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The nuclei of which one of the following pairs of nuclei are isotones :
A
$^{74}_{34}Se$, $^{71}_{31}Ga$
B
$^{84}_{38}Sr$, $^{86}_{38}Sr$
C
$^{92}_{42}Mo$, $^{92}_{40}Zr$
D
$^{40}_{20}Ca$, $^{32}_{16}S$
Detailed Solution
Isotones are nuclei having the same number of neutrons, $N = A - Z$.
$^{74}_{34}Se$: $N = 74 - 34 = 40$; $^{71}_{31}Ga$: $N = 71 - 31 = 40$. The neutron numbers are equal, so these are isotones.
$^{84}_{38}Sr$ and $^{86}_{38}Sr$ have the same atomic number (38), so they are isotopes.
$^{92}_{42}Mo$ and $^{92}_{40}Zr$ have the same mass number (92), so they are isobars.
$^{40}_{20}Ca$: $N = 20$; $^{32}_{16}S$: $N = 16$. They are neither.
So the isotones are $^{74}_{34}Se$ and $^{71}_{31}Ga$.
$^{74}_{34}Se$: $N = 74 - 34 = 40$; $^{71}_{31}Ga$: $N = 71 - 31 = 40$. The neutron numbers are equal, so these are isotones.
$^{84}_{38}Sr$ and $^{86}_{38}Sr$ have the same atomic number (38), so they are isotopes.
$^{92}_{42}Mo$ and $^{92}_{40}Zr$ have the same mass number (92), so they are isobars.
$^{40}_{20}Ca$: $N = 20$; $^{32}_{16}S$: $N = 16$. They are neither.
So the isotones are $^{74}_{34}Se$ and $^{71}_{31}Ga$.