Question

In a triad of elements, atomic mass of third element is 100 and the atomic mass of second element is 70. What is the atomic mass of this first element of this triad?

Answer 1

When elements are ordered in ascending order of atomic mass, according to Doberiner's triad, the atomic mass of the middle element is roughly equal to the arithmetic mean (average) of the third and first elements.

Elements placed in Doberiner's triad have similar properties.

The atomic mass of a middle element (B) is written as:

$\begin{array}{rcl}\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{B}& =& \frac{\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{A}+\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{C}}{2}\\ \mathrm{B}& =& \frac{\mathrm{A}+\mathrm{C}}{2}\end{array}$

The atomic mass of middle element B is $70$.

The atomic mass of third element C is $100$.

Suppose, the atomic mass of the first element A is $\mathrm{x}$.

Calculate the atomic mass of the element B:

$\begin{array}{rcl}\mathrm{B}& =& \frac{\mathrm{A}+\mathrm{C}}{2}\\ 70& =& \frac{\mathrm{x}+100}{2}\\ \mathrm{x}& =& 140-100\\ & =& 40\end{array}$

The atomic mass of the first element of the triad is $40$.

Final Answer:

Therefore, the Atomic mass of the first element in the triad is $40$