Question

The isotones of ${}_{56}^{138}Ba$ are:

• A. ${}_{57}^{139}La$ and ${}_{38}^{88}Sr$
• B. ${}_{42}^{92}Mo$ and ${}_{58}^{140}Ce$
• C. ${}_{58}^{140}Ce$ and ${}_{42}^{92}Mo$
• D. ${}_{57}^{139}La$ and ${}_{58}^{140}Ce$

Answer 1

The correct option is D ${}_{57}^{139}La$ and ${}_{58}^{140}Ce$

Isotones are species that have the same number of neutrons but different atomic numbers.
Number of neutrons = mass number – atomic number
Number of neutrons in ${}_{56}^{138}Ba$ = 138 – 56 = 82
Number of neutrons in ${}_{57}^{139}La$ = 139 – 57 = 82
Number of neutrons in ${}_{58}^{140}Ce$ = 140 – 58 = 82