In 1829 Johann Dobereiner discovered the halogen triad and the alkali metal triad. The halogen triad was made up of elements chlorine, bromine, and iodine. The alkali metal triad was made up of elements lithium, sodium and potassium. After discovering these triads, Dobereiner proposed that in nature there are triads of elements and the middle element contains properties that are an average of the other two members when ordered by the atomic weight. This is now known as the Law of Triads.