molecular potassium with buckminster fullerine (c structure

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MOLECULAR STRUCTURE

A computer representation of K3C60, a superconducting substance formed by reacting

potassium with buckminster fullerine (C60)

Copyright © Houghton Mifflin Company. All rights reserved. 13a–2

Source: Photo Researchers, Inc.

Two forms of carbon; graphite and diamond.


Source: Grant Hellman

Quartz grows in beautiful, regular crystals

SiO2

Vs


CO2

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Figure 13.1: (a) The interaction of two hydrogen atoms (b) Energy profile as a function of the distance

between the nuclei of the hydrogen atoms.


Figure 13.1: (a) The interaction of two hydrogen atoms (b) Energy profile as a function of the distance

between the nuclei of the hydrogen atoms.


WHERE DO THE ELECTRONS GO?

• Are they shared equally?• Are they more on one atom than the other?





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ANSWER:


It depends who is pulling harder



ANSWER:


It depends who is pulling harder(“Electro negativity”)


The HCL molecule has a dipole moment


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Pauling and his electronegativity


Pauling and his electronegativity


Figure 13.3: The Pauling electronegativity values as updated by A.L. Allred in 1961. (cont’d)


Arbitrarily set F as 4


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The HCL molecule has a dipole moment

Copyright © Houghton Mifflin Company. All rights reserved. 13a–17 Copyright © Houghton Mifflin Company. All rights reserved. 13a–18

X


X


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models

Now lets considerNow lets consider more than two atoms

in a molecule


Linear molecules


Planar molecules


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Tetrahedral molecules


MODELS

• These are only models


MODELS

• These are only models• But…. Models are very useful for describing

properties.


MODELS


properties.Newton:particles


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MODELS


properties.Newton:particles


Huygens:waves

models


Ball-and-stick model of a protein segment illustrating the alpha helix.



The concept of individual bonds makes it much easier to deal with complex molecules such as DNA.



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PolarityPolarity ofof MoleculesMoleculesDipole Moments of Polyatomic MoleculesDipole Moments of Polyatomic MoleculesExample: in CO2, each C-O dipole is canceled because the molecule is linear. In H2O, the H-O dipoles do not cancel because the molecule is bent.


O C O

Skeletal Structure

• Hydrogen atoms are always terminal• Hydrogen atoms are always terminal atoms.

• Central atoms are generally those with the lowest electronegativity.

• Carbon atoms are always central atoms.


Carbon atoms are always central atoms.• Generally structures are compact and

symmetrical.

Skeletal Structure

• Identify central and terminal atoms in the l l C H O ( th l l h l th l)molecule C2H6O (ethyl alcohol or ethanol).

CH

H HH

OC

NOTE: Terminal atoms are all bonded to only one other atoms. Central atoms are

Now where do the


H HCentral atoms are bonded to two or more other atomselectrons go?

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Writing Lewis Structures

• All the valence e- of atoms must appear.• Usually, the e- are paired.• Usually, each atom requires an octet.

– H only requires 2 e-.• Multiple bonds may be needed.


– Readily formed by C, N, O, S, and P.

Lewis Structures

Draw Lewis structures for:

HF H F

or H F

HF:

H2O:

NH3:

H F

or H F

H O H

or H O H

H N H

or H N H

Copyright © Houghton Mifflin Company. All rights reserved.

CH4:

H

H

H C HH

H

or H C H

H

H

Benzene

Fredrich August von Kekule (German chemist) said that heFredrich August von Kekule (German chemist) said that he discovered the ring-shaped chemical structure of benzene because of a strange, reptilian dream he had in 1865: "I turned my chair to the fire and dozed. Again the atoms were gamboling before my eyes. ... My mental eye... could not distinguish larger structures, of manifold conformation; long rows, sometimes more closely fitted together; all twining and twisting in snakelike motion. But look! What was that? One of the snakes had seized hold of its own tail and the form


the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes. As if by a flash of lighting I awoke... " ( From "Creativity, Beyond the Myth of Genius" by Robert Weisberg published by W. H. Freeman 1992 .) Although some scholars now believe that Kekule's dream was a hoax to avoid sharing credit for the discovery of the hexagonal shape of benzene, it still makes a wonderful story.

Resonance Forms

C6H6 30 ve’s

C CC C

C

C

H

H H

HH

C CC C

C

C

H

H H

HH


CH

CH

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Electron-Rich Atoms

PF5

P

FF F

FF


Exceptions to the Octet Rule

• Expanded octets.

P

Cl

Cl

••••

••

P

Cl

Cl

Cl

••••

••

Cl

ClS

F

F

F

••••

••

F

F

F••

••

••Cl


••

MolecularMolecular ShapesShapes

•Lewis structures give atomic connectivity: they tell usLewis structures give atomic connectivity: they tell us which atoms are physically connected to which.

•The shape of a molecule is determined by its bond angles.



In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimizes this repulsion.


We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.

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TheThe VSEPRVSEPR ModelModelPredicting Molecular GeometriesPredicting Molecular Geometries






experimentally we find all Cl-C-Cl bond angles are 109.5. Therefore, the molecule cannot be planar.All Cl atoms are located at the vertices of a tetrahedron with the C at its center.

TheThe VSEPRVSEPR ModelModelPredicting Molecular GeometriesPredicting Molecular GeometriesTo determine the electron pair geometry:

•draw the Lewis structure•count the total number of electron pairs around the central atom•arrange the electron pairs in one of the above geometries to minimize e-e repulsion•multiple bounds count as one bonding pair


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Molecular ShapesAB2Linear

AB3 AB2EAB3Trigonal planar AB4

Tetrahedral

AB5

AB2EAngular or Bent AB3E

Trigonalpyramidal

AB2E2Angular or Bent

AB4E AB3E2 AB2E3


AB5Trigonal bipyramidal

AB6Octahedral

AB4EIrregular tetrahedral(see saw)

AB3E2T-shaped

AB2E3Linear

AB5ESquare pyramidal

AB4E2Square planar

TheThe VSEPRVSEPR ModelModelThe valence electrons in a molecule are the bonding pairs of electrons as well as the lone pairs.

There are 11 shapes that are important to us:Number of atoms, formula Shapes

(3 atoms, AB2) linear or bent(4 atoms, AB3) trigonal planar, trigonal bipyramidal,

or T-shaped(5 atoms, AB4) tetrahedral, square planar, or see-saw


( , 4) , q p ,(6 atoms, AB5) trigonal bipyramidal or square pyramidal(7 atoms, AB6) octahedral

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TheThe VSEPRVSEPR ModelModel


TheThe VSEPRVSEPR ModelModelMolecules with More than One Central AtomMolecules with More than One Central AtomIn acetic acid, CH3COOH, there are three central atoms.We assign the geometry about each central atom separately.

Number of electron domains

C OH H

H

H O

4 3 4

C


Number of electron domains

Electron-domain geometry

Predicted bond angles

TetrahedralTrigonalplanar Tetrahedral

109.5o 120o 109.5o

4 3 4

TheThe VSEPRVSEPR ModelModelMolecules with More than One Central AtomMolecules with More than One Central AtomIn acetic acid, CH3COOH, there are three central atoms.We assign the geometry about each central atom separately.


Dipole Moment

C OO

Bond dipoles In H2O the bond dipoles are also equal inmagnitude but do not exactly oppose eachother. The molecule has a nonzero overall dipole moment

NonpolarH H

O

C OO

Overall dipole moment = 0

Bond dipoles

The overall dipole moment of a moleculeis the sum of its bond dipoles In CO the

dipole moment.


PolarOverall dipole moment

is the sum of its bond dipoles. In CO2 thebond dipoles are equal in magnitude butexactly opposite each other. The overall dipole moment is zero.

221

dqqkF

Coulomb’s law

= Q r Dipole moment,

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Figure 13.6: The carbon dioxide molecule

• Nonpolar Molecules– Dipole moments are symmetrical and cancel out.


Determining Molecular Polarity

• Nonpolar Molecules– Dipole moments are symmetrical and cancel out.

F


BF3

F F

B

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem


• Polar Molecules– Dipole moments are asymmetrical and don’t

cancel .

O


netdipolemoment

H2OH H

O



polar molecules have...asymmetrical shape (lone pairs) or

H

– asymmetrical shape (lone pairs) or – asymmetrical atoms


CHCl3Cl Cl

Cl

netdipolemoment


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Figure 13.5: (a) The structure and charge distribution of the ammonia molecule. (b) The dipole moment of the

ammonia molecule oriented in an electric field.

Copyright © Houghton Mifflin Company. All rights reserved. 13a–61 Copyright © Houghton Mifflin Company. All rights reserved. 13a–62

Sulfur has a partial positive charge


Hydrogen atoms have a partial positive charge


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Hydrogen atoms and a small partial negative charge on the carbon


..

Polar Bonds

OF

HNH Cl

Polar

H HO

PolarF F

B

Nonpolar

HH

H

N

Polar

F F FCl Cl

Copyright © Houghton Mifflin Company. All rights reserved. 13a–66A molecule has a zero dipole moment when bond dipoles cancel one another.

Polar Nonpolar

F FCl

F

F F

Xe

F FCl

ClC

Nonpolar Polar

ClH

C

H

H


Comparing fuels

• Natural gas: CH4 + 2O2→ CO2 + 2 H2OCH4 + 2O2 → CO2 + 2 H2O ΔH=-808 kJ/mol

• Coal: C + O2 → CO2ΔH=-393.5 kJ/mol

• Oil: C H 30½O 20CO 21 H O


C20H42 + 30½O2 → 20CO2 + 21 H2O ΔH=-13315 kJ/molΔH=-666 kJ/mol.CO2

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Comparing fuels

Production of 1 GigaJoule of energy releases:• Natural gas:

109 J x 0.044 kg/mol ÷ 808,000 J/mol = 54.5 kg CO2

• Coal: 112 kg CO


112 kg CO2

• Oil: 66 kg CO2

N3 high energy density

3 2


ATP energy


ATP energy

Repulsion weakens these bonds


Resonance!

molecular potassium with buckminster fullerine (c structure

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