Chapter 13, Problem 20E

Interpretation Introduction

(a)

Interpretation:

The electron-pair geometry for the molecules, ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ is to be explained and the Lewis diagram and wedge-and-dash diagram for the molecules are to be drawn.

Concept introduction:

The electron pairs in Lewis diagrams repel each other in real molecule and thus, they distribute themselves in positions around the central atoms that are as far away from one another. This arrangement of electron pairs is called electron-pair geometry. The electron pairs may be shared in covalent bond, or they may be lone pairs.

Answer to Problem 20E

The Lewis diagrams for ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ are shown as below.

and

The wedge-and-dash diagrams for ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ are shown as below.

and

The electron pair geometry for both molecules is tetrahedral.

Explanation of Solution

To write the Lewis diagram for a compound first the number of valence electrons is to be calculated. In the molecule, ${\text{BF}}_4{}^{-}$, boron has three valence electrons and each fluorine has seven valence electrons. Since, ${\text{BF}}_4{}^{-}$ has a negative molecule so, one electron is to be added in the total number of electrons. The total number of valence electron for the molecule ${\text{BF}}_4{}^{-}$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(3+4\times 7+1\right)e^{-}\\ & =& 32e^{-}\end{array}$

Similarly, in the molecule, ${\text{CCl}}_{\text{4}}$, carbon has four valence electrons and each chlorine has seven valence electron. The total number of valence electron for the molecule ${\text{CCl}}_{\text{4}}$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(4+4\times 7\right)e^{-}\\ & =& 32e^{-}\end{array}$

The atom which is least electronegative is the central atom. In ${\text{BF}}_4{}^{-}$, boron is least electronegative element. Therefore, boron is the central atom. Boron is bonded with four fluorine atoms through single bond. In Lewis diagram, each electron is placed around the atom such that the octet rule is obeyed. Therefore, the Lewis diagram of ${\text{BF}}_4{}^{-}$ is shown below.

Figure 1

In ${\text{CCl}}_{\text{4}}$, carbon is least electronegative element. Therefore, carbon is the central atom. Carbon is bonded with four chlorine atoms through single bond. In Lewis diagram, each electron is placed around the atom such that the octet rule is obeyed. Therefore, the Lewis diagram of ${\text{CCl}}_{\text{4}}$ is shown below.

Figure 2

The electron-pair geometry depends on the number of electron pairs around the central atoms. In both the molecules ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ there are four electron-pairs around the boron and carbon atom. Therefore, the electron pair geometry is tetrahedral.

The wedge-and-dash diagram for the molecules ${\text{BF}}_4{}^{-}$ is shown below.

Figure 3

The wedge-and-dash diagram for the molecules ${\text{CCl}}_{\text{4}}$ is shown below.

Figure 4

Conclusion

The Lewis and wedge-and-dash diagrams for ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ are shown in the Figure 1, Figure 2, Figure 3, aand Figure 4. The electron pair geometry for both molecules is tetrahedral.

Interpretation Introduction

(b)

Interpretation:

The molecular geometry predicted by the valence shell electron-pair repulsion theory for the molecules ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ is to be explained and the Lewis diagram and wedge-and-dash diagram for the molecules are to be drawn.

Concept introduction:

Molecular geometry is the precise term that is used to describe the shape of molecules and arrangement of atoms around the central atom. The molecular geometry of a molecule is predicted by valence shell electron-pair repulsion theory or in short VSEPR theory. VSEPR theory applies to substances in which a second period element is bonded to two, three, four, or other atoms.

Answer to Problem 20E

The Lewis diagrams for ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ are shown as below.

and

The wedge-and-dash diagrams for ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ are shown as below.

and

The molecular geometry for both molecules is tetrahedral.

Explanation of Solution

To write the Lewis diagram for a compound first the number of valence electrons is to be calculated. In the molecule, ${\text{BF}}_4{}^{-}$, boron has three valence electrons and each fluorine has seven valence electrons. Since, ${\text{BF}}_4{}^{-}$ has a negative molecule so one electron is to be added in the total number of electrons. The total number of valence electron for the molecule ${\text{BF}}_4{}^{-}$ is calculated as below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(3+4\times 7+1\right)e^{-}\\ & =& 32e^{-}\end{array}$

Similarly, in the molecule, ${\text{CCl}}_{\text{4}}$, carbon has four valence electrons and each chlorine has seven valence electron. The total number of valence electron for the molecule ${\text{CCl}}_{\text{4}}$ is calculated as below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(4+4\times 7\right)e^{-}\\ & =& 32e^{-}\end{array}$

The atom which is least electronegative is the central atom. In ${\text{BF}}_4{}^{-}$ boron is least electronegative element. Therefore, boron is the central atom. Boron is bonded with four fluorine atoms through single bond. In Lewis diagram, each electron is placed around the atom such that the octet rule is obeyed. Therefore, the Lewis diagram of ${\text{BF}}_4{}^{-}$ is shown below.

Figure 1

In ${\text{CCl}}_{\text{4}}$, carbon is least electronegative element. Therefore, carbon is the central atom. Carbon is bonded with four chlorine atoms through single bond. In Lewis diagram, each electron is placed around the atom such that the octet rule is obeyed. Therefore, the Lewis diagram of ${\text{CCl}}_{\text{4}}$ is shown below.

Figure 2

The molecular geometry depends on the number of electron pairs as well as number of unpaired electron on the central atoms. In both the molecules ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ there are four electron-pairs around the boron and carbon atom and no loane pair on the central atoms. Therefore, the molecular geometry would be same as electron-pair geometry that is tetrahedral.

The wedge-and-dash diagram for the molecules ${\text{BF}}_4{}^{-}$ is shown below.

Figure 3

The wedge-and-dash diagram for the molecules ${\text{CCl}}_{\text{4}}$ is shown below.

Figure 4

Conclusion

The Lewis and wedge-and-dash diagrams for ${\text{BF}}_4{}^{-}$, and ${\text{CCl}}_{\text{4}}$ are shown in the Figure 1, Figure 2, Figure 3, aand Figure 4. The molecular geometry for both molecules is tetrahedral.