1. Johann Dobereiner• 1817-1829 Dobereiner began grouping elements with similar properties into groups of three, also called triads.• Law of Triads - Started in 1817 when Dobereiner noticed that the atomic weight of strontium, Sr, was at halfway between the weights of calcium and barium. These elements also had similar chemical properties.• In 1829, Dobereiner had discovered the halogen triad (chlorine, bromine, and iodine) and an alkali metal triad (lithium, sodium, and potassium). He proposed that nature contained triads of elements where the middle element had properties that were an average of those of the other two elements. This occurred when elements were classified in terms of atomic weight.• Later, other scientists discovered other triads and recognized that elements could be grouped into larger sets (more than three). However, the low precision of atomic weight measurements made it difficult to group these elements together.2. John Newlands• Law of Octaves – 1863 Newlands proposed the Law of Octaves, which stated that elements repeated their chemical properties each time. ...