Mike Brock polishes the entry tiles that are laid out in the pattern of the periodic table of the elements at the National Museum of Nuclear Science & History. Proclaimed the “International Year of the Periodic Table” by the United Nations General Assembly and UNESCO, 2019 marks the 150th anniversary of Dimitri Mendeleev’s publication of a periodic table that arranged the elements by atomic mass.

National Periodic Table Day is observed annually on Feb. 7 to commemorate John Newlands’ 1863 publication of a table that grouped the 56 known elements following his Law of Octaves, which asserted that when the elements were arranged by increasing atomic weight, those with similar characteristics would occur after every seventh element.

Of the table’s 118 elements, 22 have been discovered by Department of Energy scientists.