120oo))4 domains - tetrahedral (109.54 domains - tetrahedral (109.5oo))5 domains - trigonal bipyramidal (905 domains - trigonal bipyramidal (90oo & 120 & 120oo))6 domains - octahedral (906 domains - octahedral (90oo))

VSEPRVSEPR09m15an109m15an1

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Shapes of some simple ABShapes of some simple ABnn molecules molecules

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What is wrong with Jason’s model of water?

What is wrong with Jason’s model of water?

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Electron domains can be occupied by atoms Electron domains can be occupied by atoms (bonding electron pairs) or by a (bonding electron pairs) or by a nonbonding pair of electrons, giving various nonbonding pair of electrons, giving various structuresstructures

Water has a tetrahedral Water has a tetrahedral arrangement of electron arrangement of electron pairs, but a bent pairs, but a bent arrangement of atomsarrangement of atoms

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Electron-Domain GeometryElectron-Domain Geometry

This is not the same as molecular geometry!This is not the same as molecular geometry!

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Molecular GeometryMolecular Geometry

Molecular geometry describes the shape of Molecular geometry describes the shape of only those electron domains that contain only those electron domains that contain bonding electrons.bonding electrons.

Molecular geometry is predicted by writing Molecular geometry is predicted by writing a Lewis structure, then determining the a Lewis structure, then determining the electron-domain geometry, and finally electron-domain geometry, and finally determining the molecular geometry.determining the molecular geometry.

1111


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4egroups.exe.lnk

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Why do COWhy do CO22 and SO and SO22 have different have different

shapes?shapes?

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Possible ShapesPossible Shapes

ABAB linearlinearABAB22 linear, bentlinear, bent

ABAB33 trigonal planar, trigonal trigonal planar, trigonal

pyramidal, T-shapedpyramidal, T-shapedABAB44 tetrahedral, see-saw (distorted tetrahedral, see-saw (distorted

tetrahedral), square planartetrahedral), square planarABAB55 trigonal bipyramidal, square trigonal bipyramidal, square

pyramidalpyramidalABAB66 octahedraloctahedral

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What are the structures of the following molecules and ions?What are the structures of the following molecules and ions?

Group Work:Group Work:

HCNHCN XeFXeF22

NONO22-- SFSF22

NHNH33 ClFClF33

NONO33-- SFSF44

XeFXeF44 SOSO4422--

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The Effect of Nonbonding The Effect of Nonbonding Electrons and Multiple Bonds Electrons and Multiple Bonds

on Bond Angleson Bond Angles



By experiment, the H-X-H bond angle By experiment, the H-X-H bond angle decreases on moving from C to N to O:decreases on moving from C to N to O:

Since electrons in a bond are attracted by two Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs.nuclei, they do not repel as much as lone pairs.

Therefore, the bond angle decreases as the Therefore, the bond angle decreases as the number of lone pairs increases.number of lone pairs increases.

OHH

104.5O107O

NHH

HC

H

HHH109.5O

2222

Similarly, electrons in multiple bonds Similarly, electrons in multiple bonds repel more than electrons in single repel more than electrons in single bonds.bonds.





C OCl

Cl111.4o

124.3o

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Molecules with More than Molecules with More than One Central AtomOne Central Atom

Molecules with More than Molecules with More than One Central AtomOne Central Atom

In acetic acid, CHIn acetic acid, CH33COOH, there are COOH, there are three central atoms.three central atoms.

We assign the geometry about each We assign the geometry about each central atom separately.central atom separately.

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Polarity of Polyatomic Molecules

Polarity of Polyatomic Molecules

Bonds can be polar if electrons are shared Bonds can be polar if electrons are shared unequallyunequally

If molecules are polar, they have a separation If molecules are polar, they have a separation of positive and negative charge within the of positive and negative charge within the molecule, and they exhibit a dipole moment, molecule, and they exhibit a dipole moment, which is the product of the charge and the which is the product of the charge and the distance between centers of positive and distance between centers of positive and negative chargenegative charge

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For diatomic molecules, the bond is more For diatomic molecules, the bond is more polar (greater dipole moment), the greater polar (greater dipole moment), the greater the difference in electronegativity between the difference in electronegativity between the two atoms.the two atoms.

++ --H : FH : F Polarity Polarity is in the direction is in the direction of the bondof the bond

Polar BondsPolar Bonds

4:514:51

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Yes, if they are diatomic.Yes, if they are diatomic.Maybe, if they are polyatomic. Bond Maybe, if they are polyatomic. Bond

polarity of two or more bonds can cancel polarity of two or more bonds can cancel one another.one another.

-- + + ++ --Cl : Be : ClCl : Be : Cl

Are molecules polar if they have polar bonds?

Are molecules polar if they have polar bonds?

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Polarity of MoleculesPolarity of Molecules

Can a molecule be polar if its bonds are all Can a molecule be polar if its bonds are all nonpolar?nonpolar?

What structures will give rise to nonpolarity What structures will give rise to nonpolarity even when bonds are polar?even when bonds are polar?

Any completely symmetrical molecule (a Any completely symmetrical molecule (a symmetrical structure with identical outside symmetrical structure with identical outside atoms) is always nonpolar. atoms) is always nonpolar.

Many properties of molecules depend on Many properties of molecules depend on whether they are polar or nonpolar.whether they are polar or nonpolar.

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Always NonPolarAlways NonPolar

ABAB linearlinearABAB22 linearlinear, bent, bentABAB33 trigonal planartrigonal planar, trigonal , trigonal

pyramidal, T-shapedpyramidal, T-shapedABAB44 tetrahedraltetrahedral, see-saw (distorted , see-saw (distorted

tetrahedral), tetrahedral), square planarsquare planarABAB55 trigonal bipyramidaltrigonal bipyramidal, square , square

pyramidalpyramidalABAB66 octahedraloctahedral

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Polarity of MoleculesPolarity of Molecules

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Which of these molecules is polar?

Which of these molecules is polar?

HH22OO

CClCCl44CHCH44 CHCH22ClCl22

COCO22 SOSO22

SFSF22 SFSF44 SFSF66

XeFXeF22

PClPCl33 BClBCl33

ElectronegativitiesElectronegativitiesFF 4.04.0OO 3.53.5Cl, NCl, N 3.03.0BrBr 2.82.8C, I, SC, I, S 2.52.5H, PH, P 2.12.1BB 2.02.0Polarity.exe.lnk

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Partner QuizPartner Quiz

For each of the following:For each of the following:Determine the molecular geometryDetermine the molecular geometryIdentify the molecule as polar or nonpolarIdentify the molecule as polar or nonpolar

HH22SS

CHCH33ClCl

SOSO33

PFPF55

NBrNBr33

ElectronegativitiesElectronegativitiesFF 4.04.0OO 3.53.5Cl, NCl, N 3.03.0BrBr 2.82.8C, I, SC, I, S 2.52.5H, PH, P 2.12.1BB 2.02.0

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Please don’t groanPlease don’t groan

Which bear would be most likely to dissolve Which bear would be most likely to dissolve in water – one from California, or one from in water – one from California, or one from Alaska?Alaska?

The one from Alaska, of course, since it is a The one from Alaska, of course, since it is a polarpolar bear. bear.

Documents

1 Molecular Shapes lCO 2 is linear, SO 2 is bent. Why are they different? lThe shape of a molecule is determined by its bond angles, the angles made by