Answer : The Lewis structure of [tex]CN^-[/tex] is shown below.
Explanation :
Lewis-dot structure : It shows the bonding between the atoms of a molecule and it also shows the unpaired electrons present in the molecule.
In the Lewis-dot structure the valance electrons are shown by 'dot'.
The representation of valence electrons of an atom by dots around the symbol of an element is said to be the Lewis-dot symbol.
In the representation of Lewis-symbol, one electron dot must put on each side of the element symbol then paired the electrons.
The given molecule is, [tex]CN^-[/tex]
As we know that carbon has '4' valence electrons and nitrogen has '5' valence electrons.
Therefore, the total number of valence electrons in [tex]CN^-[/tex] = 4 + 5 + 1 = 10
According to Lewis-dot structure, there are 6 number of bonding electrons and 4 number of non-bonding electrons.
Now we have to determine the formal charge for each atom.
Formula for formal charge :
[tex]\text{Formal charge}=\text{Valence electrons}-\text{Non-bonding electrons}-\frac{\text{Bonding electrons}}{2}[/tex]
For structure 1 :
[tex]\text{Formal charge on C}=4-3-\frac{2}{2}=0[/tex]
[tex]\text{Formal charge on N}=5-5-\frac{2}{2}=-1[/tex]
For structure 2 :
[tex]\text{Formal charge on C}=4-2-\frac{4}{2}=0[/tex]
[tex]\text{Formal charge on N}=5-4-\frac{4}{2}=-1[/tex]
For structure 3 :
[tex]\text{Formal charge on C}=4-2-\frac{6}{2}=-1[/tex]
[tex]\text{Formal charge on N}=5-2-\frac{6}{2}=0[/tex]
