Post on 30-May-2018
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