A and B are two elements which have the same atomic weight and are having atomic numbers $27$ and $30$ respectively. If the atomic numbers are $27$ and $30$ respectively. If the atomic weight of A is $57$ then number of neutron in B is-
A.$27$
B.$33$
C.$30$
D.$40$

Hint: We can say that in an element, atomic number is represented by the amount of protons, and mass number is represented by the amount of nucleons. We can say that nucleons are together known as protons and neutrons.

Complete answer:
We can say that the atomic number of A is $27$ and the atomic number of B is $30$ . We can say that the atomic number is indicated by the amount of protons in the element. The amount of electrons in an atom is equal to the amount of protons found in the nucleus of a specific element. We are given the atomic weight of element A as $57$ . So, the atomic weight is equal to the mass number that gives the total sum of protons and neutrons. Nucleons are nothing but a sum of protons and neutrons. Thus, the number of neutrons in element A would be $57 - 27 = 30$.
We are provided that atomic weights of A as well as B are the same, whereas there would be a difference in atomic number. So, the number of neutrons in element B would be $57 - 30 = 27$.
Isobars are the elements that contain the same mass number whereas there would be different atomic numbers. So, the amount of neutrons in the element is $27$ .

Option (A) is correct.

Note:
We have to know that as atomic numbers would be different, the chemical properties of two elements are different. But their structures would be different. We can take the examples of \[{}_{27}C{o^{64}}\] and ${}_{28}N{i^{64}}$ are isobars. In this atomic number of cobalt is $27$ and the number of neutrons found is $37$ . In nickel, the atomic number is $28$ and the number of neutrons found is 36 but for both these compounds the mass number is the same.