The species Ar, K+, and Ca2+ contain the same number of electrons. In which order do their ionic radii increase?

The species Ar, K+, and Ca2+ are isoelectronic species, which means they contain the same number of electrons. The number of protons present in Ar, K+, and Ca2+ are 18, 19, and 20, respectively. The number of electrons is the same, but the number of protons (and the effective nuclear charge) increases in the order Ar < K+ < Ca2+. Since argon has fully filled shells, it will have high electron-electron repulsions in its shell. Thus, it has the largest radius. Among K+ and Ca2+, K+ has a larger size. On moving from left to right across a period, the atomic size decreases. Thus, the radii increase in the order Ca2+ < K+ < Ar. Therefore, the correct option is B.