Chart that reflects the periodic recurrence of chemical and physical
properties of the Elements when the elements are arranged in order of increasing Atomic Number. The periodic table was devised by Dmitri Mendeleev and revised by Moseley, Henry. It is divided into vertical columns, or groups, numbered from I to VIII,
with a final column numbered 0. Each group is divided into two categories, or
families, one called the a series (the representative, or main group, elements) the other the b series (the Transition Elements, or subgroup elements). All the elements in a
group have the same number of Valence Electrons and have similar chemical properties. The horizontal rows of the table are
called periods. The elements of a particular period have the same number of
electron shells; the number of electrons in these shells, which equals the
element's atomic number, increases from left to right within each period. In each
period the lighter Metals appear on the left, the heavier metals in the center, and the nonmetals on
the right. Elements on the borderline between metals and nonmetals are called
metalloids. Elements in group Ia are called the Alkali Metals; in group IIa, the Alkaline-Earth Metals; in group VIIa, the Halogens; and
in group 0, the Inert Gases (see States Of Matter).