Question

Answer the following (any three):
State Dobereiner's law of triads. Give one example of it.

Hint:

Dobereiner’s law of triads was an early attempt to classify elements based on their atomic masses. Although limited in scope, it laid the foundation for future periodic table developments.

Answer 1

Step 1: Understanding Dobereiner’s law of triads
In 1829, Johann Wolfgang Dobereiner proposed the law of triads as an early attempt to classify elements. He observed that certain elements with similar chemical properties could be grouped into sets of three, called triads. The key pattern in a triad is: \[ \text{Atomic mass of middle element} \approx \frac{\text{Atomic mass of first element} + \text{Atomic mass of third element}}{2} \]
Step 2: Example of a Dobereiner triad
One of the well-known triads is the alkali metal triad: \[ \begin{array}{|c|c|} \hline Element & Atomic Mass (u) \\ \hline Lithium (Li) & 7 \\ Sodium (Na) & 23 \\ Potassium (K) & 39 \\ \hline \end{array} \] Here, the atomic mass of sodium (Na) is approximately the average of lithium (Li) and potassium (K): \[ \frac{7 + 39}{2} = 23 \] This follows Dobereiner’s rule. Thus, Dobereiner’s triads provided early evidence of periodic relationships between elements.