Question

Identify the correct order of the size of the following:
(a) Ca²⁺ < K⁺ < Ar < Cl⁻ < S²⁻
(b) Ar < Ca²⁺ < K⁺ < Cl⁻ < S²⁻
(c) Ca²⁺ < Ar < K⁺ < Cl⁻ < S²⁻
(d) Ca²⁺ < K⁺ < Ar < S²⁻ < Cl⁻

Answer 1

Correct order of the size will be $Ca^{2+}$ < $K^{+}$ < Ar < $Cl^{-}$ < $S^{2-}$.

Explanation:

Isoelectronic species are defined as the species which have same electronic structure and contains same number of valence electrons.

• For $Ca^{2+}$

Electrons in calcium are 20 and on losing two electrons there will be total 18 electrons.

[Ca] : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}$

$[Ca^{2+}]$ : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}$

• For $K^{+}$

Electrons in potassium are 19 and on losing one electron there will be total 18 electrons.

[K] : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{1}$

$[K^{+}]$ : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}$

• For Ar, electrons in argon are 18. As it is given as a neutral atom so it will contains a total of 18 electrons only.

[K] : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}$

• For $Cl^{-}$, electrons in chlorine are 17 and on gaining one electron there will be total 18 electrons.

[Cl] : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{5}$

$[Cl^{-}]$ : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}$

• For $S^{2-}$, electrons in sulfur are 16 and on gaining two electrons there will be total 18 electrons.

[S] : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}$

$[S^{2-}]$ : $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}$

More is the atomic number of an atom more will be the number of protons present in it. Hence, more strongly it will attract the electrons towards itself. As a result, smaller will be the size of atom.

Therefore, correct order of the size will be $Ca^{2+}$ < $K^{+}$ < Ar < $Cl^{-}$ < $S^{2-}$.

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