Early Periodic Table

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In the period around 1870, Dmitri Mendeleev, following and greatly extending the work of earlier chemists, arranged the chemical elements, whose composition was entirely unknown at the time, essentially in order of their increasing atomic mass. He noted that certain periodic properties occurred, which suggested that the elements not only should be listed, but also arranged in rows with elements having similar chemical and physical properties above or below one another. In doing so, he found gaps and misalignment, which suggested missing elements. Mendeleev famously predicted not only the existence of the elements which we now call "gallium," "germanium," and "technetium," but quite accurately predicted many of their chemical and physical properties. (Unit: 6)