Chapter 9, Problem 9.59QP

Interpretation Introduction

Interpretation:

The three resonance structures for the given molecule $BC{l}_3$ should be drawn in which the octet rule is satisfied for both $\text{B}\text{and}\text{Cl}$ atoms. The formal charges for atoms should be shown.

Concept Introduction:

Octet rule: According to octet rule, when an atom contains eight electrons in its valence shell (outermost shell), it will be stable. Most of the atom will donate or accept electrons to its outer most orbital to satisfy the octet rule.

The Lewis structure is based on the concept of the octet rule so that the electrons shared in each atom should have 8 electrons in its outer shell.

Sometimes the chemical bonding of a molecule cannot be represented using a single Lewis structure. In these cases, the chemical bonding are described by delocalization of electrons and is known as resonance.

All the possible resonance structures are imaginary whereas the resonance hybrid is real.

Resonance is a method used to describe about delocalized electrons inside certain molecules or polyatomic ions since the Lewis structure can’t express it. A molecule or ion containing delocalized electrons can be represented by using several similar structures such structures are called as resonance structures or canonical structures.

The general rules to be followed for drawing resonance structures are given below:

1. 1. The position of the atoms does not change only the pi electrons and a lone pair of electrons can change their positions.
2. 2. The total number of valence electrons in all the resonance structures remains the same.
3. 3. Octet rule must not be violated. No atom can have electrons more than 8 in its valence shell except sulphur.
4. 4. Transfer of electrons between the bonds is shown by curved arrows.

Answer to Problem 9.59QP

The resonance structure with the formal charges is represented as follows,

Explanation of Solution

The molecule of ${\text{BCl}}_{\text{3}}$ has incomplete octet around $\text{B}$. The resonance structures of molecule where $\text{B}\text{and}\text{Cl}$ satisfies the octet rule can be given as,

The formal charges for atoms in the resonance structures are calculated as follows,

• Boron atom

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=3}\\ \text{Number}\text{of}\text{bonding}\text{electron=}8\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}0\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}3\text{-}\left(\frac{\text{1}}{\text{2}}\text{×8}\right)\text{-0}\\ \text{=-1}\end{array}$

• Chlorine atom in double bond

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=7}\\ \text{Number}\text{of}\text{bonding}\text{electron=}4\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}4\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}7\text{-}\left(\frac{\text{1}}{\text{2}}\text{×4}\right)\text{-4}\\ \text{=+1}\end{array}$

• Chlorine atom in both single bond

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=7}\\ \text{Number}\text{of}\text{bonding}\text{electron=}2\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}6\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}7\text{-}\left(\frac{\text{1}}{\text{2}}\text{×2}\right)\text{-6}\\ \text{=0}\end{array}$

• Boron atom

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=3}\\ \text{Number}\text{of}\text{bonding}\text{electron=}8\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}0\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}3\text{-}\left(\frac{\text{1}}{\text{2}}\text{×8}\right)\text{-0}\\ \text{=-1}\end{array}$

• Chlorine atom in double bond

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=7}\\ \text{Number}\text{of}\text{bonding}\text{electron=}4\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}4\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}7\text{-}\left(\frac{\text{1}}{\text{2}}\text{×4}\right)\text{-4}\\ \text{=+1}\end{array}$

• Chlorine atom in both single bonds

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=7}\\ \text{Number}\text{of}\text{bonding}\text{electron=}2\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}6\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}7\text{-}\left(\frac{\text{1}}{\text{2}}\text{×2}\right)\text{-6}\\ \text{=0}\end{array}$

• Boron atom

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=3}\\ \text{Number}\text{of}\text{bonding}\text{electron=}8\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}0\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}3\text{-}\left(\frac{\text{1}}{\text{2}}\text{×8}\right)\text{-0}\\ \text{=-1}\end{array}$

• Chlorine atom in double bond

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=7}\\ \text{Number}\text{of}\text{bonding}\text{electron=}4\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}4\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}7\text{-}\left(\frac{\text{1}}{\text{2}}\text{×4}\right)\text{-4}\\ \text{=+1}\end{array}$

• Chlorine atom in both single bond

$\begin{array}{c}\text{Number}\text{of}\text{valence}\text{electron=7}\\ \text{Number}\text{of}\text{bonding}\text{electron=}2\\ \text{Number}\text{of}\text{non-bonding}\text{electron}\text{=}6\end{array}$

Substituting these values to the equation,

$\begin{array}{c}\text{FC}\text{=}7\text{-}\left(\frac{\text{1}}{\text{2}}\text{×2}\right)\text{-6}\\ \text{=0}\end{array}$

Conclusion

The three resonance structures for the given molecule $BC{l}_3$ were drawn in which the octet rule is satisfied for both $\text{B}\text{and}\text{Cl}$ atoms and the formal charges were determined.