The scientist who discovered the arrangement of the periodic table is Dmitri Mendeleev. He produced one of the first practical periodic tables in the 19th century by arranging elements in order of increasing atomic mass, organizing them into horizontal periods and vertical groups. Mendeleev left gaps for undiscovered elements and successfully predicted their properties, which made his periodic table widely accepted as the foundation of the modern periodic table. Although the modern table arranges elements by atomic number, Mendeleev's work remains the basis of the periodic law and the periodic table we use today.