Question

A and B are two elements which have the same atomic weight and are having atomic number 27 and 30 respectively. If the atomic weight of A is 57, then the number of neutrons in B is:
(A) 27
(B) 33
(C) 30
(D) 40

Answer 1

Hint: In an element, the number of protons is denoted by the atomic number and the number of nucleons (protons and neutrons) is denoted by the mass number.

Complete step by step solution:
It is given that the two elements A and B have:
The atomic number of A = 27
The atomic number of B = 30
This atomic number represents the number of protons in the element. Such that, the number of electrons is equal to the number of protons present in the nucleus of a particular element.
The atomic weight of element A is given to be 57. This atomic weight is approximately equal to the mass number, which represents the total sum of the number of protons and neutrons in the nucleus, that is the number of nucleons. Then, the number of neutrons in element A will be (57 – 27) = 30.
It is given that the atomic weight of both element A and B are the same, but different atomic numbers. Then, the number of neutrons in element B will be (57-30) = 27.
These elements with the same mass number but different atomic numbers are known as isobars. That is, the number of protons and neutrons is different, but the number of nucleons is equal in both the elements.

Therefore, the number of neutrons in element B is option (A)- 27.

Note: The chemical properties of both elements are different due to different atomic numbers. Also, they have completely different structures. An example of isobars is ${}_{27}C{{o}^{64}}\,and\,{}_{28}N{{i}^{64}}$, where the atomic number of cobalt is 27 and the number of neutrons is 37, whereas in nickel, the atomic number is 28 and the number of neutrons is 36. But the mass number (nucleons) is the same in both, that is, 64.