The periodic table is a tabular display of the chemical elements, organized on a basis of their properties. Elements are presented in increasing atomic number; while rectangular in general outline, gaps are included in the rows or periods to keep elements with similar properties together, such as the halogens and the noble gases, in columns or groups, forming distinct rectangular areas or blocks. Because the periodic table accurately predicts the properties of various elements and the relations between properties, its use is widespread within chemistry, providing a useful framework for analysing chemical behavior, as well as in other sciences.
Although precursors exist, the current table is generally credited to Dmitri Mendeleev, who developed it in 1869 to illustrate periodic trends in the properties of the then-known elements; the layout has been refined and extended as new elements have been discovered and new theoretical models developed to explain chemical behavior. Mendeleev’s presentation also predicted some properties of then-unknown elements expected to fill gaps in his arrangement; these predictions were proved right when those elements were discovered and found to have properties close to the predictions.
All elements from atomic numbers 1 (hydrogen) to 118 (ununoctium) have been isolated. Of these, all up to californium exist naturally; the rest have only been artificially synthesised in laboratories, along with numerous synthetic radionuclides of naturally occurring elements. Production of elements beyond ununoctium is being pursued, with the question of how the periodic table may need to be modified to accommodate these elements being a matter of ongoing debate.