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. $^{40}Ca^{2+}$ and $^{40}Ar$
• B. $^{39}K^{+}$ and $^{40}K^{+}$
• C. $^{24}Mg^{2+}$ and $^{25}Mg$
• D. $^{23}Na$ and $^{24}Na^{-}$
• E. $^{56}Fe^{3+}$ and $^{57}Fe^{2+}$

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