Question

How should we know if a group of elements forms dobreniers triad

Answer 1

Dobereiner had found other groups of three elements (hence "triads") whose physical properties were similarly related.[2] He also noted that some quantifiable properties of elements (e.g. atomic weight and density) in a triad followed a trend whereby the value of the middle element in the triad would be exactly or nearly predicted by taking the arithmetic mean of values for that property of the other two elements.

Predicted vs actual atomic mass of the central atom of each triadTriad name[2]Elements and atomic masses[2][3]Element 1
MassElement 2
Mean of 1 and 3
Actual massElement 3
MassAlkali-forming elementsLithium
6.94Sodium
23.02
22.99Potassium
39.10Alkaline-earth-forming elements
[atomic masses verification needed]Calcium
40.1Strontium
88.7
87.6Barium
137.3Salt-forming elementsChlorine
35.470Bromine
80.470
78.383Iodine
126.470Acid-forming elementsSulfur
32.239Selenium
80.741
79.263Tellurium
129.243-
[atomic masses verification needed]Iron
55.8Cobalt