Question

Elements A (35.5u), B (81u) and C follow the Döbereiner's triad. Determine the atomic mass of Element C.

• A. 58.5
• B. 106.5
• C. 126.5
• D. 128

Answer 1

The correct option is C 126.5

In a triad, the atomic mass of the central atom is nearly an average of the other two atoms. In the triad of elements A, B and C, the mass of ‘B’ will be the average mass of ‘A’ and ‘C’.

Atomic mass of the middle element (B) = (Atomic mass of A + Atomic mass of C)/2

Atomic mass of C = (Atomic mass of B X 2) - Atomic mass of A

Atomic mass of C = (81 X 2) - 35.5 = 126.5 ≃ 127u.