Sample elements arranged in periodic table formation, late 20th century.

The periodic table displays all the elements, arranged in order of their atomic number, summarising their major properties. The vertical columns (groups) are numbered I-VII and 0 to reflect the number of electrons in the outermost unfilled shell. A gradation of properties may be traced along the horizontal rows (periods). The first periodic table was devised by Rusian chemist Dmitry Mendeleyev (1834-1907) in 1869.