Chapter 5, Problem 29E

Draw a Lewis structure and use VSEPR theory to determine the geometry of each molecule. If the molecule has more than one central atom, indicate the geometry about each of these and draw the three-dimensional structure.

$\begin{array}{l}\begin{array}{l}\text{a}\text{. ClNO }\left(\text{nitrogen is the central atom}\right)\hfill \\ \text{b}{\text{. H}}_{\text{3}}{\text{CCH}}_{\text{3}}\left(\text{two carbon atoms in the middle, each with three hydrogen atoms attached}\right)\hfill \\ \text{c}{\text{. N}}_{\text{2}}{\text{F}}_{\text{2}}\left(\text{nitrogen atoms in the center and fluorine atoms on the ends}\right)\hfill \end{array}\\ \text{d}{\text{. N}}_{\text{2}}{\text{H}}_{\text{4}}\left(\text{nitrogen atoms in the center and two hydrogen atoms attached to each nitrogen}\right)\end{array}$

Interpretation Introduction

Interpretation:

The Lewis structures for the given compounds are to be drawn and the geometry of each of the given molecules is to be determined and the geometry of more than one central atom is to be indicated using the VSEPR theory. The three-dimensional structures of the given molecules are to be drawn.

Concept Introduction:

According to the Lewis theory, in covalent bonds, atoms share their electrons. The steps for drawing a covalent Lewis structure are as follows:

Write the skeletal structure of the molecule.

Add the number of valence electrons of each of the atoms in the molecule to determine the total number of electrons in the molecule.

Place the electrons by dots to complete the octets of the atoms.

If the central atom has not obtained an octet, then its multiple bonds can be formed.

The VSEPR theory is helpful in predicting the shapes of molecules from their Lewis structures. The geometry of a molecule can be determined on the basis of the number of electron groups, lone pairs and bonding pairs in the molecule.

Answer to Problem 29E

Solution:

a)

Lewis structure:

Molecular geometry is bent.

b)

Lewis structure:

Molecular geometry is tetrahedral.

c)

Lewis structure:

Molecular geometry is bent.

d)

Lewis structure:

Molecular geometry is pyramidal.

Explanation of Solution

a) $\text{ClNO}$(nitrogen is central atom)

The skeletal structure for the compound is as follows:

$\text{Cl N O}$

Now, the total number of electrons for the molecule is determined as follows:

$\begin{array}{c}\text{Total electrons = valence electrons for Cl + valence electrons for N + valence electrons for O}\\ \text{= 7 + 5 + 6}\\ \text{= 18}\end{array}$

Place the electrons as dots to give octet to each of the atoms in the molecule. Draw a single bond between the atoms. The central atom has not obtained an octet. Hence, nitrogen will form a double bond with oxygen to get its octet completed. Thus, the Lewis structure of $\text{ClNO}$ is as follows:

As all the atoms in the structure have obtained their octet, and thus, structure is complete. There are three electron pairs around the central atom. A double bond counts as a single electron group. Thus, there are two bonding pairs and one lone pair. Hence, according to the VSEPR theory, the molecular geometry is bent.

b) ${\text{H}}_3{\text{CCH}}_{\text{3}}$(two carbon atoms in the middle, each with three hydrogen atoms attached)

The two carbon atoms are in the middle, each with three hydrogen atoms attached.

The skeletal structure for the compound is:

$\begin{array}{l}\text{ H H}\\ \text{H C C H}\\ \text{ H H}\end{array}$

Now, the total number of electrons for the molecule is determined as follows: $\begin{array}{c}\text{Total electrons = valence electrons for C + valence electrons for H}\\ \text{= 2}\left(4\right)+6\left(1\right)\\ \text{= 14}\end{array}$

Place the electrons as dots to give octet or duet to each of the atoms in the molecule. Draw a single bond between the atoms. Hence, the Lewis dot structure for the molecule will be as follows:

As all the hydrogen atoms in the structure have obtained their duet and both the carbon atoms have obtained their octet, thus, structure is complete. There are two central carbon atoms and so, the geometry is considered at each. There are four electron pairs around each carbon atom and no lone pair. Hence, according to the VSEPR theory, the molecular geometry is tetrahedral at each carbon atom.

c) ${\text{N}}_2{\text{F}}_2$(nitrogen atoms in the center and fluorine atoms on the ends)

The two nitrogen atoms are in the centre and two fluorine atoms at the ends.

The skeletal structure for the compound is as follows:

$\text{F N N F}$

Now, the total number of electrons for the molecule is determined as follows: $\begin{array}{c}\text{Total electrons = valence electrons for N + valence electrons for F}\\ \text{= 2}\left(5\right)+2\left(7\right)\\ \text{= 24}\end{array}$

Place the electrons as dots to give octet to each of the atoms in the molecule. Draw a single bond between the atoms. Hence, the Lewis dot structure for the molecule will be as follows:

As all the atoms in the structure have obtained an octet, the structure is complete. There are two central nitrogen atoms and so, the geometry is considered at each. There are three electron groups around each nitrogen: two bonding groups and a lone pair. Hence, according to the VSEPR theory, the electron geometry is trigonal planar but the correct molecular geometry is bent.

d) ${\text{N}}_{\text{2}}{\text{H}}_{\text{4}}$ (nitrogen atoms in the center and two hydrogen atoms attached to each nitrogen)

The two nitrogen atoms are in the centre and the two hydrogen atoms are attached to each nitrogen.

The skeletal structure for the compound is:

$\begin{array}{l}\text{H N N H}\\ \text{ H H}\end{array}$

Now, the total number of electrons for the molecule is determined as follows: $\begin{array}{c}\text{Total electrons = valence electrons for N + valence electrons for H}\\ \text{= 2}\left(5\right)+4\left(1\right)\\ \text{= 14}\end{array}$

Place the electrons as dots to give octet or duet to each of the atoms in the molecule. Draw a single bond between the atoms. Hence, the Lewis dot structure for the molecule will be as follows:

As all the hydrogen atoms in the structure have obtained their duet and the nitrogen atoms have completed their octets, the structure is complete. There are two central nitrogen atoms and so, the geometry is considered at each one. There are four electron groups around each nitrogen: three bonding groups and a lone pair. Hence, according to the VSEPR theory, the molecular geometry is trigonal pyramidal at each nitrogen atom.