All versions of the periodic table include only chemical elements, not mixtures, compounds, or subatomic particles.[n 1] Each chemical element has a unique atomic number representing the number of protons in its nucleus. 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. 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 (the number of protons in the nucleus of an atom). A new row (period) is started when a new electron shell has its first electron. Columns (groups) are determined by the electron configuration of the atom; elements with the same number of electrons in a particular subshell fall into the same columns (e.g. oxygen and selenium are in the same column because they both have four electrons in the outermost p-subshell). Elements with similar chemical properties generally fall into the same group in the periodic table, although in the f-block, and to some respect in the d-block, the elements in the same period tend to have similar properties, as well. Thus, it is relatively easy to predict the chemical properties of an element if one knows the properties of the elements around it.
As of 2012, the periodic table contains 118 confirmed chemical elements. Of these elements, 114 have been officially recognized and named by the International Union of Pure and Applied Chemistry (IUPAC). A total of 98 of these occur naturally, of which 84 are primordial. The other 14 natural elements only occur in decay chains of primordial elements All elements from einsteinium to copernicium, as well as flerovium and livermorium, while not occurring naturally in the universe, have been duly synthesized and officially recognized by the IUPAC. Elements 113, 115, 117 and 118 have reportedly been synthesized in laboratories but these reports have not yet been confirmed. As such these elements are currently known only by their systematic element names, based on their atomic numbers. No element heavier than einsteinium (element 99) has ever been observed in macroscopic quantities in its pure form. No elements past 118 have been synthesized as of 2012.