If the ion is negatively charged, anion, it means that it gains two
electrons from its surroundings. The number of electrons of an argon (neutral
atom) is 18. Two of the electrons in our anion is from the surrounding, so
originally it only has 16 electrons.
In the periodic table, the 16th element is sulphur. The
chemical symbol of sulphur is S. Thus, the anion is the negatively charged
sulphur with chemical symbol of S2-.