In Dobereiner’s triad, elements showed similarities in properties.
Law of Triads
In 1817, Johann Dobereiner noticed that Strontium had similar chemical properties to Calcium and Barium and that its atomic weight fell midway. More work in this area led to discovering the halogen triad of Chlorine, Bromine and Iodine, and the alkali metal triad of Lithium, Sodium and Potassium. In 1829, he proposed the Law Of Triads.
The Law Of Triads states that,
Nature contains triads of elements where the middle element has properties that are an average of the other two triad members when ordered by the atomic weight.