Question

Cl, Br, I, if this is a Dobereiner's triad and the atomic masses of Cl and I are 35.5 and 127 respectively, then the atomic mass of Br is -

• A. 162.5
• B. 91.5
• C. 81.25
• D. 45.625

Answer 1

The correct option is C

81.25

Step 1: Dobereiner's triad

One of the earliest attempts of arrangement of elements was done by Dobereiner in which he arranged elements with similar properties in group of three called triads. Some examples of triads are:

• Lithium (Li), Sodium (Na), Potassium (K).
• Sulphur (S), Selenium (Se), Tellurium (Te).
• Chlorine (Cl), Bromine (Br), Iodine (I)

He proposed that the properties of the element in the middle can be predicted from the properties of the other two elements in the triad.

Step 2: Determine the atomic mass of Bromine $\begin{array}{rcl}& & \left(\mathrm{Br}\right)\end{array}$

According to Dobereiner's theory, the atomic mass of central element can be calculated as the arithmetic mean of atomic masses of the other two elements . In the triad of Chlorine, Bromine and Iodine, Bromine is the central element and therefore, its atomic mass can be calculated as the mean of atomic masses of Chlorine and Iodine.

Therefore,

$\begin{array}{rcl}{\mathrm{M}}_{\mathrm{Br}}& =& \frac{{\mathrm{M}}_{\mathrm{Cl}}+{\mathrm{M}}_{\mathrm{I}}}{2}\\ & =& \frac{35.5+127}{2}\\ & =& \frac{162.5}{2}\\ & =& 81.25\end{array}$

The atomic mass of Bromine is 81.25.

Final Answer:

Therefore, the correct option is (C).