b The noble gases were not discovered at the time of Mendeleev's original table. Later (1902), Mendeleev accepted the evidence for their existence, and placed them in a separate "group 0".
c Hydrogen (H), while placed in column 1, is not considered to be in the group alkali metals.
d Group 3 begins with scandium (Sc) and yttrium (Y) but different sources define the rest of the group as either (1) lutetium (Lu) and lawrencium (Lr), or (2) lanthanum (La) and actinium (Ac), or (3) the whole set of 15 لانتانیدهاs and 15 آکتینیدهاs. IUPAC has initiated a project to standardize the definition as either (1) Sc, Y, Lu and Lr, or (2) Sc, Y, La and Ac.[۱]