CBSE Class 10 Answered
No its the traids.
Between 1817-1829, Johann Dobereiner began to group elements with simi liar properties in to groups of three or triads. This began in 1817 when he noticed that the atomic weights of strontium, Sr, was halfway between the weights of calcium and barium. These elements possessed similar chemical properties. By 1829, he had discovered the a halogen triad made up of chlorine, bromine, and iodine and a alkali metal triad of lithium, sodium and potassium. He postulated that nature contained triads of elements in which the middle element had properties that were an average of the other two elements. Later, other scientists found other triads and recognized that elements could be grouped into set large than three. The poor accuracy of measurements such as that of atomic weights hindered grouping more elements.