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Lewis Formula and Molecular Bonding

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Need answers to all highlighted questions in the file regarding Lewis structures, molecular structure, 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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The expert examines the Lewis formulas and molecular bonding.

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Bonding, Lewis Structures and Molecular Geometry

Consider the following elements and arrange them in order of increasing number of electrons depicted in electron dot symbols. Which one occupies the intermediate position in the series? P, Se, Si, I and B

A. P
B. B
C. Si
D. I

Consider the following elements and select the one which has the smallest number of electrons shown in an electron dot symbol. Al, Se, Si, Sb and Br

A. Al
B. Br
C. Si
D. Sb

Consider the following species. Which contain an odd number of electrons? BrO3, HO2-, N2O, SO3 and SO32-

A. 2 and 5
B. only 2
C. 3 and 4
D. only 1

Consider the following species. Which contain an odd number of electrons? O, O2-, O22-, O2- and O3

A. only 3
B. only 2
C. 2 and 3
D. only 4

Consider the following species. Which contain an odd number of electrons? NO2, CO, O3-, O22- and SO2

A. 2 and 4
B. 2 and 5
C. 2, 4 and 5
D. 1 and 3

The ion, CS32- consists of a central carbon atom bonded to the three other atoms. Write a resonance form conforming to the Lewis octet rule and determine the number of double bonds in this species.

A. zero
B. three
C. two
D. one

Write resonance structures for SO2 and SO3. Based on these structures one can conclude that

A. both have the same bond length
B. SO2 has longer bonds than SO3
C. SO2 has shorter bonds than SO3
D. bond lengths are variable in both compounds

What types of hybrid orbitals are involved in bonding of BH4-?

A. sp
B. sp2
C. sp3
D. dsp3

What types of hybrid orbitals are involved in bonding of BCl3?

A. sp
B. sp2
C. sp3
D. dsp3

Which of the species in the following list is tetrahedral?

BF4-

SF4

XeF4

SiF4

A. 1 and 3
B. 1 and 4
C. 1 only
D. 2 and 3

Which of the species in the following list is square planar?

BF4-

SF4

XeF4

SiF4

A. 2 and 3
B. 3 only
C. 1 only
D. 1 and 4

Which of the following molecules is nonlinear?

A. HCN
B. C2H2
C. SO2
D. CO2

The largest of the following species is:

A. As3-
B. Se2-
C. S2-
D. Br-

Draw the Lewis formula for IF2+. How many lone pair(s) of electrons are there in the valence shell of the central atom?

A. 0
B. 1
C. 2
D. 3

Draw the Lewis formula for NF4+. How many lone pair(s) of electrons are there in the valence shell of the central atom?

A. 0
B. 1
C. 2
D. 3

The bond angles in IF4- are:

A. slightly less than 109.5º
B. slightly less than 90º
C. exactly 109.5º
D. exactly 90º

The bond angles in IF2+ are:

A. exactly 109.5º
B. exactly 120º
C. slightly less than 90º
D. slightly less than 109.5º

Calculate the Ho for the following reaction using the given bond energies. (H-Cl = 431 kJ; O-O = 494 kJ; H-O = 463, Cl-Cl = 243).

4 HCl(g) + O2(g) → 2 H2O(g) + 2 Cl2(g)

A. -102
B. -120
C. -139
D. -152

Use the following bond energies to calculate Ho for the given reaction. (H - C = 414; C - C = 347; C = C = 619; H - O = 464; C - O = 351).

CH3CH3(g) + H2O(g) → CH3CH2OH(g)

A. -22
B. -33
C. -45
D. -56

Calculate the Ho for the following reaction using the given bond energies. (C - H = 414 kJ; F - F = 155 kJ; H - F = 431, C - F = 485).

CH4(g) + 4 F2(g) → CF4(g) + 4 HF(g)

A. -1388
B. -1422
C. -1542
D. -1598

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