Explain the "Law of Triads" with examples.

(N/A) The German chemist,Johann Dobereiner,was the first to consider the idea of trends among the properties of elements.
He noted a similarity among the physical and chemical properties of groups of three elements,which he called "Triads".
"Law of Triads":
- "The atomic weight of the middle element of each of the Triads was approximately the arithmetic mean of the atomic weights of the other two elements."

Triad Number	Elements and Atomic Weights	Average of $1^{st}$ and $3^{rd}$ element
$(1)$	$Li(7), Na(23), K(39)$	$(7+39) / 2 = 23$
$(2)$	$Ca(40), Sr(88), Ba(137)$	$(40+137) / 2 = 88.5$
$(3)$	$Cl(35.5), Br(80), I(127)$	$(35.5+127) / 2 = 81.25$

Conclusion: The atomic weight of the middle element is approximately equal to the average of the atomic weights of the first and third elements.