Question

If chlorine, bromine and iodine form a Dobereiner's triad and the atomic masses of chlorine and iodine are 35.5 u and 127.0 u respectively, then the atomic mass of bromine is _____.

• A. 162 u
• B. 91 u
• C. 81 u
• D. 45 u

Answer 1

The correct option is C

81 u

In a triad, the atomic mass of the central atom is nearly an average of the other two atoms. In the triad of chlorine, bromine and iodine, the mass of bromine will be average mass of chlorine and iodine.
$=\frac{35.5+127.0}{2}=81.25\text{u}$ ~ 81 $\text{u}$