Question

Which of the following structures is the most preferred and hence of lowest energy for $S{O}_3$?

A.

B.

C.

D.

Answer 1

Hint:. Remember, the lowest energy structure for a compound would be the one that will have the smallest value of formal charges on its central atom. Try to find the formal charge of the central atom in each option and see which one has the lowest value.

Complete step by step answer:
As per the given question, the various Lewis structures of the compound are given and we have to find out which structure has the lowest energy and is the most preferred one for $S{O}_3$.
So, to find out which structure has the lowest energy, we must calculate the formal charge of the central atom and will see which structure has lowest value of formal charge. We should know that the central atom in the structure having lowest formal charge will be more stable and will have lowest energy and so it will be mostly preferred one. Let us see, what is formal charge.
Formal charge over an atom of a polyatomic molecule or ion is the difference between the valence electron of that atom in the elemental state and the number of electrons assigned to that atom in Lewis structure.

It is calculated by the formula,
Formal charge of an atom $=V-N-{\displaystyle \frac{1}{2}}\times B$
Where, V is the total number of valence electrons, N is the non-bonding electrons and B is the bonded electrons.
Let us find the formal charge of the sulphur atom (atomic number is $16$) from each option using the above formula.
In the first option, the formal charge of the sulphur atom as per the Lewis structure will be $=6-0-{\displaystyle \frac{1}{2}}\times 8=+2$.
In the second option, the formal charge of the sulphur atom will be $=6-2-{\displaystyle \frac{1}{2}}\times 6=+1$.
In the third option, the formal charge of sulphur will be $=6-0-{\displaystyle \frac{1}{2}}\times 8=+2$.
And in the last option, the formal charge of sulphur will be $=6-0-{\displaystyle \frac{1}{2}}\times 12=0$
So, we can see that the last option is having the lowest value of formal charge, so it will be more stable than other structures and will have the lowest energy.
So, the correct answer is “Option D”.

Note: Important thing to remember, whenever we want to predict the energy of any substance, formal charge should be considered first. Less is the formal charge; more will be the stability and lower will be the energy (as stability is inversely proportional to the formal charge). Formal charge should not be confused with the actual charge on atoms in a molecule.