<Tr> <Td_colspan="6"> Metal </Td> <Td> Metalloid </Td> <Td_colspan="3"> Nonmetal </Td> <Td> Unknown chemical properties </Td> </Tr> <Tr> <Td> Alkali metal </Td> <Td> Alkaline earth metal </Td> <Td> Lanthanide </Td> <Td> Actinide </Td> <Td> Transition metal </Td> <Td> Post - ​ transition metal </Td> <Td> Polyatomic nonmetal </Td> <Td> Diatomic nonmetal </Td> <Td> Noble gas </Td> </Tr> <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>

What's the atomic number on the periodic table