Chapter 13, Problem 19E

Interpretation Introduction

(a)

Interpretation:

The electron-pair geometry for the molecules, ${\text{BCl}}_3,{\text{PH}}_3$, and ${\text{H}}_2\text{S}$ 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 therefore, they distribute themselves in positions around the central atoms that are as far away from one another. This arrangement of electron pairs is known as electron-pair geometry. The electron pairs may be shared in covalent bond, or they may be lone pairs.

Answer to Problem 19E

The Lewis diagrams for ${\text{BCl}}_3,{\text{PH}}_3$, and ${\text{H}}_2\text{S}$ are shown as below.

The wedge-and-dash diagrams for ${\text{BCl}}_3,{\text{PH}}_3$, and ${\text{H}}_2\text{S}$ are shown as below.

The electron pair geometry for ${\text{BCl}}_3$ is trigonal planar and for ${\text{PH}}_3$ and ${\text{H}}_2\text{S}$ 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, ${\text{BCl}}_3$, boron has three valence electrons and each chlorine has seven valence electrons. The total number of valence electron for the molecule ${\text{BCl}}_3$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(3+3\times 7\right)e^{-}\\ & =& 24e^{-}\end{array}$

Similarly, in the molecule, ${\text{PH}}_3$, phosphorus has five valence electrons and each hydrogen has one valence electron. The total number of valence electron for the molecule ${\text{PH}}_3$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(5+3\times 1\right)e^{-}\\ & =& 8e^{-}\end{array}$

In the molecule, ${\text{H}}_2\text{S}$, each hydrogen has one valence electron and sulfur has six valence electron. The total number of valence electron for the molecule ${\text{H}}_2\text{S}$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(6+2\times 1\right)e^{-}\\ & =& 8e^{-}\end{array}$

The atom which is least electronegative is the central atom. In ${\text{BCl}}_3$, boron is least electronegative element. Therefore, boron is the central atom. Boron is bonded with three 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 ${\text{BCl}}_3$ is shown below.

Figure 1

In ${\text{PH}}_3$, hydrogen is least electronegative element. But hydrogen cannot be a central atom. Therefore, phosphorus is the central atom. Phosphorus is bonded with three hydrogen 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 ${\text{PH}}_3$ is shown below.

Figure 2

In ${\text{H}}_2\text{S}$, hydrogen is least electronegative element. But hydrogen cannot be a central atom. Therefore, sulfur is the central atom. Sulfur is bonded with two hydrogen 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 ${\text{H}}_2\text{S}$ is shown below.

Figure 3

The electron-pair geometry depends on the number of electron pairs around the central atoms. In the molecule ${\text{BCl}}_3$, there are three electron-pairs around the boron atom. Therefore, the electron pair geometry is trigonal planar.

In the molecule ${\text{PH}}_3$, there are four electron-pairs around the phosphorus atom. Therefore, the electron pair geometry is tetrahedral.

In the molecule ${\text{H}}_2\text{S}$, there are four electron-pairs around the sulfur atom. Therefore, the electron pair geometry is tetrahedral.

The wedge-and-dash diagram for the molecule ${\text{BCl}}_3$ is shown below.

Figure 4

The wedge-and-dash diagram for the molecules ${\text{PH}}_3$ is shown below.

Figure 5

The wedge-and-dash diagram for the molecules ${\text{H}}_2\text{S}$ is shown below.

Figure 6

Conclusion

The Lewis and wedge-and-dash diagrams for ${\text{BCl}}_3,{\text{PH}}_3$, and ${\text{H}}_2\text{S}$ are shown in the Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, and Figure 6. The electron pair geometry for ${\text{BCl}}_3$ is trigonal planar and for ${\text{PH}}_3,{\text{H}}_2\text{S}$, it is tetrahedral.

Interpretation Introduction

(b)

Interpretation:

The molecular geometry predicted by the valence shell electron-pair repulsion theory for the molecules ${\text{BCl}}_3,{\text{PH}}_3$, and ${\text{H}}_2\text{S}$ 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 19E

The Lewis diagrams for ${\text{BCl}}_3,{\text{PH}}_3$, and ${\text{H}}_2\text{S}$ are shown as below.

The wedge-and-dash diagrams for ${\text{BCl}}_3,{\text{PH}}_3$, and ${\text{H}}_2\text{S}$ are shown as below.

The molecular geometry for ${\text{BCl}}_3$ is trigonal planar, for ${\text{PH}}_3$ it is trigonal pyramidal and for ${\text{H}}_2\text{S}$ it is bent.

Explanation of Solution

To write the Lewis diagram for a compound first the number of valence electrons is to be calculated. In the molecule, ${\text{BCl}}_3$, boron has three valence electrons and each chlorine has seven valence electrons. The total number of valence electron for the molecule ${\text{BCl}}_3$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(3+3\times 7\right)e^{-}\\ & =& 24e^{-}\end{array}$

Similarly, in the molecule, ${\text{PH}}_3$, phosphorus has five valence electrons and each hydrogen has one valence electron. The total number of valence electron for the molecule ${\text{PH}}_3$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(5+3\times 1\right)e^{-}\\ & =& 8e^{-}\end{array}$

In the molecule, ${\text{H}}_2\text{S}$, each hydrogen has one valence electron and sulfur has six valence electron. The total number of valence electron for the molecule ${\text{H}}_2\text{S}$ is calculated below.

$\begin{array}{rll}\text{Total}\text{number}\text{of}\text{valence}\text{electron}& =& \left(6+2\times 1\right)e^{-}\\ & =& 8e^{-}\end{array}$

The atom which is least electronegative is the central atom. In ${\text{BCl}}_3$, boron is least electronegative element. Therefore, boron is the central atom. Boron is bonded with three 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 ${\text{BCl}}_3$ is shown below.

Figure 1

In ${\text{PH}}_3$, hydrogen is least electronegative element. But hydrogen cannot be a central atom. Therefore, phosphorus is the central atom. Phosphorus is bonded with three hydrogen 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 ${\text{PH}}_3$ is shown below.

Figure 2

In ${\text{H}}_2\text{S}$, hydrogen is least electronegative element. But hydrogen cannot be a central atom. Therefore, sulfur is the central atom. Sulfur is bonded with two hydrogen 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 ${\text{H}}_2\text{S}$ is shown below.

Figure 3

The molecular geometry depends on the number of electron pairs as well as number of unpaired electron on the central atoms. In the molecule ${\text{BCl}}_3$, there is no lone pair of electron around the boron. Therefore, the molecular geometry is trigonal planar.

In the molecule ${\text{PH}}_3$, there is one lone pair of electrons around the phosphorus atom. The lone pair and bonding electrons will try to repel each other. Therefore, the molecular geometry is trigonal pyramidal.

In the molecule ${\text{H}}_2\text{S}$, there are two lone pair of electrons and two pair of bonding electrons. The lone pairs will try to repel each other. Therefore, the molecular geometry is bent.

The wedge-and-dash diagram for the molecule ${\text{BCl}}_3$ is shown below.

Figure 4

The wedge-and-dash diagram for the molecule ${\text{PH}}_3$ is shown below.

Figure 5

The wedge-and-dash diagram for the molecule ${\text{H}}_2\text{S}$ is shown below.

Figure 6

Conclusion

The Lewis and wedge-and-dash diagrams for ${\text{BCl}}_3,{\text{PH}}_3$, and ${\text{H}}_2\text{S}$ are shown in the Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6. The molecular geometry for ${\text{BCl}}_3$ is trigonal planar, for ${\text{PH}}_3$ it is trigonal pyramidal and for ${\text{H}}_2\text{S}$ it is bent.