Russian chemist who arranged the 63 known elements (A substance composed of a single type of atom (e.g., iron)) into a periodic table based on atomic mass, which he published in Principles of Chemistry in 1869. This organization surpassed attempts at classification by Beguyer de Chancourtois and Newlands and was published a year before the work of Lothar Meyer. Mendeléev left space for new elements, and predicted three yet-to-be-discovered elements including eka-silicon and eka-boron. His table did not include any of the noble gases, however, which had not yet been discovered. His table placed elements in their correct position by atomic number, thus showing variance from atomic weight in a number of places. The periodic table displays all chemical elements systematically in order of increasing atomic number, i.e., the number of protons in the nucleus. In 1869, Russian chemist Dmitri Mendeleev first described an arrangement of the chemical elements now known as the periodic table. Mendeleev wrote, "The elements, if arranged according to their atomic weights, show a distinct periodicity of their properties.... Elements exhibiting similarities in their chemical behavior have atomic weights which are approximately equal (as in the case of Pt, Ir, Os) or they possess atomic weights which increase in a uniform manner (as in the case of K, Rb, Cs)." Isotopes of an element have the same number of protons but a different number of neutrons. They are chemically very similar but have different atomic masses.