Question

If A, B and C are the three hypothetical elements of Dobereiner's Triads and atomic mass of A and B are 7 and 15 respectively then the atomic mass of C is :

• A. $1$
• B. $11$
• C. $23$
• D. $25$

Answer 1

The correct option is C $23$

Dobereiner stated in his law of triads that the arithmetic mean of the atomic masses of the first and third element in a triad would be approximately equal to the atomic mass of the second element in that triad.
$\therefore M_B=\frac{M_A+M_C}{2}$
$⟹15=\frac{7+M_C}{2}$
$\therefore M_C=23$
Hence, the correct answer is option (c)