The periodic table is a chart of the chemical elements, ordered by atomic number and electron configuration. The elements in each group (column) often have similar chemical properties. The table also shows four rectangular blocks with some similarities in physical and chemical properties. Six groups have generally accepted names, including the halogens of group 17 and the noble gases of group 18. The table provides a framework for analyzing chemical behaviour, and is extensively used in chemistry and other sciences. The Russian chemist Dmitri Mendeleev published the first widely recognized periodic table in 1869, and correctly predicted some properties of then-unknown elements that would be expected to fill in the gaps. Mendeleev's periodic table has been expanded and refined over time; elements 1–94 have all been found to occur naturally, and elements 95–118 have been synthesized in nuclear reactors or laboratories.
