In the early 1800's the German chemist J.W Doberiner proposed that some elements could be classified into sets of 3 called triads. He was the first to recognize the relation between atomic weights and chemical properties. The fact that he saw this relation was very remarkable because at the else ever did. Dobereiner observed that, in a set of three elements, whoseof the chemical properties were similar, the atomic weight of the second member of the "triad" was about the average atomic weight of the first and third element. Due to his thorough observation we were able to conclude that there is some type of numeric law overriding chemical behavior. Dobereiner published an explanation for triads some time in the 1800's in, " An Attempt to Group Elementary Substances According to Their Analogies."(Davis 132) .
This chemist explain that when these elements were arranged in increasing order of their atomic mass, they genarally occured in groups of three. In addition to that helpful discovery, he observed that the atomic mass of the middle element of the triad was the average atomic mass of the other two elements. The elements of a triad are similar in different way. They happen to share both physical and mental properties. Some triads were lithium, sodium, potassium; calcium, strontium, barium; and clorine, bromine, iodine. That explains his law of triads which states: When elements are placed in order of the accending order of atomic masses, groups of three elements having similar properties are obtained. For example, lithium, sodium, and potassium have the atomic masses of about 7, 23, 39. .
Those elements are also all, part of the alkali metal group, which means that they are all highly reactive and have I valence electron.
This example proves Dobereiner's theory that the 2nd element was the average of the other 2, meaning 7+39/ 2= 23 .(Davis 134) Although you might think his law was flawless it was hard to believe because of the limited amount of elements discovered at the time.