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

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.