Group (periodic table)
Group names
v t e Groups in the Periodic table 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 rarely exhibits behaviour comparable to theirs, 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 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. c 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. Some other names have been proposed and used without gaining wide acceptance: "volatile metals" for group 12; "icosagens" for group 13; "crystallogens", "adamantogens", and "merylides"[citation needed] for group 14; and "aerogens" for group 18.
Group (periodic table)-5