<P> The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties, whose structure shows periodic trends . Generally, within one row (period) the elements are metals to the left, and non-metals to the right, with the elements having similar chemical behaviours placed in the same column . Table rows are commonly called periods and columns are called groups . Six groups have accepted names as well as assigned numbers: for example, group 17 elements are the halogens; and group 18 are the noble gases . Also displayed are four simple rectangular areas or blocks associated with the filling of different atomic orbitals . </P> <P> The organization of the periodic table can be used to derive relationships between the various element properties, but also the predicted chemical properties and behaviours of undiscovered or newly synthesized elements . 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 . Most of his forecasts proved to be correct . 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 . The modern periodic table now provides a useful framework for analyzing chemical reactions, and continues to be widely used in chemistry, nuclear physics and other sciences . </P> <P> All the elements from atomic numbers 1 (hydrogen) through 118 (oganesson) have been either discovered or synthesized, completing the first seven rows of the periodic table . The first 98 elements exist in nature, although some are found only in trace amounts and others were synthesized in laboratories before being found in nature . Elements 99 to 118 have only been synthesized in laboratories or nuclear reactors . The synthesis of elements having higher atomic numbers is currently being pursued: these elements would begin an eighth row, and theoretical work has been done to suggest possible candidates for this extension . Numerous synthetic radionuclides of naturally occurring elements have also been produced in laboratories . </P> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul>

How many elements in the periodic table 2018