Cl$^{-}$ is not isoelectronic with

Cl$^{-}$ is not isoelectronic with

K$^{+}$

Mg$^{2+}$

S$^{2-}$

P$^{3-}$

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Answer is Wrong!

This question was previously asked in

UPSC CDS-2 – 2022

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Isoelectronic species are atoms or ions that have the same number of electrons. Cl$^{-}$ has an atomic number of 17, so Cl has 17 electrons. Cl$^{-}$ has gained one electron, giving it 17 + 1 = 18 electrons. We need to find the species among the options that does not have 18 electrons.

– K$^{+}$: Atomic number of K is 19. K$^{+}$ has lost one electron, so it has 19 – 1 = 18 electrons.
– Mg$^{2+}$: Atomic number of Mg is 12. Mg$^{2+}$ has lost two electrons, so it has 12 – 2 = 10 electrons.
– S$^{2-}$: Atomic number of S is 16. S$^{2-}$ has gained two electrons, so it has 16 + 2 = 18 electrons.
– P$^{3-}$: Atomic number of P is 15. P$^{3-}$ has gained three electrons, so it has 15 + 3 = 18 electrons.
Thus, Cl$^{-}$ (18 electrons) is not isoelectronic with Mg$^{2+}$ (10 electrons).

Other common isoelectronic series with 18 electrons include Ar, Ca$^{2+}$, Sc$^{3+}$. The electronic configuration for all species with 18 electrons is 1s²2s²2p⁶3s²3p⁶, which is the electron configuration of the noble gas Argon.

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