Periodic table three naturally occurring isotopes: all of its atoms have six protons and most have six neutrons as well, but about one per cent have seven neutrons, and a very small fraction have eight neutrons. Isotopes are never separated in the periodic table; they are always grouped together under a single element. Elements with no stable isotopes have the atomic masses of their most stable isotopes, where such masses are shown, listed in parentheses. In the standard periodic table, the elements are listed in order of increasing atomic number "Z" (the number of protons in the nucleus of an atom). A
Alternative periodic tables each element has with respect to another element in the table. Other tables aim to emphasize the chemical element isolations by humans over time. Charles Janet's Left step periodic table (1928) is considered to be the most significant alternative to the traditional depiction of the periodic system. It organizes elements according to orbital filling (instead of valence) and is widely used by physicists. Compared to the common layout, the Left step table has these changes: A modern version of the periodic table is constructed by Valery Tsimmerman, named the ADOMAH periodic table (2006). Its structure is based on the four