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Lewis Formula and Molecular Bonding

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Assignment Chapter 10
Concept Explorations

10.21. Best Lewis Formula and Molecular Geometry
A student writes the Lewis electron-dot formula for the carbonate anion, CO32−, as

a. Does this Lewis formula obey the octet rule? Explain. What are the formal charges on the atoms? Try describing the bonding for this formula in valence bond terms. Do you have any difficulty doing this?
Answer )

The structure violates the octet rule. In order to avoid violating the octet rule, the conversion of a non-bonding pair of electrons to a bonding pair must be followed by the conversion of a bonding pair to a non-bonding pair. The net result is the transfer of the negative charge from one oxygen atom to another.

The 3 structures are equivalent since they have equal formal charges on the elements. It has been proven experimentally that the C-O bond in the carbonate ion is a hybrid of a single and a double bond (the length of the C-O bond in the carbonate ion is approximately half of the sum of the lengths of a normal C-O and C=O bond).
Number of valence electrons of atoms in molecules:
In Lewis structures,each atom contributes : 2 electrons for each lone pair,1 electron/bond
Formal charge of an atom in a molecule :
(Number of valence electrons in the free atom) - ( Number of valence electrons in the Lewis structure)
According to valence bond theory, the carbonate ion contains two types of C-O bonds; one of them is a double bond and the other two are single bonds. In other words, valence bond theory predicts that two of the C-O bonds should be the same length, while the other should be different. Specifically, the C-O double bond should be shorter than the two C-O single bonds. Also the O=C-O bond angle would be different than the O-C-O angle. These predictions are contradicted by the experimental facts: all three C-O bonds in carbonate are the same length; all three O-C-O bond angles are 120o.
This discrepency required modification of valence bond theory. That modification is what we now call resonance theory. Whenever there are two or more valid Lewis structures for the same compound, the true structure of the molecule is best represented as a "hybrid" of the individual structures. Figure below presents the ...

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