Final answer:
Among O, K, K+, Rb, and Rb+, the atom with the largest size is Rb (rubidium) because it has the most electron shells. The cationic forms (K+ and Rb+) are smaller than their respective atoms.
Step-by-step explanation:
The size variation among O, K, K+, Rb, and Rb+ depends on their electron configuration and atomic radius. O (oxygen) is smaller because it has fewer energy levels for its electrons. K (potassium) and Rb (rubidium) are larger as they have more energy levels. However, when they lose an electron to form ions, K+ and Rb+, they get smaller due to a decrease in electron-electron repulsion. But among them, Rb (rubidium) has the largest size because it has more electron shells compared to others.
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