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Question

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

A
Ca2+<K+<Ar
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B
K+<Ar<Ca2+
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C
Ar<K+<Ca2+
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D
Ca2+<Ar<K+
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Solution

The correct option is A Ca2+<K+<Ar
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 Ca, K has a larger size and on moving from left to right, the atomic size decreases.

Thus, the radii increases in the order Ca2+<K+<Ar.
Therefore, the correct option is A.

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