Final answer:
Ions arranged by decreasing size, considering the number of electron shells and the nuclear charge, lead to the order I- > Br- > Ca²⁺ > Mg²⁺, thus option c) is correct.
Step-by-step explanation:
Order of Ions by Decreasing Size
To arrange the ions Br-, I-, Mg²⁺, and Ca²⁺ in order of decreasing size, we must consider both the number of protons in the nucleus (which affects the nuclear charge) and the number of electron shells. The more electron shells, the larger the ion will be due to the increased distance from the nucleus to the outer electrons. Also, ions with a higher positive charge will be smaller because they have more protons pulling on the same number of electron shells.
Starting with the ions listed, I- is the largest because it has more electron shells than Br-. Among the cations, Ca²⁺ has more electron shells than Mg²⁺, but also a greater nuclear charge, making them fairly close in size. Arranging based on these principles, the correct order from largest to smallest is: I- > Br- > Ca²⁺ > Mg²⁺. Therefore, the correct answer is option c) I⁻ > Br⁻ > Ca²⁺ > Mg²⁺.