Problem: Choose the best Lewis structure for SeO42-.

For this problem, we need to compare the formal charge of each structure. 

Recall that the formula for the formal charge is:

$\boxed{\mathbf{F}\mathbf{.}\mathbf{C}\mathbf{.}\mathbf{=}\mathbf{Group}\mathbf{ }\mathbf{Number}\mathbf{-}\mathbf{(}\mathbf{Bonds}\mathbf{+}\mathbf{Nonbonding}\mathbf{ }{\mathbf{e}}^{\mathbf{-}}\mathbf{)}}$

SeO₄^2- 

           Group              Valence Electrons

Se        6A                   1 x 1 e^- = 6 e^-

O          6A                  4 x 6 e^- = 6 e^-

-2 charge                                    +2 e^-

                                    Total: 32 valence e^-

Structure 1: The given Lewis structure is:

Total # of electrons = (2 e^- x 6 bonds) + (2 e^- x 9 lone pairs)

Total # of electrons = 30 e^-

The structure is not the best Lewis structure for SeO₄^2-.

Structure 2: The given Lewis structure is:

Total # of electrons = (2 e^- x 4 bonds) + (2 e^- x 13 lone pairs)

Total # of electrons = 34 e^-

The structure is not the best Lewis structure for SeO₄^2-.

Structure 3: The given Lewis structure is:

Total # of electrons = (2 e^- x 6 bonds) + (2 e^- x 9 lone pairs)

Total # of electrons = 30 e^-

The structure is not the best Lewis structure for SeO₄^2-.

Structure 4: The given Lewis structure is: