Question

In which one of the following pairs the two species are both isoelectronic and isotopic? (Atomic numbers: $Ca=20,Ar=18,K=19,Mg=12,Fe=26,Na=11)$

• A. ${}^{40}C{a}^{2+}$ and ${}^{40}Ar$
• B. ${}^{39}{K}^{+}$ and ${}^{40}{K}^{+}$
• C. ${}^{24}M{g}^{2+}$ and ${}^{25}Mg$
• D. ${}^{23}Na$ and ${}^{24}N{a}^{-}$
• E. ${}^{56}F{e}^{3+}$ and ${}^{57}F{e}^{2+}$

Answer 1

Answer: (B)

Isotopes means Same atomic number but difference mass number (same number of protons but different nutrons
${}^{39}{K}^{+}$ and ${}^{40}{K}^{+}$
both have same number of electrons (18) and are isotopes