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 neutron drip lines and stability toward alpha decay and spontaneous fission. One calculation by Y. Gambhir et al., analyzing nuclear binding energy and stability in various decay channels, suggests a limit to the existence of bound nuclei at "Z" = 146. Some, such as Walter Greiner, predicted that there may not be an end to the periodic table. Other predictions of an end to the periodic table include "Z" = 128 (John Emsley) and "Z" = 155 (Albert Khazan). It is a "folk legend" among physicists that Richard Feynman suggested that neutral atoms could not exist for atomic numbers greater