a Group 1 is composed of hydrogen (H) and the alkali metals. Elements of the group have one s-electron in the outer electron shell. Hydrogen is not considered to be an alkali metal as it is not a metal, though it is more analogous to them than any other group. This makes the group somewhat exceptional. n/a Do not have a group number 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.
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).