The periodic table, also known as the periodic table of elements, is a tabular display of the chemical elements, which are arranged by atomic number, electron setup, and repeating chemical properties. The structure of the table shows occasional (popular things/general ways things are going). The seven rows of the table, called periods, generally have metals on the left and nonmetals on the right. The columns, called groups, contain elements with almost the same chemical behaviours. 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 connected with the filling of different atomic orbitals.
The elements from atomic numbers 1 (hydrogen) to 118 (oganesson) have all been discovered or synthesized, completing seven full rows of the periodic table. The first 94 elements, hydrogen to plutonium, all happen naturally, though some are found only in trace amounts and a few were discovered in nature only after having first been created. Elements 95 to 118 have only been created in laboratories, nuclear reactors, or nuclear explosions. The synthesis of elements having higher atomic numbers is now being chased after: these elements would begin an eighth row, and theoretical work has been done to suggest possible candidates for this extension. Many synthetic radioisotopes of naturally happening elements have also been produced in laboratories.
The organization of the periodic table can be used to get relationships between the different element properties, and also to predict chemical properties and behaviours of undiscovered or newly created elements. Russian chemist Dmitri Mendeleev published the first recognizable periodic table in 1869, developed mainly to illustrate occasional 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 soon proved to be correct, ending with the discovery of gallium and germanium in 1875 and 1886, which backed up his predictions. Mendeleev’s idea has been slowly expanded and fine tuned with the discovery of further new elements and the development of new theoretical models to explain chemical behaviour. The modern periodic table now provides a useful base for carefully studying chemical reactions, and continues to be widely used in chemistry, nuclear physics and other sciences. Some ongoing discussion remains about the placement and categorisation of particular elements, the future extension and limits of the table, and whether there is the best form of the table.