Chapter 4, Problem 4.105QA

Interpretation Introduction

To find how formal charges are used to choose between possible molecular structures.

Explanation of Solution

1) Concept :

A Formal charge (FC) is not a real charge to calculate the charge of an individual atom in a molecule or polyatomic ion. It is used to assign the number of electrons to an atom in a structure.

2) Formula:

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

Using this formula FC on an atom in a molecule or polyatomic ion can be calculated.

3) Calculation:

Let’s take the example of N₂O molecule. The various resonance structures possible for N₂O molecule are:

In structure (I),

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

$Formal charge on middle N=\left(5\right)-\left(0+ \frac{6}{2}\right)= +1$

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

Therefore, the sum of FC on three atoms = (0) + (+1) + (-1) = 0

In structure (II),

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

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

$Formal charge on O=\left(6\right)-\left(4+ \frac{4}{2}\right)= 0$

Therefore, the sum of FC on three atoms = (-1) + (+1) + (0) = 0

In structure (III),

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

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

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

Therefore, the sum of FC on three atoms = (-2) + (+1) + (+1) = 0

Now to decide the best or most contributing resonance structures, some rules or criteria have to be followed.

Criteria to choose best molecular structure

a) The best structure shall be one in which each atom has formal charge equal to zero.

b) If this is not possible, then structure having most of the atoms carrying zero or nearly zero charge shall be considered.

c) Negative charge should reside on more electronegative element(s)

Considering these three criteria, it can be concluded that, among the structures I, II and III, structure (I) is best structure to represent N₂O molecule.

Structure (III) is least contributing structure and (II) can be considered as the intermediate between (I) and (III).

Conclusion:

To choose the best resonance structure in a molecule or in a polyatomic ion, the above three criteria should be followed.