The correct order of the decreasing ionic radii among the following isoelectronic species are:
Ca2+ > K+ > S2- > Cl-
Cl- > S2- > Ca2+ > K+
S2- > Cl- > K+ > Ca2+
K+ > Ca2+ > Cl- > S2-
First, we note that this is an isoelectronic series of ions, with all ions having 18 electrons. In such a series, size decreases as the nuclear charge (atomic number) of the ion increases. The atomic numbers of the ions are S (16), Cl (17), K (19), and Ca (20). Thus, the ions decrease in size in the order S2–> Cl–> K+> Ca2+.
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