Question

In which one of the following pairs, the two species are both isoelectronic and isotopic? (Atomic number: $Cr=24,Ar=18,K=19,Mg=12,Fe=26,Na=11)$

• A. ${}^{24}M{g}^{2+}$ and ${}^{40}Ar$
• B. ${}^{39}{K}^{+}$ and ${}^{40}{K}^{+}$
• C. ${}^{24}M{g}^{2+}$ and ${}^{25}Mg$
• D. ${}^{23}Na$ and ${}^{24}N{a}^{+}$

Answer 1

Answer: (B)

${}^{39}{K}^{+}$ and ${}^{40}{K}^{+}$ both contain 18 electrons; i.e., they are isoelectronic and isotopic as they have same atomic number.