Question

Add formal charges to each resonance form of HCNO below., Please add formal charges for each pictures..  for A , B and C..

Add formal charges to each resonance form of HCNO below.  

Based on the formal charges you added above, which structure is favored?

• A

• B

• C

Answer 1

Concepts and reason

Each atom in a Lewis structure for a molecule or ion will have formal charge. Formal charge on each atom is due to difference in valence electrons in an isolated atom and number of electrons assigned to that atom in a Lewis structure.

We use the following equation to calculate formal charge on an atom in Lewis structure.

$FC{\rm{ = }}V - N - \frac{B}{2}$

Here, $FC$ is formal charge on an atom, $V$ are valence electrons of the atom, $N$ are non-bonding electrons on the atom, and $B$ are bonding electrons assigned to the atom.

If a molecule has more Lewis structures, a Lewis structure with no formal charge on any of its atom is favorable than a Lewis structure with formal charge. A Lewis structure with formal charges +2, -2, or +3, -3 are not favorable.

A favorable Lewis structure will have negative formal charge on more electronegative atom and positive formal charge on less electronegative atom.

Fundamentals

Number of valence electrons in an atom is equal its group number in the periodic table.

A single bond is formed with two electrons and they called bonding electrons. A double bond is formed using four electrons. A triple bond is formed using six electrons.

Non-bonding electrons are placed on atoms as pairs.

A molecule can have more than one Lewis structure. A Lewis structure with no formal charge on its atom is favorable structure.

Each available Lewis structure for a molecule or ion is a resonance form. A new resonance form will be obtained by changing bonds and electron pairs around an atom in a resonance form.

The Lewis structure for the resonance form A is as follows:

$\begin{array}{c}\\FC\left( {{\rm{on H}}} \right) = 1 - 0 - \frac{2}{2}\\\\ = 0\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on C}}} \right) = 4 - 4 - \frac{4}{2}\\\\ = - 2\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on N}}} \right) = 5 - 0 - \frac{8}{2}\\\\ = + 1\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on O}}} \right) = 6 - 2 - \frac{6}{2}\\\\ = + 1\\\end{array}$

The resonance form A with formal charge on each atom is as follows:

The Lewis structure for the resonance form B is as follows:

$\begin{array}{c}\\FC\left( {{\rm{on H}}} \right) = 1 - 0 - \frac{2}{2}\\\\ = 0\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on C}}} \right) = 4 - 0 - \frac{8}{2}\\\\ = 0\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on N}}} \right) = 5 - 0 - \frac{8}{2}\\\\ = + 1\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on O}}} \right) = 6 - 6 - \frac{2}{2}\\\\ = - 1\\\end{array}$

The resonance form B with formal charge on each atom is as follows:

The Lewis structure for the resonance form C is as follows:

$\begin{array}{c}\\FC\left( {{\rm{on H}}} \right) = 1 - 0 - \frac{2}{2}\\\\ = 0\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on C}}} \right) = 4 - 2 - \frac{6}{2}\\\\ = - 1\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on N}}} \right) = 5 - 0 - \frac{8}{2}\\\\ = + 1\\\end{array}$

$\begin{array}{c}\\FC\left( {{\rm{on O}}} \right) = 6 - 4 - \frac{4}{2}\\\\ = 0\\\end{array}$

The resonance form C with formal charge on each atom is as follows:

The resonance form B is favored structure for ${\rm{HCNO}}$ . The resonance form B is as follows:

Ans: