Question

Three elements A,B and C with similar properties have atomic masses X,Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called? Give one example of such a set of elements. [2 MARKS]

Answer 1

Each point: 1 Mark

1. The arrangement of these elements is known as Dobereiner’s triad. Dobereiner showed that when the three elements in a triad were written in the order of increasing atomic masses, the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
2. Example: lithium, sodium and potassium form a Dobereiner triad.