Question

Add formal charges to each resonance form of HCNO below.

1/ H---:C:----N-triplebond-O: 2/ H---C-triplebond-N----:O::(three lone pairs on the O) 3/ H---:C==N==:O:

Based on the formal charges you added above, which structure is favored? Am I correct in beliving it is the 2nd One?

Answer 1

Concepts and reason

The problem is based on the concepts of formal charge and resonating structure. Formal charge is defined as the difference between the numbers of valence electrons in Free State to the number of electrons of atom in Lewis structure. Resonance structures are the energetically identical structure.

Fundamentals

Formal charge is calculated by using the relation as:

$F = v - n - \frac{B}{2}$

Here, F is the formal charge, n is the number of nonbonding electrons and B is the number of bonding electrons. Non-bonding electrons are the electrons which do not participates in bonding.

Part 1

The first Lewis structure of ${\rm{HCNO}}$ molecule is as follows:

Formal charge on H is calculated as follows:

${F_{\rm{H}}} = v - n - \frac{B}{2}$

Substitute 1 for v, 0 for n and 2 for B as follows:

$\begin{array}{c}\\{F_{\rm{H}}} = 1 - 0 - \frac{2}{2}\\\\ = 0\\\end{array}$

Formal charge on C is calculated as follows:

${F_{\rm{C}}} = v - n - \frac{B}{2}$

Substitute 4 for v, 4 for n and 4 for B as follows:

$\begin{array}{c}\\{F_{\rm{C}}} = 4 - 4 - \frac{4}{2}\\\\ = - 2\\\end{array}$

Formal charge on N is calculated as follows:

${F_{\rm{N}}} = v - n - \frac{B}{2}$

Substitute 5 for v, 0 for n and 8 for B as follows:

$\begin{array}{c}\\{F_{\rm{N}}} = 5 - 0 - \frac{8}{2}\\\\ = + 1\\\end{array}$

Formal charge on O is calculated as follows:

${F_{\rm{O}}} = v - n - \frac{B}{2}$

Substitute 6 for v, 2 for n and 6 for B as follows:

$\begin{array}{c}\\{F_{\rm{O}}} = 6 - 2 - \frac{6}{2}\\\\ = + 1\\\end{array}$

Part 2

The second Lewis structure of ${\rm{HCNO}}$ molecule is as follows:

Formal charge on H is calculated as follows:

${F_{\rm{H}}} = v - n - \frac{B}{2}$

Substitute 1 for v, 0 for n and 2 for B as follows:

$\begin{array}{c}\\{F_{\rm{H}}} = 1 - 0 - \frac{2}{2}\\\\ = 0\\\end{array}$

Formal charge on C is calculated as follows:

${F_{\rm{C}}} = v - n - \frac{B}{2}$

Substitute 4 for v, 0 for n and 8 for B as follows:

$\begin{array}{c}\\{F_{\rm{C}}} = 4 - 0 - \frac{8}{2}\\\\ = 0\\\end{array}$

Formal charge on N is calculated as follows:

${F_{\rm{N}}} = v - n - \frac{B}{2}$

Substitute 5 for v, 0 for n and 8 for B as follows:

$\begin{array}{c}\\{F_{\rm{N}}} = 5 - 0 - \frac{8}{2}\\\\ = + 1\\\end{array}$

Formal charge on O is calculated as follows:

${F_{\rm{O}}} = v - n - \frac{B}{2}$

Substitute 6 for v, 6 for n and 2 for B as follows:

$\begin{array}{c}\\{F_{\rm{O}}} = 6 - 6 - \frac{2}{2}\\\\ = - 1\\\end{array}$

Part 3

The third Lewis structure of ${\rm{HCNO}}$ molecule is as follows:

Formal charge on H is calculated as follows:

${F_{\rm{H}}} = v - n - \frac{B}{2}$

Substitute 1 for v, 0 for n and 2 for B as follows:

$\begin{array}{c}\\{F_{\rm{H}}} = 1 - 0 - \frac{2}{2}\\\\ = 0\\\end{array}$

Formal charge on C is calculated as follows:

${F_{\rm{C}}} = v - n - \frac{B}{2}$

Substitute 4 for v, 2 for n and 6 for B as follows:

$\begin{array}{c}\\{F_{\rm{C}}} = 4 - 2 - \frac{6}{2}\\\\ = - 1\\\end{array}$

Formal charge on N is calculated as follows:

${F_{\rm{N}}} = v - n - \frac{B}{2}$

Substitute 5 for v, 0 for n and 8 for B as follows:

$\begin{array}{c}\\{F_{\rm{N}}} = 5 - 0 - \frac{8}{2}\\\\ = + 1\\\end{array}$

Formal charge on O is calculated as follows:

${F_{\rm{O}}} = v - n - \frac{B}{2}$

Substitute 6 for v, 4 for n and 4 for B as follows:

$\begin{array}{c}\\{F_{\rm{O}}} = 6 - 4 - \frac{4}{2}\\\\ = 0\\\end{array}$

The most favored structure is the third one which is shown as follows:

Formal charges for the structure is shown as follows:

Ans: Part 1

The formal charge to the given Lewis dot structure is as follows:

Part 2

The formal charge to the given Lewis dot structure is as follows:

Part 3

The formal charge to the given Lewis dot structure is as follows:

The most favored structure is shown as follows: