Question

In the following electron dot structure, calculate the formal charge from left to right nitrogen atom (shown)

A. ${{ - 1, - 1, + 1}}$
B.\[{{ - 1, + 1, - 1}}\]
C.${{ + 1, - 1, - 1}}$
D.${{ + 1, - 1, + 1}}$

Answer 1

Hint:Charge assigned to an atom in a molecule is the formal charge. It is also called a fake charge. We know that when an atom gives away its electrons, it gains a positive charge, and when an atom accepts electrons, it gains a negative charge. Even if it is a neutral molecule, the atom may possess a charge.

Complete step by step answer:
-The formal charge can be calculated by the formula:-
\[\text{formal charge = valence electrons - Non bonding valence electrons - }\dfrac{{{{bonding electrons}}}}{{{2}}}\]
-Let us take a simple example to understand this.
-In ${{C}}{{{H}}_{{4}}}$,
   

, let us find the formal charge by using the above-mentioned formula. The valence electrons in Carbon is 4. All the 4 valence electrons of carbon in methane are bonded. Thus, there are no nonbonding electrons. The bonding electrons are 8.
-On giving these values in the equation,
${{F}}{{.C of C = 4 - 0 - }}\dfrac{8}{{{2}}}$

${{F}}{{.C = 0}}$
-So now let us look at
 

-Let the nitrogen in the left end be considered first.
The valence electrons in Nitrogen is 5. The non-bonding electrons are 4. The bonding electrons are 4.
-On substituting these values to the formal charge formula
${{F}}{{.C = 5 - 4 - }}\dfrac{{{4}}}{{{2}}}$
$ \Rightarrow {{ F}}{{.C = - 1}}$
It is the same for the rightmost nitrogen atom.
-On checking the formal charge on the middle nitrogen, the valence electrons on nitrogen as we know is 5. The non-bonding electrons present here is 0 because the octet is satisfied here in the middle nitrogen. The bonding electrons are 8.
${{F}}{{.C = 5 - 0 - }}\dfrac{8}{2}$
$ \Rightarrow {{ F}}{{.C = 1}}$
Therefore, the formal charge is in the order ${{ - 1, + 1, - 1}}$
The correct answer is option (B).


Note:
Formal charge helps to find the electrons in a molecule, whether it has more electrons or protons associated with it. In some molecules in which resonance structures are possible, the resonance structure which has the least number of atoms with a non-zero formal charge would be preferred. The lower formal charge would be favored than one with a high formal charge.