* Atomic number defines the number of protons in an atom's nucleus.

* Electrons typically balance out the protons in a neutral atom.

Since the atom has an atomic number of 16, it has 16 protons. If it has 18 electrons, it has two more electrons than protons. This means the atom is an anion (negatively charged ion).

The element with atomic number 16 is Sulfur (S). Therefore, the element is a sulfide ion (S²⁻).