Question

Atomic weights of three elements in a Doberenier triad are x, 81, and 127. Find the missing atomic weight.

• A. 104
• B. 35
• C. 46
• D. 23

Answer 1

The correct option is B

35

Dobereiner's Law of Triads

• When elements are arranged in the order of increasing atomic masses, groups of three elements (known as triads), having similar chemical properties are obtained. The atomic mass of the middle element of the triad is equal to the arithmetic mean of the atomic masses of the other two elements.
• For example - The elements lithium, sodium, and potassium have similar chemical properties and form a triad.

Atomic weight of an element 1	Atomic weight of an element 2	Atomic weight of an element 3
x	81	127

The atomic weight of an element 1 can be calculated as

$\frac{\mathrm{x}+127}{2}=81$

$\Rightarrow \mathrm{x}+127=2\left(81\right)=162$

$\Rightarrow \mathrm{x}=35$

Hence, the missing atomic weight ‘x’ is 35.