lecture 5.4 - chemical bonding 2- dipoles & resonance

8/14/2019 Lecture 5.4 - Chemical Bonding 2- Dipoles & Resonance

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Chemical BondinII:

Bond Polarity &


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Bond Polarity &

Formal Charge

The result of a method of electron

bookkeeping that tells whether an atom in a

molecule has gained or lost electrons

compared to an isolated atom.

Formal charge = # valence electrons(in free atom) (# bonds + #electrons as lone pairs)


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Formal Charge

For molecules and polyatomic ionsthat exceed the octet there areseveral different structures.

Use charges on atoms to helpdecide which.

Trying to use the oxidation numbers

to put charges on atoms inmolecules doesnt work.


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Formal ChargeThe difference between the number

of valence electrons on the free atomand that assigned in the molecule orion.

We count half the electrons in eachbond as belonging to the atom.

Molecules try to achieve as low aformal charge as possible.

Negative formal charges should beon electronegative elements.


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Polar Covalent Bonds

and ElectronegativityPolar bonds are bonds where the

electron densityis not shared equally

between the two bonded atoms.

In polar bonds there is a positive and a

negative end to the bond.


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Electronegativity

difference

Bond

Type

Zero

Intermediate

Large

Covalent

Polar

Covalent

Ionic

Co

valent

Ch

a racter

decr

ease

s

Ionic

Charact e

rincrea

ses


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Dipole Moments

A molecule with a center of negativecharge and a center of positivecharge is dipolar (two poles),

or has a dipole moment.

Center of charge doesnt have to beon an atom.

Will line up in the presence of anelectric field.


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How It is drawn

H - F

+ -


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H - F

+ -

H-F

+

-

H-F

+

-

H-

F

+

-

H

-F +

-

H-F+-

H

-F

+

-

H-

F

+

-

+-


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Bond Polarity

NaCl Cl-ClHCl


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Resonance

Sometimes there is more than onevalid structure for an molecule or ion.

NO3-

Use double arrows to indicate it is theaverage of the structures.

It doesnt switch between them.

NO2- Localized electron model is based on

pairs of electrons, doesnt deal with

odd numbers.


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Resonance

In some Lewis structures, the multiplebonds can be written in severalequivalent locations. All structureshave the exact same energy. Whichis the correct Lewis structure??

Answer : None alone are correct

the true molecule is a hybrid of thepossible structures. The electrons aredelocalized.


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Definitions

Sigma bond

the first bond toform betweenany two atoms

forms betweenatoms

Pi bond

second or thirdbond to formbetween twoatoms

forms above

and belowplane of themolecule

lecture 5.4 - chemical bonding 2- dipoles & resonance

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