Question

The nuclei of which one of the following pairs of nuclei are isotones?

• A. ${ }_{34} Se ^{74},{ }_{31} Ga ^{71}$
• B. ${ }_{38} Sr ^{84},{ }_{38} Sr ^{86}$
• C. ${ }_{42} Mo ^{92},{ }_{40} Zr ^{92}$
• D. ${ }_{20} Ca ^{40},{ }_{16} S ^{32}$

Answer 1

Answer: (A)

The isotones have same number of neutrons.
In given options ${ }_{34} Se ^{74}$ has $74-34=40,$
${}_{31} Ga ^{71}$ has $71-31=40,$
${}_{38} Sr ^{84}$ has $84-38=46,$
${}_{38} Sr ^{86}$ has $86-38=48,$
${}_{42} Mo ^{92}$ has $92-42=50,$
${}_{40} Zr ^{92}$ has $92-40=52,$
${}_{20} Ca ^{40}$ has $40-20=20$,
and ${}_{34} Se ^{74}$ has $32-16=16$ neutrons.
Hence isotonic pair is of $_{34} Se ^{74}, {}_{31} Ga ^{71}$