periodic table of the elements

A table in which the elements are arranged in order of their atomic number. There are eight groups of elements, plus a block of transition metals in the centre. Group 1 contains the alkali metals; Group 2 the alkaline-earth metals; Group 7 the halogens; and Group 0 the noble gases (rare gases). A zigzag line through the groups separates the metals on the left-hand side of the table from the non-metals on the right. The horizontal rows in the table are called periods. The table summarizes the major properties of the elements and enables predictions to be made about their behaviour.

There are striking similarities in the chemical properties of the elements in each of the groups (vertical columns), which are numbered 1-7 (I-VII) and 0 to reflect the number of electrons in the outermost unfilled shell and hence the maximum valency. Reactivity increases down a group. A gradation (trend) of properties may be traced along the periods. Metallic character increases across a period from right to left, and down a group. A large block of elements, between Groups 2 and 3, contains the transition elements, characterized by displaying more than one valence state.

The properties of elements and the trends within the periodic table are a direct consequence of the electronic structure of the atoms. The relationships established between the positions of elements in the periodic table and their major properties has enabled scientists to predict the properties of other elements - for example, technetium, atomic number 43, first synthesized in 1937.

The current arrangement of the periodic table by atomic numbers was devised by English physicist Henry Moseley in 1913-14; the original tables were set up by atomic weights (masses), first proposed by English chemist John Newlands in 1863 and expanded on by Russian chemist Dmitri Mendeleyev and German chemist Lothar Meyer in 1869.