<P> Each chemical element has a unique atomic number (Z) 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 . </P> <P> 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 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 . </P> <P> As of 2016, the periodic table has 118 confirmed elements, from element 1 (hydrogen) to 118 (oganesson). Elements 113, 115, 117 and 118, the most recent discoveries, were officially confirmed by the International Union of Pure and Applied Chemistry (IUPAC) in December 2015 . Their proposed names, nihonium (Nh), moscovium (Mc), tennessine (Ts) and oganesson (Og) respectively, were announced by the IUPAC in June 2016 and made official in November 2016 . </P> <P> The first 94 elements occur naturally; the remaining 24, americium to oganesson (95--118), occur only when synthesized in laboratories . Of the 94 naturally occurring elements, 83 are primordial and 11 occur only in decay chains of primordial elements . No element heavier than einsteinium (element 99) has ever been observed in macroscopic quantities in its pure form, nor has astatine (element 85); francium (element 87) has been only photographed in the form of light emitted from microscopic quantities (300,000 atoms). </P>

How many are the elements of the periodic table