If a species has 18 electrons, 16 protons and 16 neutrons, the symbol of the species is 

At. no. = No. of protons = 16

Mass number = p + n = 16 + 16 = 32

16 p < 18 ${\mathrm{e}}^{-}$$\Rightarrow$$2^{-}\mathrm{charge}$

Symbol is ${}_{16}{}^{32}\mathrm{S}^{2-}.$