Chapter 4, Problem 4.175QA

Interpretation Introduction

To draw:

a) The Lewis structures for four resonance forms of linear N₅^-.

b) Calculate formal charges and identify the structures that contribute the most to the bonding in N₅^-.

c) By comparing Lewis structures for N₅^- and N₃^-, find out the ion in which the nitrogen-nitrogen bonds have the higher average bond order.

Answer to Problem 4.175QA

Solution:

a)

b) The last four structures having the lowest formal charge. These structures contribute the most to the bonding in N₅^-.

c)

From these resonance structures we see that each bond is predicted to be of double bond character in N₃^-. Therefore, in N₅^- there are two longer nitrogen-nitrogen bonds than in N₃^-. N₃^- has the higher average bond order.

Explanation of Solution

The ground state electronic configuration of N is 1s² 2s² 2p³.

Element		Valence electrons
Symbol	# of atoms	In one atom	Total
N	5	5	25
One electron from -1 charge			1
Valence electrons in molecule			26

Each N atom is attached with single bond.

N - N - N - N - N

Eight electrons get involved in the bond formation. Now we put remaining electrons as lone pairs on the atoms.

The octet of terminal N atoms is get completed. The octet of remaining N is not get completed. For that we will convert some lone pairs into bond pairs.

Now we calculate the formal charge on each atom in second structure.

$Formal charge = \left(valence electrons in atom\right)-\left(non bonding electrons+\frac{bonding electrons}{2}\right)$

$Formal charge on first N=\left(5\right)-\left(6+ \frac{2}{2}\right)= -2$

$Formal charge on second N=\left(5\right)-\left(2+ \frac{6}{2}\right)= 0$

$Formal charge on third N=\left(5\right)-\left(0+ \frac{8}{2}\right)= +1$

$Formal charge on first N=\left(5\right)-\left(0+ \frac{8}{2}\right)= +1$

$Formal charge on first N=\left(5\right)-\left(4+ \frac{4}{2}\right)= -1$

The Lewis structure with formal charge on N atom is as follows.

Now we draw the resonating structures of this molecule.

The molecules which having minimum formal charge will contribute the most to the bonding in N₅^-. Following structures having least formal charge and will contribute the most to the bonding in N₅^-.

Let’s we draw the Lewis structure of N₃^-.

Element		Valence electrons
Symbol	# of atoms	In one atom	Total
N	3	5	15
One electron from -1 charge			1
Valence electrons in molecule			16

From these resonance structures we see that each bond is predicted to be of double bond character. Therefore, in N₅^- there are two longer nitrogen-nitrogen bonds than in N₃^-.

Bond order is the number of bonds between atoms. For single bond the bond order is 1. For double bond the bond order is 2. For triple bond the bond order is 3. From the resonance structure of N₅^- and N₃^-, we can say that N₃^- has the higher average bond order.

Conclusion:

From the electronic configuration of atoms in the molecule we can draw Lewis structure of the molecule. From the Lewis structure we can find out formal charge and bond order.