Question

Chlorine, Y and iodine form a Dobereiner's triad. Identify the atomic weight of Y.
(A)162.5
(B)81.25
(C)121.5
(D)90.5

Answer 1

In the year 1829 a German scientist named Johann dobereiner placed various elements into groups and called them triads. To solve this question we should have prior knowledge about the Dobereiner's law of triads.

Complete answer:
Basically dobereiner observe that certain elements having similar chemical properties can be grouped together. When these particular similar elements were arranged in the increasing order of their atomic masses then they generally occur in the group of three which means that the atomic mass of the middle element is equal to the arithmetic mean of the other two elements. The group of the three elements were called triads.
Therefore this law states that when the elements are placed in the order of their increasing atomic mass then the group of three elements having similar properties are obtained known as triads and the atomic mass of the middle element is equal to the arithmetic mean of the other two elements.
According to the question:
We have to find the atomic mass of unknown element Y
The atomic mass of chlorine = 35.5
The atomic mass of iodine = 127
The atomic mass of the middle element y is equal to the arithmetic mean of the other two elements i.e. chlorine and iodine
The atomic mass of Y= (35.5+127)/2=81.25

Hence the correct answer is option (B)

Note:
There were some drawbacks of this law, all the elements could not be arranged in the form of triads. This law is only valid for some elements. This law does not hold good for the elements having very low or very high atomic masses.