Chapter 4, Problem 4.109QA

Interpretation Introduction

To draw:

a) Lewis structure for hydrogen isocyanide (HNC) and assign formal charges to each atom.

b) Explain how the formal charges on atoms differ in the Lewis structures for HCN and HNC.

Answer to Problem 4.109QA

Solution:

a) A Lewis structure for HNC is

b) The formal charges for each of the atoms in HNC are

Atom	Formal Charge
H	0
N	1+
C	1-

A comparison of formal charges in Lewis structures for HCN and HNC is given below. Atoms in HCN all have zero formal charge, while the carbon atom in HNC has a formal charge of -1, in spite of having lower electronegativity.

Atom	Formal Charges in
	HNC	HCN
H	0	0
N	1+	0
C	1-	0

Explanation of Solution

1) Formula: The formal charge on an atom in a molecule is calculated using the following formula.

$formal charge=number of valence e^{-}-\left[number of e^{-}in lone pairs+\frac{1}{2}\left(number of shared e^{-}\right)\right]$

2) Calculations:

a) The electronic configuration of the elements H, N and C is as follows.

H: 1s¹

C: 1s² 2s² 2p²

N: 1s² 2s² 2p³

Element		Valence electrons
Symbol	# of atoms	In one atom	Total
H	1	1	1
C	1	4	4
N	1	5	5
Valence electrons in molecule			10

The skeletal structure of the molecule HNC is

There are two bond pairs. Therefore 2 x 2 = 4 electrons get utilized in bond formation. Now we put the remaining electrons as lone pairs

To complete octet of N we will convert some of the lone pairs on C into bond pairs.

A Lewis structure for HNC is as follows.

Based on this Lewis structure, the formal charges on the atoms can be calculated as follows.

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

$Formal charge on H=\left(1\right)-\left(0+ \frac{2}{2}\right)= 0$

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

$Formal charge on C=\left(4\right)-\left(2+ \frac{6}{2}\right)= -1$

b) The skeletal structure of the molecule HCN is

There are two bond pairs. Therefore 2 x 2 = 4 electrons get utilized in bond formation. Now we put the remaining electrons as lone pairs

To complete octet of C we will convert some of the lone pairs on N into bond pairs.

A Lewis structure for HCN is as follows.

Based on this Lewis structure, the formal charges on the atoms can be calculated as follows.

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

$Formal charge on H=\left(1\right)-\left(0+ \frac{2}{2}\right)= 0$

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

$Formal charge on C=\left(4\right)-\left(0+ \frac{8}{2}\right)= 0$

The Lewis structure of HCN is as follows.

Conclusion: Thus, atoms in HCN all have zero formal charge, while the carbon atom in HNC has a formal charge of -1, in spite of having lower electronegativity.