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.
Numerous synthetic radionuclides of naturally occurring elements have also been produced in laboratories.
Most elements have differing numbers of neutrons among different atoms, with these variants being referred to as isotopes.
For example, carbon has 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.
Russian chemist Dmitri Mendeleev was the first to publish a recognizable periodic table in 1869, developed mainly to illustrate periodic trends of the then-known elements.
He also predicted some properties of unidentified elements that were expected to fill gaps within the table. Mendeleev's idea has been slowly expanded and refined with the discovery or synthesis of further new elements and the development of new theoretical models to explain chemical behaviour.