Question
The correct order of the decreasing ionic radii among the following isoelectronic species is
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Ca2+ > K+ >S2- > Cl-
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Cl- > S2- > Ca2+ > K+
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S2- > Cl- > K+ > Ca2+
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K+ > Ca2+ > Cl- > S2-
Solution
C.
S2- > Cl- > K+ > Ca2+
During the formation of a cation, the electrons are lost from the outer shell and the remaining electrons experience a great force of attraction by the nucleus, ie, attracted more towards the nucleus. In other words, nucleus holds the remaining electrons more tightly and result in decreased radii.
However, in the case of anion formation, the addition of an electron (s) takes place in the same outer shell, thus the hold of the nucleus on the electrons of outer shell decreases and this result in increased ionic radii.
Thus, the correct order of ionic radii is
S2- > Cl- > K+ > Ca2+
