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$^+$
Answer is Right!
Answer is Wrong!
This question was previously asked in
UPSC Geoscientist – 2022
– 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.
– The number of electrons in an ion is calculated by subtracting the charge from the number of protons (atomic number).