Question

Number of nucleons in ${{D}_{2}}$ molecule is
A.$1$
B.$2$
C.$3$
D.$4$

Answer 1

Hint: Nucleons are the subatomic particles that reside inside the nucleus of any atom. The subatomic particles which reside inside the nucleus are protons and neutrons. Hence nucleons are the total number of neutrons and protons inside the nucleus. The nucleon number in a nucleus defines the atomic mass number.

Formula used: A general expression of an atom can be represented by :
$_{Z}^{A}X$ ; here X is any atom.
Z= atomic or proton number of atom
A=mass number of atom
Number of neutrons, n = (A-Z)
Hence, the number of nucleons of an atom $=(Z+n)$.

Complete Step by Step Answer:
Isotopes are atoms of the same element which contain a different number of neutrons and the same number of atomic or protons. Some isotopes are unstable as they will decay over time and they will turn into another isotope element. These unstable isotopes are referred to as radioactive species. Hydrogen has three different isotopes named protium ($_{1}^{1}H$), Deuterium [D] ($_{1}^{2}H$), and ($_{1}^{3}H$).

Here we have a ${{D}_{2}}$ molecule or two deuterium atoms as two D atoms combined to form ${{D}_{2}}$ molecule. One deuterium (D) atom contains one proton and mass number$2u$.
The number of neutrons, n $=(2-1)=1$.
The number of nucleons of one deuterium $=(1+1)=2$.
Therefore two deuterium atoms have nucleons $=2\times 2=4$.
Hence the number of nucleons in ${{D}_{2}}$molecule are $4$.
Thus, option (D) is correct.

Note: There are both neutrons and protons residing in the nucleus. But it is found that their mass is less than the sum of their individual masses. This difference arises due to the binding energy per nucleon. The binding energy per nucleon is the amount of energy required to remove a nucleon from a nucleus.