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
Periodic table form (as shown at various other points in this article), is attributable to Horace Groves Deming. In 1923, Deming, an American chemist, published short (Mendeleev style) and medium (18-column) form periodic tables. Merck and Company prepared a handout form of Deming's 18-column medium table, in 1928, which was widely circulated in American schools. By the 1930s Deming's table was appearing in handbooks and encyclopaedias of chemistry. It was also distributed for many years by the Sargent-Welch Scientific Company. With the development of modern quantum mechanical theories of electron configurations within atoms, it became apparent that each period (row) in the