Section 10.1: Which is the more important resonance structure

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Section 10.1 Which is the more important
resonance structure?
All resonance forms contribute equally when
central atom has surrounding atoms that are all
the same.
When atoms surrounding the central atom are not
the same One resonance form may weight the
average (In other words, it counts more than
the other forms.) Determining the most important
resonance form Determine each atoms formal
charge the charge it would have if the bonding
electrons were equally shared,
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Section 10.1 Which is the more important
resonance structure? Formal charge Determined
for each atom in a compound
valence e- - ( unshared valance e- ½
shared valence e-)
Example Ozone (O3) Formal charge for OA 6
(4 ½4) 0 Formal charge for OB 6 (2
½6) 1 Formal charge for OC 6 (6 ½2)
-1 In the case of ozone, both resonance
structures (I and II) have the same
formal charges (but on different O atoms) so they
contribute equally to the resonance hybrid.
Formal charges must sum to the total charge on
the chemical species.
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Section 10.1 Which is the more important
resonance structure? Formal charge Determined
for each atom in a compound
Recall In nearly all compounds H atoms form
1 bond C forms 4 bonds N forms 3 bonds O
forms 2 bonds Halogens (F, Cl, Br, I,) form 1
bond
When formal charge is 0, an atom has its usual
of bonds.
Ozone Example Formal charge for OA 6 (4
½4) 0 Formal charge for OB 6 (2 ½6)
1 Formal charge for OC 6 (6 ½2) -1
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Section 10.1 Which is the more important
resonance structure? What about a case where the
resonance structures do not contribute equally?
Occurs when there are different atoms around the
central atom.
Example Cyanate ion, NCO-
Criteria for choosing the more important
resonance structure (1) Smaller formal
charges ( or -) are preferable to larger
ones. Resonance form I is out. (2) The
same nonzero formal charges on adjacent atoms are
not preferred. Not applicable to this
example. (3) A more negative formal charge
should reside on a more electronegative atom. O
is more EN than N, so Resonance form III is the
winner.
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Section 10.1 Formal charge is not the same as
the ON
What is the difference?
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Section 10.1 Exceptions (Limitations) to the
Octet and Formal Charge Rules
(1) e- deficient molecules gaseous molecules
containing Be or B as central atom
Halogens much more EN than Be or B Formal
charge rules make sharing of extra lone pairs by
halogens unlikely
Be 4 e- B 6 e-
(2) Odd e- molecules central atom has odd of
valence e- free radicals very reactive (b/c
very unstable) often react with each other to
pair up their lone e- (make you age)
(3) Expanded valence shells more than 8 valence
e- occurs only where d orbitals are available ?
Row 3 or higher (Review of orbital
types p289 295 s 2 e-, p 6 e-, d 10 e-,
f 14 e-)
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Resonance Structures 10.14a 10.16b