A species that is not isoelectronic with $^{35}_{17}$Cl$^-$ is

A species that is not isoelectronic with $^{35}_{17}$Cl$^-$ is

$^{31}_{15}$P$^{3-}$
$^{32}_{16}$S$^{2-}$
$^{40}_{18}$Ar$^{2+}$
$^{39}_{19}$K$^+$
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UPSC Geoscientist – 2022
Isoelectronic species are atoms or ions that have the same number of electrons. First, determine the number of electrons in $^{35}_{17}$Cl$^-$:
– Atomic number (Z) = 17 (number of protons)
– Charge = -1
– Number of electrons = Protons – Charge = 17 – (-1) = 18 electrons.
Now check the electron count for each option:
– A) $^{31}_{15}$P$^{3-}$: Z=15. Electrons = 15 – (-3) = 18 electrons. (Isoelectronic)
– B) $^{32}_{16}$S$^{2-}$: Z=16. Electrons = 16 – (-2) = 18 electrons. (Isoelectronic)
– C) $^{40}_{18}$Ar$^{2+}$: Z=18. Electrons = 18 – (+2) = 16 electrons. (Not isoelectronic)
– D) $^{39}_{19}$K$^+$: Z=19. Electrons = 19 – (+1) = 18 electrons. (Isoelectronic)
The species that is not isoelectronic with Cl$^-$ is $^{40}_{18}$Ar$^{2+}$ with 16 electrons.
– Isoelectronic species have the same electron configuration.
– The number of electrons in an ion is calculated by subtracting the charge from the number of protons (atomic number).
Noble gases, like Argon (Ar), are often found as isoelectronic species with neighboring elements when they form ions, as they achieve a stable electron configuration. For instance, Cl$^-$ and K$^+$ are isoelectronic with neutral Argon (Ar), which has 18 electrons (Z=18). The mass number (superscript, e.g., 35) indicates the total number of protons and neutrons and is not relevant for determining the number of electrons.