Question

The nuclei of which one of the following pairs of nuclei are isotones
(A) $ _{34}\text{S}{{\text{e}}^{74}}{{,}_{31}}\text{G}{{\text{a}}^{71}} $
(B) $ _{38}\text{S}{{\text{r}}^{84}}{{,}_{38}}\text{S}{{\text{r}}^{86}} $
(C) $ _{42}\text{M}{{\text{o}}^{92}}{{,}_{40}}\text{Z}{{\text{r}}^{92}} $
(D) $ _{20}\text{C}{{\text{a}}^{40}}{{,}_{16}}{{\text{S}}^{32}} $

Answer 1

Hint: Isotones: Isotones are defined as any two or more elements having the same number of neutrons. Atom is made up of the three particles i.e. electron neutron, & proton. Neutrons and protons are present in the nucleus of an atom while electrons revolve around the nucleus in stationary orbits.

Complete Step By Step Solution
Isotones: The species having the same number of neutrons present in their nucleus is called Isotones. The word ‘iso’ means the same.
To calculate the number of neutrons firstly we subtract the number of protons from the mass number i.e.
Number of neutrons = mass number $ - $ Number of proton
In the given question
Option [A] is correct.
As
Number of neutron in $ _{34}\text{S}{{\text{e}}^{74}}=74-34=40 $
Number of neutron in $ _{31}{{49}^{71}} $
 $ =71-31=40 $
So
 $ _{34}\text{S}{{\text{r}}^{74}},{{\text{ }}_{31}}{{49}^{71}} $ is an Isotonic pair.

Note
Note that the number of protons gives the atomic number of the element. Mass no is defined as the total no. of protons and neutrons in an element i.e. sum of no. of protons and neutrons, so no. of neutrons can be easily found by subtracting mass number and atomic number. One should have the knowledge of Isotones, Isobars, and Isoelectronic pairs to conclude the solution. Elements having the same electronic configuration have the same properties. The atomic no. of an element is represented by Z and mass number is represented by A. There also exists Isotopes which have the same no. of protons i.e. their atomic no. is same but due to different neutron no. their mass no. varies.