Question

Which one of the following groups represents a collection of isoelectronic species?
(At. nos.: Cs-55, Br-35)

• A. $Na^+$, $Ca^{2+}$, $Mg^{2+}$
• B. $N^{3-}$, $F^-$, $Na^+$
• C. $Be$, $Al^{3+}$, $Cl^-$
• D. $Ca^{2+}$, $Cs^+$, $Br$

Answer 1

Answer: (B)

$N ^{3-}, F ^{-}, Na ^{+}$ represents a collection of iso-electronic species. They have 10 electrons each. Iso-electronic species have the same number of electrons.
The atomic numbers of $N , F$ and $Na$ are 7,9 and 11 respectively.
Number of electrons in $N ^{3-}=7+3=10$.
Number of electrons in $F ^{-}=9+1=10$.
Number of electrons in $Na ^{+}=11-1=10$.