Sample elements arranged in periodic table formation, late 20th century.
© Science Museum / Science & Society Picture Library
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.