Periodic Table of Elements

The Russian chemist Dmitry Mendeleyev proposed the periodic table of elements, which grouped elements in rows and columns by shared chemical properties. The elements were initially arranged by atomic weight. Many elements were unknown in the middle of the 19th century when Mendeleyev made the configuration; subsequent discoveries have led to the completion of the table, which is now arranged in terms of an element’s atomic number (the number of protons it contains).