Question

An isotone of $${{}_{32}}^{76}Ge$$ is:
(a)- $${{}_{34}}^{78}Se$$
(b)- $${{}_{33}}^{78}Se$$
(c)- $${{}_{33}}^{77}Se$$
(d)- $${{}_{32}}^{77}Ge$$

Answer 1

Hint: Isotones are those elements that have the same number of neutrons. The number of neutrons can be calculated by subtracting the number of protons from the mass number of the element. The number of protons present is equal to the atomic number of the element.

Complete answer: It may be noted that isotopes differ in the number of neutrons only whereas isobars differ in the number of neutrons as well as protons. However, some atoms of different elements are found to have the same number of neutrons.
Such atoms of different elements that contain the same number of neutrons are called isotones.
Total number of protons = total number of electrons = Atomic number of the element.
The atomic number of the element is written in the subscript of the element symbol.
The mass number of an element = number of protons + number of neutrons.
The mass number of the element is written in the superscript of the element symbol.
So, for finding the number of neutrons atomic number is subtracted from the mass number.
Example of isotones, $${{}_6}^{14}C{{;}_7}^{15}N{{;}_8}^{16}O$$
Each of them contains 8 neutrons.
In $${{}_{32}}^{76}Ge$$ the number of neutrons,
76 – 32 = 44
There are 44 neutrons in $${{}_{32}}^{76}Ge$$

Let us study all the options one by one:
(a)- The number of neutrons in $${{}_{34}}^{78}Se$$ = 78 – 34 = 44
(b)- The number of neutrons in $${{}_{33}}^{78}Se$$ = 77 – 34 = 43
(c)- The number of neutrons in $${{}_{33}}^{77}Se$$ = 78 – 33 = 45
(d)- The number of neutrons in $${{}_{32}}^{77}Ge$$ = 77 – 32 = 45
Hence $${{}_{32}}^{76}Ge$$ and $${{}_{34}}^{78}Se$$ have the same number of neutrons. $${{}_{34}}^{78}Se$$ is the isotone of $${{}_{32}}^{76}Ge$$.

So, the correct answer is “Option A”.

Note: Don’t get confused between isotopes, isobars, and isotones of the elements. Isotones cannot be calculated just by seeing the option whereas isotopes and isobars can be easily observed. Isotopes are the elements that have the same number of protons but different mass numbers while isobars are those who have the same mass numbers but different number of protons.