Period (periodic table) across periods. Seven periods of elements occur naturally on Earth. For period 8, which includes elements which may be synthesized after 2016, see the extended periodic table. A group in chemistry means a family of objects with similarities like different families. There are 7 periods, going horizontally across the periodic table. The first period contains the least elements than any other, with only two, hydrogen and helium. They therefore do not follow the octet rule. Chemically, helium behaves like a noble gas, and thus is taken to be part of the group 18 elements. However, in terms of its nuclear
Extended periodic table periodic table would quite possibly end around "Z" = 130. Beyond element 164, the fissility line defining the limit of stability with respect to spontaneous fission may converge with the neutron drip line, posing a limit to the existence of heavier elements. Nevertheless, further magic numbers have been predicted at "Z" = 210, 274, and 354 and "N" = 308, 406, 524, 644, and 772, with two beta-stable doubly magic nuclei found at 210 and 274; the same calculation method reproduced the predictions for Fl and 164. Although additional stability toward alpha decay and fission are predicted for these nuclei,