1. Johann Dobereiner• 1817-1829 Dobereiner began grouping elements with similar properties into groups of three also known as triads.• Law of Triads: Began in 1817 when Dobereiner noticed that the atomic weight of strontium, Sr, was halfway between the of calcium and barium. These elements also had similar chemical properties.• In 1829 Dobereiner had discovered the triad of halogens (chlorine, bromine, and iodine) and a triad of alkali metals (lithium, sodium, and potassium). He proposed that nature contained triads of elements in which the central element had properties that were an average of the other two elements. This occurred when the elements were ordered in terms of atomic weight. • Later, other scientists found other triads and recognized that elements could be grouped into larger sets (more than three). However, the poor accuracy of atomic weight measurements has made it difficult to group these elements.2. John Newlands• Law of Octaves – 1863 Newlands proposed the Law of Octaves, which stated that elements repeat their chemical properties every ...