Question
The nuclei of which one of the following pairs of nuclei are isotones ?
A.
$$_{34}S{e^{74}},{\,_{31}}G{a^{71}}$$
B.
$$_{42}M{o^{92}},{\,_{40}}Z{r^{92}}$$
C.
$$_{38}S{r^{84}},{\,_{38}}S{r^{86}}$$
D.
$$_{20}C{a^{40}},{\,_{16}}{S^{32}}$$
Answer :
$$_{34}S{e^{74}},{\,_{31}}G{a^{71}}$$
Solution :
The nuclei which have same number of neutrons but different atomic number and mass number are known as isotones. In choice (A) nuclei of $$_{34}S{e^{74}}$$ and $$_{31}G{a^{71}}$$ are isotones as
$$A - Z = 74 - 34 = 71 - 31 = 40$$