a Group 3 has scandium (Sc) and yttrium (Y). For the rest of the group, sources differ as either being (1) lutetium (Lu) and lawrencium (Lr),or(2) lanthanum (La) and actinium (Ac),or(3) the whole set of 15+15 lanthanides and actinides. IUPAC has initiated a project to standardize the definition as either (1) Sc, Y, Lu and Lr, or (2) Sc, Y, La and Ac.[1] b Group 18, the noble gases, were not discovered at the time of Mendeleev's original table. Later (1902), Mendeleev accepted the evidence for their existence, and they could be placed in a new "group 0", consistently and without breaking the periodic table principle. r Group name as recommended by IUPAC. h Hydrogen (H), while placed in group 1, is not considered to be part of the alkali metals.
Column counts are columns only that contain elements. "Compact cells" denote cells with element symbol only, have navbox style and the table fits basic wiki page width (1024×768). "Detailed cells" have more data, and may cause page width excess (scroll bar added). "Extended" periodic tables are about theoretical periods 8 and higher (element 119 and higher).