Question

One of the resonating structures of $S{O}_4^{-2}$ is as shown. Which set of formal charge on oxygen and bond order is correct?

(A) -0.5 and 1.5
(B) 1.5 and 3
(C) 2 and 3
(D) 1.5 and 1.5

Answer 1

Hint: To solve this question we should know the formula to calculate set formal charge and bond order as well as the theory behind it. The formal charge on oxygen in $S{O}_4^{-2}$ is equal to the formal charge on each oxygen and takes an average of all.

Complete step by step answer:
The formal charge is the number of charges on an atom in a molecule, under assumption that electrons in all bonds are equally shared irrespective of the relative electronegativity.
Formula of formal charges:
$$Formalcharge=numberofvalenceelectroninfreeatom-numberoflonepairelectrons-{\displaystyle \frac{1}{2}}numberofbondpairelectrons.$$

Carefully refer to the diagram, where each oxygen is numbered.
-The formal charges of oxygen 1:
$$Formalcharge=numberofvalenceelectroninfreeatom-numberoflonepairelectrons-{\displaystyle \frac{1}{2}}numberofbondpairelectrons.$$
= 6 - 4 - 2
  = 0
-The formal charges of oxygen 2:
$$Formalcharge=numberofvalenceelectroninfreeatom-numberoflonepairelectrons-{\displaystyle \frac{1}{2}}numberofbondpairelectrons.$$
= 6 - 6 - 1
  = -1
 -The formal charges of oxygen 3:
$$Formalcharge=numberofvalenceelectroninfreeatom-numberoflonepairelectrons-{\displaystyle \frac{1}{2}}numberofbondpairelectrons.$$
= 6 - 4 - 2
  = 0

-The formal charges of oxygen 4:
$$Formalcharge=numberofvalenceelectroninfreeatom-numberoflonepairelectrons-{\displaystyle \frac{1}{2}}numberofbondpairelectrons.$$
 = 6 - 6 - 1
 = -1
The total formal charges on oxygen in $S{O}_4^{-2}$ is:

$$Formalchargesinoxygen={\displaystyle \frac{sumofformalchargeonoxygen1,2,3and4}{Totalnumberofoxygen}}$$
   $$={\displaystyle \frac{0+(-1)+0+(-1)}{4}}$$
    = - 0.5
Thus, formal charge on $S{O}_4^{-2}$ is - 0.5

Bond order is the number of chemical bonds present between the pairs of atoms
$$bondorder={\displaystyle \frac{numberofbonds}{Totalnumberofatomsharingthebond}}$$
  $$={\displaystyle \frac{6}{4}}$$
  = 1.5
So, the correct answer is “Option A”.

Note: Generally, Formula to calculate bond pair:
$$Bondorder={\displaystyle \frac{1}{2}}(N_b-N_a)$$
Where, $N_b$ = number of bonding electrons
             $N_a$ = number of antibonding electrons
But for compound that has resonating structures, then the formula will be
$$bondorder={\displaystyle \frac{numberofbonds}{Totalnumberofatomsharingthebond}}$$