Final answer:
The arrangement from largest to smallest by size for the ions Na+, Mg2+, Ca2+, and Rb+ is Rb+, Ca2+, Mg2+, Na+. This order is based on their respective positions in the periodic table, taking into account the period and charge of each ion.
Step-by-step explanation:
The size of ions depends on their positions in the periodic table, with ions getting smaller as you move across a period (from left to right) and larger as you move down a group (from top to bottom). When comparing ions with the same charge, the size typically increases as you move down a group due to the addition of electron shells. However, when ions have different charges, the one with a higher charge generally pulls its electrons in more tightly, resulting in a smaller ionic radius. Given the ions Na+, Mg2+, Ca2+, and Rb+, we arrange them in descending order of size. Rb+ is the largest as it is located in the fifth period, followed by Ca2+ and Mg2+ in the fourth and third periods, respectively. Na+ is the smallest because it is located in the third period and has a +1 charge compared to the +2 charge of Mg2+. Therefore, the correct order is Rb+, Ca2+, Mg2+, Na+. Option (a) Rb+, Ca2+, Mg2+, Na+ is the correct choice.