The formal charge of an atom In a polyatomic molecule or 100 may be defined as the difference between the number of valence electrons of that atom in an isolated or free state and the number of electrons assigned to that atom in the Lewis structure. [tis expressed as :
$\left[\begin{array}{c}\mathrm{Formal} \mathrm{charge} \left[\mathrm{F}.\mathrm{C}.\right]\\ \mathrm{on} \mathrm{an} \mathrm{atome} \mathrm{in} \mathrm{Lewis}\\ \mathrm{Structure}\end{array}\right]=\left[\begin{array}{c}\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{valence}\\ \mathrm{electrons} \mathrm{in} \mathrm{the} \mathrm{free} \\ \mathrm{atom}\end{array}\right]$

                      $-\left[\begin{array}{c}\mathrm{Total} \mathrm{number} \mathrm{of} \mathrm{non} -\\ \mathrm{bonding} (\mathrm{lone} \mathrm{pair})\\ \mathrm{electrons} \end{array}\right]-\left(\frac{1}{2}\right) \left[\begin{array}{c}\mathrm{Total} \mathrm{numb} \mathrm{of}\\ \mathrm{bonding} \mathrm{shared}\\ \mathrm{electrons} \end{array}\right]$

Select correct feature about ${\mathrm{CO}}_3^{2-}$ carbonate ion in one of the Lewis structure based on the presence or two single bonds and one double bond between carbon and oxygen atoms.