Question

P, Q, and R are the three elements in a Dobereiner's triad. If the atomic weight of P is 35.5 and R is 127 then calculate the atomic weight of Q.

Answer 1

• Dobereiner’s triads were groups of elements with similar properties that were identified by the German chemist Johann Wolfgang Dobereiner.
• He observed that groups of three elements (triads) could be formed in which all the elements shared similar physical and chemical properties.
• 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.

Here,

Since the atomic mass of a P: 35.5 and the atomic mass of an R: 127

Then the atomic mass of Q will be: $\frac{35.5+127}{2}$

= $\frac{162.5}{2}$

= $81.25$

So, the atomic no. of P is 81.5