Final answer:
Atoms of the element argon (Ar) have the same electronic structure as a sulfur ion (S²-) because both have 18 electrons, making them isoelectronic species with a complete octet in the valence shell.
Step-by-step explanation:
The element that has atoms with the same electronic structure as a sulfur ion (S²-) is argon (Ar). When sulfur (Z = 16) gains two electrons, it ends up with a total of 18 electrons, which is the same number of electrons as the noble gas argon. This is because argon has an atomic number of 18, meaning it naturally has 18 electrons in its neutral state. The complete octet in the valence shell of both S²- and argon atoms results in the same electronic structure. In chemistry, ions or atoms that have the same number of electrons and thus the same electronic structure are known as isoelectronic species. This similarity in electronic structure can lead to similar chemical properties, a concept which forms the basis of the organization of the periodic table and has implications in many fields including medicine and science.