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Based on his findings, Dobereiner proposed the Law of Triads in 1829. The pattern seemed too remarkable to be a coincidence. In each group of three, the atomic weight of one element fell halfway between the atomic weights of the other two elements. In 1817 Dobereiner noticed that certain elements that were chemically similar could be grouped together in threes, for example, calcium, strontium, and barium lithium, sodium, and potassium chlorine, bromine, and iodine. One of the earliest attempts to organize the elements based on their chemical and physical properties was made by German chemist Johann Dobereiner. This arrangement developed side by side with atomic theory over about 200 years, and it continues to evolve as new elements are discovered. The elements are also organized in vertical columns, or groups, based on similar physical characteristics and chemical behavior. Today, the periodic table organizes the elements in horizontal rows, or periods, by order of increasing atomic number, which equals the number of protons in the atomic nucleus of each element. Only 12 elements were known prior to 1700, but as more and more elements were discovered-by 1900 there were more than 80-scientists tried to find a way to organize them systematically, according to their physical and chemical properties. Although some elements, such as gold and iron, have been known to humans since prehistoric times, it wasn't until the 17th century that the first scientific discovery of an element (phosphorus) was made.