Question

Three elements A, B and C form a Döbereiner’s triad. Atomic weights of first element A and middle element C are 35 u and 79 u respectively. If the atomic number of element B is 61, then the total number of neutrons in its nucleus is (Assume atomic weight = Mass number)

• A. 61
• B. 63
• C. 60
• D. 62

Answer 1

The correct option is D 62

Since it is a Döbereiner’s triad, the atomic mass of middle element C should roughly be the average of the atomic masses of the elements A and B

158 = 35 + Atomic mass of B
Atomic mass of B = 158 − 35 = 123 u
∴ Mass number = 123
Atomic number of B = 61
Atomic number = Number of protons
Number of neutrons = Mass number − Number of protons
∴ Number of neutrons = 123 − 61 = 62
Hence, the correct answer is option (d).