What is the Law of Triads?

The Law of Triads was an attempt to find similar physical and chemical behaviour among elements. It was a forerunner of Dmitiri Mendeleev's Periodic Table.

There were 53 known elements when German chemist Johann Wolfgang Dobereiner (1780-1849) started noticing that for certain groups of three elements (the "triads"), atomic weight and density followed a pattern where the middle element of the triad had predictable properties based on the first and third elements. This was called his Law of Triads.

https://en.wikipedia.org/wiki/D%C3%B6bereiner%27s_triads