# Which one of the following sets of ions represents the collection of isoelectronic species?(Atomic nos.: F = 9, Cl = 17, Na = 11, Mg = 12, Al = 13, K = 19, Ca = 20, Sc = 21) Option 1) $K^{+},Ca^{2+},Sc^{3+},Cl^{-}\;$ Option 2) $\; Na^{+},Ca^{2+},Sc^{3+},F^{-}\;$ Option 3) $\; K^{+},Cl^{-},Mg^{2+},Sc^{3+}\;$ Option 4) $\; Na^{+},Mg^{2+},A1^{3+},Cl^{-}$

As we learnt in

Isoelectronic species -

A series of atom, ions and molecules in which each species contains the same number of electrons but different nuclear charge.

$e.g.\:\:N^{3-},O^{2-},F^{-},Ne,Na^{+},Mg^{2+},Al^{3+}$

Let us consider the number of electrons in all species:

$K^{+}: 18\:\:\:\:\:\:\:\:\:\:\:\:Na^{+}:10\:\:\:\:\:\:\:\:\:\:\:\:K^{+}:18\:\:\:\:\:\:\:\:\:\:\:\:Na^{+}:10$

$Ca^{2+}: 18\:\:\:\:\:\:\:\:\:\:Ca^{2+}:18\:\:\:\:\:\:\:\:\:\:Cl^{-}:18\:\:\:\:\:\:\:\:\:\:Mg^{2+}:10$

$Sc^{3+}: 18\:\:\:\:\:\:\:\:\:\:Sc^{3+}:18\:\:\:\:\:\:\:\:\:\:Mg^{2+}:10\:\:\:\:\:\:\:\:\:\:Al^{3+}:10$

$Cl^{-}: 18\:\:\:\:\:\:\:\:\:\:\:\:\:F^{-}:10\:\:\:\:\:\:\:\:\:\:\:\:Sc^{3+}:18\:\:\:\:\:\:\:\:\:\:\:\:Cl^{-}:18$

Clearly, the answer is $K^{+},Ca^{2+},Sc^{3+},Cl^{-}\;$

Option 1)

$K^{+},Ca^{2+},Sc^{3+},Cl^{-}\;$

This option is correct.

Option 2)

$\; Na^{+},Ca^{2+},Sc^{3+},F^{-}\;$

This option is incorrect.

Option 3)

$\; K^{+},Cl^{-},Mg^{2+},Sc^{3+}\;$

This option is incorrect.

Option 4)

$\; Na^{+},Mg^{2+},A1^{3+},Cl^{-}$

This option is incorrect.

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