Question

What are the characteristics of Dobereiner's triads?

Answer 1

• German chemist Johann Dobereiner classified elements having similar chemical properties into groups of three. These groups were called triads.
• He proposed a law known as Dobereiner's law of triads.
• According to this law, when elements are arranged in the order of increasing atomic mass in a triad, the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the atomic masses of the other two elements.

Examples of Dobereiner’s triad are Lithium(Li), Sodium(Na), and Potassium(K) with atomic masses of 7, 23, and 39 respectively.

• The atomic mass of sodium(Na) is the arithmetic mean of the mass of lithium(Li) and potassium(K).
• $$\begin{gathered} \mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{sodium}\left(\mathrm{Na}\right)=\frac{\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Lithium}\left(\mathrm{Li}\right)+\mathrm{Atomic}\mathrm{mass}\mathrm{of}\mathrm{Potassium}\left(\mathrm{K}\right)}{2} \\ =\frac{7+39}{2} \\ =\frac{46}{2}=23 \end{gathered}$$

Hence, the mass of sodium(Na) is equal to the arithmetic mean of the atomic masses of lithium(Li) and potassium(K).