Answer: In a triad of elements with similar properties, the atomic mass of the middle element is about the average of the atomic masses of the other two.
Dobereiner arranged elements with similar chemical properties in groups of three, called triads. According to his law of triads, the atomic mass of the middle element is nearly the mean of the atomic masses of the first and the third elements.
Example: the triad Li (7), Na (23), K (39). Average of 7 and 39 is \(\dfrac{7+39}{2} = 23\), which is the atomic mass of sodium.