11

Chapter 9&10: Structure and BondingChapter 9&10: Structure and Bonding

Coverage:1. Electron Configurations.2. Lewis Structures3. Covalent and Ionic bonds4. Atomic and Molecular Orbitals5. Hybridization

Goals:

1. Write the electron configuration for any element in the periodic table.

2. Write Lewis line bond structures for simple organic molecules.3. Know how to calculate formal charges on atoms.4. Know the definitions of a sigma and pi bonds.5. Draw energy diagrams for formation of pi and sigma bonds.5 Predict the number of sigma and pi bonds in a molecule.6. Predict the hybridization of an atom in a molecule.7. Predict the approximate bond angles in a molecule.

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Linus Pauling 1901-1994

Nobel Prize Chemistry 1954Nobel Prize Peace 1963

Chemical bonding, medicine, biology

For more see: http://www.chemheritage.org/EducationalServices/chemach/home.html

Both independently recogized that carbon atoms link together to form carbon chains. Kekule’ proposes that carbon is tetravalent

33

1. The first step in drawing Lewis structures is to determine the number of electrons to be used to connect the atoms. This is done by simply adding up the number of valence electrons of the atoms in the molecule.

Consider carbon dioxide CO2 Carbon (C) has four valence electrons x 1 carbon = 4 e- Oxygen (O) has six valence electrons x 2 oxygens = 12 e- There are a total of 16 e- to be placed in the Lewis structure.

2. Connect the central atom to the other atoms in the molecule with single bonds.

Carbon is the central atom, the two oxygens are bound to it and electrons are added to fulfill the octets of the outer atoms.

3. Complete the valence shell of the outer atoms in the molecule.

Lewis Structures

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4. Place any remaining electrons on the central atom.

There are no more electrons available in this example.

o If the valence shell of the central atom is complete, you have drawn an acceptable Lewis structure.

Carbon is electron deficient - it only has four electrons around it. This is not an acceptable Lewis structure.

o If the valence shell of the central atom is not complete, use a lone pair on one of the outer atoms to form a double bond between that outer atom and the central atom. Continue this process of making multiple bonds between the outer atoms and the central atom until the valence shell of the central atom is complete.

becomes

 

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The central atom is still electron deficient, so share another pair.

becomes

5. Double check to make sure that you have used the correct number of electrons in the Lewis structure and that no atom that cannot exceed its valence shell, does not.

The best Lewis structure that can be drawn for carbon dioxide is:  

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Formal Charge

Formal charge is an accounting procedure. It allows chemists to determine the location of charge in a molecule as well as compare how good a Lewis structure might be. The formula for calculating formal charge is shown below:

Consider the molecule H2CO2. There are two possible Lewis structures for this molecule. Each has the same number of bonds. We can determine which is better by determining which has the least formal charge. It takes energy to get a separation of charge in the molecule (as indicated by the formal charge) so the structure with the least formal charge should be lower in energy and thereby be the better Lewis structure.

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The two possible Lewis structures are shown below. They are connected by a double headed arrow and placed in brackets. The non-zero formal charge on any atoms in the molecule have been written near the atom.

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Orbitals and Wave Functions

Bohr and Quantum Mechanical Method

de Broglie – electrons have wave properties

Wave Function that describes orbitalElectron Density around Nucleus

+-

Analogy: Guituar String

+ and – indicate mathematical sign of wave, not charges!

Electron Density+

-

Nucleus

Distance from Nucleus

1s orbital

2s 2p1s

1010

Electron Density

Node

Node - region of zero electron density

Nucleus

Distance from Nucleus

+ +

- -

2p orbital

See HyperChem 3-D Model

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Atomic and Molecular Orbitals

Guidelines for Combining Atomic Orbitals

1. Atomic Orbitals (AOs) on different atoms combine to produce Molecular Orbitals (Mos) that are bonding or antibonding. This is referred to as a Linear Combination of Atomic Orbitals (LCAO).

MO1 = aAO1 + bAO2 + c AO3 where a,b,c are coefficients.

2. Each AO used results in an MO. For example, if three AOs are used to construct the MOs, then three MOs must result.

3. When AOs on the same atom combine they produce hybrid AOs.

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H2 Molecule

Energy

Antibonding MO

Bonding MO

1s AO1s AO

In the Ground State, theAntibonding MO is empty.

In the Ground State, the Bonding MO has two electrons

+ +

+ -

+ or - + or -

. .

. .

..

1313

1414

Sigma Bond ( – covalent bond that is symmetric about the bond axis.

1515

Pi () Bond

Energy

Antibonding MO

Bonding MO

2p AO2p AO

In the Ground State, theAntibonding MO is empty.

In the Ground State, the Bonding MO has two electrons

. .

..

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Two atomic p orbitals Pi (Bond

3D view of Pi (Bond

(pi) Bond – overlap of two p orbitals oriented perpendicular to the line connecting the nuclei.

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Hybridization of Atomic OrbitalsHybridization of Atomic Orbitals

1. AOs on a single atom mix to form new, hybrid orbitals. These hybrid orbitals have characteristics of both s and p orbitals.

2. Provides a means of explaining observed bond angles in organic molecules.

Acetylene C1

C2

HH

180o

spsp

E

__ s

__ __ __ p

__

__ __ __ __ __ p __ __ sp hydrid

promote e- Mix orbitals

D:\rw32b2a.exe

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s + p = sp + sp

Two sp orbitals

Remember, there are two p orbitalsleftover and these would be locatedon the y and z axes.

y

z

xEach sp orbital possesses 50% s characterand 50% p character.

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Orbital Picture of AcetyleneOrbital Picture of Acetylene

D:\rw32b2a.exe

2020

Ethylene sp2 sp2

CH

HC

H

H116.6o

121.7o

The carbon atom is sp2 hybridized to obtain trigonal planar geometry

E

__ s

__ __ __ p

__

__ __ __ __ p __ __ __ sp2 hydrid

Promote e- Mix orbitals

2121

s + 2 p

=

3 sp2 orbitals

sp2 Hybridization

There is one p orbital left over,and it would be along the z axis.

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Orbital Picture of Ethylene

D:\rw32b2a.exe

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Methane sp3

CH

H

HH

109.50

The carbon atom is sp3 hybridized to obtain tetrahedral geometry. All bond angles are equal at 109.50.

E

__ s

__ __ __ p

__

__ __ __ __ __ __ __sp3

Promote e- Mix orbitals

2424

s + 3 p

=

4 sp3 orbitals

D:\rw32b2a.exe

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Predicting Hybridization of Atoms in Molecules

1. Determine the number of other atoms (X) bonded to the atom of interest (A). 2. Determine the number of nonbonded pairs of electrons (E) on the atom.

If X + E = 2 then hybridization is sp.

System Hybridization Example Lewis Structure

AX2 sp CO2 :O=C=O:

AXE sp CO

If X + E = 3 then hybridization is sp2

AX3 sp2 H2C=O

AX2E sp2 H2C=NH

AXE2 sp2 H2C=O

:C O :- +

CHH

O: :

CHH

NH

:

CHH

O: :

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If X + E = 4 then hybridization is sp3

AX4 sp3 CH4

AX3E sp3 CH3NH2

AX2E2 sp3 CH3OCH3

C H

H

H

H

C N H

H

H

H

H

C O

H

H

H

C

H

H

H..

..