Question
In which one of the following pairs the two species are both is0electronic and isotopic ? ( Atomic numbers : $$Ca = 20,Ar = 18,K = 19,$$ $$Mg = 12,Fe = 26,Na = 11$$ )
A.
$$^{40}C{a^{2 + }}\,\,{\text{and}}\,{\,^{40}}Ar$$
B.
$$^{39}{K^ + }\,\,{\text{and}}\,{\,^{40}}{K^ + }$$
C.
$$^{24}M{g^{2 + }}\,\,{\text{and}}\,{\,^{25}}Mg$$
D.
$$^{23}Na\,\,{\text{and}}\,{\,^{24}}N{a^ + }$$
Answer :
$$^{39}{K^ + }\,\,{\text{and}}\,{\,^{40}}{K^ + }$$
Solution :
$$^{39}{K^ + }$$ and $$^{40}{K^ + }$$ contains same number of electrons so they are isoelectronic. They have same atomic number but different mass numbers so they are also isotopes.