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Week 6 CHEM 1310 - Sections L and M 1

Bonding: General Concepts

More on Lewis Structures– Formal Charges– Resonance– Breakdown of the Octet Rule

VSEPR Theory– Steric Number

Chapter Review

Week 6 CHEM 1310 - Sections L and M 2

Formal Charges

For some compounds, there are differentLewis structures possible.

How can we tell which is the correct Lewisstructure? Consider the formal charge…

Image from chemprofessor.com

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Week 6 CHEM 1310 - Sections L and M 3

Formal Charge

FormalCharge = # valence e-

on free atom -# valence e- assigned tothe atom in the molecule

# valence e-

assigned to theatom in themolecule

= # lone pair e- + 0.5 x (# shared e-)

Week 6 CHEM 1310 - Sections L and M 4

Formal Charge

FormalCharge = # valence e-

on free atom -# valence e- assigned tothe atom in the molecule

# valence e-

assigned to theatom in themolecule

= # lone pair e- + 0.5 x (# shared e-)

What is the formal charge onNitrogen in this compound?

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Week 6 CHEM 1310 - Sections L and M 5

Formal Charge

FormalCharge = # valence e-

on free atom -# valence e- assigned tothe atom in the molecule

# valence e-

assigned to theatom in themolecule

= # lone pair e- + 0.5 x (# shared e-)

# valence e- on free N = 5

1 lone pair e-

0.5 x (6 shared e-) = 3

5 - (1+3) = 5-4 = +1

Week 6 CHEM 1310 - Sections L and M 6

Resonance

Resonance is a way to depict average ofmultiple Lewis structures

O O O OOO

Electrons are not moving…representation means realstructure is average of these extremes

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Week 6 CHEM 1310 - Sections L and M 7

Other Examples of Resonance

C

O

OO

-2

C

O

OO

-2

C

O

OO

-2

Carbonate, CO3-2

N C S-1

N C S-1

Thiocyanate ion, NCS-1

N N O N N O N N O

N2O, laughing gas

Week 6 CHEM 1310 - Sections L and M 8

Breakdown of Octet Rule

Case 1Odd electron molecules (stable free radicals)

such as NO

# of valence e- = 5 (N) + 6 (O) = 11 e-

Lewis structure does not satisfy the octet rule butthe molecule is stable, although somewhat reactive

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Week 6 CHEM 1310 - Sections L and M 9

Breakdown of Octet Rule

Case 2Octet deficient molecules (mainly B compounds)

such as BF3

Lewis structure doesnot satisfy the octetrule for B, but themolecule is stable,although somewhat

reactive

B

F

FF

Week 6 CHEM 1310 - Sections L and M 10

Breakdown of Octet RuleCase 3

Valence Shell ExpansionThird and higher period elements can exhibit bonding

where an octet on the central atom is exceeded -can expand up 12 e-!

S

F

F

F

F

F

F

S has 12 e-

SF4 S

F

F

F

F

34 total valence e-

8 e- in bonds24 e- in lone pairs2 left overS gets extra lone pair

SF6

If electrons remainafter satisfying theoctet rule, add lonepairs to the central

atom!

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Week 6 CHEM 1310 - Sections L and M 11

Molecular Shapes

Molecular shapes are built from 5 basicarrangements:– Linear

– Planar Tetragonal

– Tetrahedral

– Trigonal Bipyramidal

– Octahedral

What do these shapeslook like?

How can we predictthe shape in which a compound is found?

Week 6 CHEM 1310 - Sections L and M 12

Linear Molecules

In linear molecules, all atoms lie in a straightline.

All diatomic molecules are linear.When 3 atoms are present in the molecule,

the atoms have a bond angle of 180°.

Example: CO2

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Week 6 CHEM 1310 - Sections L and M 13

Trigonal Planar Molecules

Central atom lies at thecenter of a triangleformed by the other 3atoms

All atoms in the sameplane

Bond angles = 120° Example: BF3

Week 6 CHEM 1310 - Sections L and M 14

Tetrahedral Molecules

Central atom at the center of the tetrahedronThree-dimensional, pyramid shape

– not planar like linear and planar trigonal molecules

Bond angles = 109.5°

Example: CH4

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Week 6 CHEM 1310 - Sections L and M 15

Trigonal Bipyramidal Molecules

Has 2 trigonal pyramids w/triangular facesCentral atom lies at middle of triangular plane

shared by upper and lower pyramidsBond angles = 90° and 120°

Example: PCl5

Week 6 CHEM 1310 - Sections L and M 16

Octahedral Molecules

2 square pyramids sharing a square baseCentral atom lies at middle of square plane

shared by upper and lower pyramidsBond angles = 90°

Example: SF6

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Week 6 CHEM 1310 - Sections L and M 17

Predicting Molecular Shapes

How can we predict the shape of aparticular molecule?

Valence Shell Electron Pair Repulsion(VSEPR) Theory

Electron pairs in the valence shell of an atom repel eachother on a spherical surface formed by the underlying

core of the atom.

How can we apply VSEPR theory to helppredict the shape of a molecule?

Week 6 CHEM 1310 - Sections L and M 18

Steric Number and VSEPR Theory

StericNumber

(SN)

# atoms bonded

to a centralatom

= +

# lone pairs oncentral

atom

O HHSN = 2 + 2 = 4 when SN = 4, shape = tetrahedral

This is why water is bent and not linear!

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Week 6 CHEM 1310 - Sections L and M 19

Steric Number and VSEPR Theory

StericNumber

(SN)

# atoms bonded

to a centralatom

= +

# lone pairs oncentral

atom

Steric Number

2

3

4

5

6

Shape (Geometry)

Linear

Trigonal Planar

Tetragonal

Trigonal Bipyramidal

Octahedral

Week 6 CHEM 1310 - Sections L and M 20

What To Study - Ch. 12

Characteristics of WavesQuantum NumbersAufbau Principle

– Electron Configuration

Trends in Periodic Table– Ionization Energy– Electron Affinity– Atomic Radius

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Week 6 CHEM 1310 - Sections L and M 21

What To Study - Ch. 13

Types of Chemical Bonds– Ionic– Covalent: Polar and Nonpolar

ElectronegativityBond Polarity and Dipole MomentsCalculation of Bond Energy for ReactionLewis StructuresVSEPR Theory to deduce molecular shape