Question

Composition of the nuclei of two atomic species A and B is given as:
                 $A\,\,\,\,\,\ B$
Protons: $18\,\,\,\,\,\ 20$
Neutrons: $22\,\,\,\,\,\ 20$
(i)- What are the mass numbers of A and B?
(ii)- How are they related to each other?

Answer 1

Hint: The mass number of the element can be calculated by adding the number of protons and number of neutrons in it. To compare, if their atomic numbers are the same and mass numbers are different they will be categorized as isotopes and if their atomic numbers are different and the mass numbers are the same then they will be categorized as isobars.

Complete answer:
We know that the three major particles in an atom are electrons, protons, and neutrons. With the help of these three, we can find much information about the element.
In the question, the number of protons in the A element is 18 and the number of neutrons in A is 22. The number of protons in the B element is 20 and the number of neutrons in B is 20.
(i)- The mass number of the element can be calculated by adding the number of protons and number of neutrons in it. The mass number of A will be:
$A=18+22=40$
The mass number of B will be:
$B=20+20=40$
The mass number of A is 40 and the mass number of B is also 40.
(ii)- With the help of a number of protons in the element we can find the atom as a number of protons is equal to a number of electrons. So, the number of protons in A is 18, therefore, its atomic number is 18. Argon in the periodic table has the atomic number 18. And the number of protons in B is 20, therefore, its atomic number is 20. Calcium in the periodic table has the atomic number 20.
Since, their mass numbers are the same so, $_{18}^{40}Ar$ and $_{20}^{40}Ca$ are isobars.

Note:
The number of electrons is always equal to the number of protons in an atom because the electrical neutrality should be maintained, if they are not the same then the atoms will be considered as ions.