Question

Composition of the nuclei of two atomic species A and B is given as under

	A	B
Protons	18	20
Neutrons	22	20

How A and B are related to each other?

Answer 1

Step 1: Understand Mass number:

• The total number of protons and neutrons in an atomic nucleus is known as the mass number, often known as the atomic mass number or nucleon number.
• The letter 'A' is used to signify it.
• Because protons and neutrons coexist in the nucleus of an atom, they are collectively referred to as nucleons.
• A Carbon ($\mathrm{C}$) atom, for example, has six protons and six neutrons. As a result, its mass number is $12$.

Step 2: Find the Mass number of A and B

• According to the question, A has $18$ protons and $22$ neutrons.
• The mass number can be found as:

$\mathrm{Mass}\mathrm{number}=\mathrm{Number}\mathrm{of}\mathrm{protons}+\mathrm{Number}\mathrm{of}\mathrm{neutrons}$

$$\begin{gathered} \mathrm{Mass}\mathrm{number}\mathrm{of}\mathrm{A}=18+22 \\ =40 \end{gathered}$$

• Similarly, B has $20$ protons and $20$ neutrons.
• The mass number of B can be found as:

$$\begin{gathered} \mathrm{Mass}\mathrm{number}\mathrm{of}\mathrm{B}=20+20 \\ =40 \end{gathered}$$

Step 4: Analyze relation between A and B

• From step 2 and step 3, A and B can be represented as:
  • A represents $\left({}_{18}{}^{40}\mathrm{Ar}\right)$ Argon.
  • B represents $\left({}_{20}{}^{40}\mathrm{Ca}\right)$ Calcium.
• Both have same Mass number, thus A and B are Isobars.
• Isobars are those elements that have a different atomic number but the same mass number.

Thus, species A and B are Isobars.