<P> The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties, whose adopted structure shows periodic trends . Generally, within one row (period) the elements are metals on the left, and non-metals on the right, with the elements having similar chemical behaviours being 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 halogens; and group 18 are noble gases . Also displayed are four simple rectangular areas or blocks associated with the filling of different atomic orbitals . </P> <P> Importantly, the organization of the periodic table can be utilized to derive relationships between various element properties, but also predicted chemical properties and behaviours of undiscovered or newly synthesized elements . Russian chemist Dmitri Mendeleev was 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 this 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 by developing 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 adopted in chemistry, nuclear physics and other sciences . </P> <P> All elements ranging from atomic numbers 1 (hydrogen) to 118 (oganesson) have been either discovered or synthesized, completing the first seven rows of the periodic table . The first 98 elements exist naturally, although some are found only in trace amounts and were synthesized in laboratories before being found in nature . Atomic numbers for 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 appearances 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>

What is the 1st element in the periodic table