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WBCS Preliminary Examination 2022 — General Studies · Question 137 of 200

Q137
General ScienceChemistry
$O^{2-}$ is isoelectronic with
  1. a.$F^{-}$
  2. b.$Cl^{-}$
  3. c.$Li^{+}$
  4. d.$K^{+}$

Answer: (A) $F^{-}$

Isoelectronic species have the same number of electrons. $O^{2-}$ has 8 protons + 2 extra electrons = 10 electrons. $F^{-}$ (fluorine) has 9 protons + 1 extra electron = 10 electrons. Both have the electronic configuration of neon [He] 2s² 2p⁶. $Cl^{-}$ has 18 electrons, $Li^{+}$ has 2, and $K^{+}$ has 18.