Periodic Table.

The periodic table is a chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number. There are 18 columns, or groups, in the standard periodic table. The periodic table uses Roman numerals followed by an A or B. This method labeled columns 1 and 2 as IA and IIA, …

…in the center, and the nonmetals on the right. Elements on the borderline between metals and nonmetals are called metalloids. Group Ia (with one valence electron) and group IIa (with two valence electrons) are called the alkali metals and the alkaline-earth metals. The nonmetals in group VIIa (with seven valence electrons) are called the halogens. The elements grouped in the final column have no valence electrons and are called the inert gases, or noble gases.