Final answer:
Sulfur (S) is the element in the 3rd period most likely to form an ion with a -2 charge. It is in Group 6 of the periodic table and tends to gain two electrons to become an anion with a 2- charge.
Step-by-step explanation:
Among the given options, the element that is most likely to form an ion with a -2 charge in the 3rd period is sulfur (S). Sulfur is in Group 6 of the periodic table, which means it tends to gain two electrons to achieve a full outer shell, resulting in a 2- charge. The periodic table helps predict ion formation and charge; elements toward the right side, like nonmetals, often form negative ions (anions), while elements toward the left side typically form positive ions (cations). Calcium (Ca) and magnesium (Mg) are in Group 2 and typically form 2+ ions, whereas chlorine (Cl) is in Group 7 and usually forms a 1- ion. Selenium (Se) might also be considered, as it is in the same group as sulfur, but sulfur is more commonly encountered with a 2- charge.