Which of these species is the largest? --
Cl⁻
K⁺
Ar
Cl-, K+ and Ar are isoelectronic species. That is the species have equal number of electrons. All this Cl-, K+ and Ar contains 18 electrons.
In the case of isoelectronic species, the size is determined by the charge on the species. Size of anion is greater than the parent atom and size of cation is less than parent atom. This is because they are having almost same nuclear charge, but different number of electrons. So the effective nuclear charge on electrons will be higher for cations and hence the atom will Shrink.
So, cations are lesser is size, follwed by neutral atom to bigger anions. As the charge on cation increases, the size decreases and charge on anion increase, then size increases.
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