The modern periodic table of chemical elements was discovered by Dmitry Mendeleev, a Russian scientist in the 19th century. He arranged the 63 known elements in a tabular form at that time, placing those with similar chemical properties in the same vertical column, which became the prototype of the periodic table. This table was revised many times over the years before it evolved into the contemporary periodic table. In the periodic table, elements are arranged according to their atomic numbers, with the smallest ones ranked first. A horizontal row in the table is called a period, while a vertical column is called a group.