i)  Valence electrons:  4 + 2 x 6 = 16 ( 8 pairs)

ii)  Central atom  C;             O -- C -- O

iii)  Give octet to carbon

                                    --

                             O -- C -- O

                                    --

 Try to fill octet to O

 iv)  Count electrons:

4 bond pairs = 4 pairs

       4 lone pairs  = 4 pairs

                               8 electron pairs