Periodic Table of Elements

The periodic table of elements is a systematic arrangement of all chemical elements. An element is a substance that cannot be broken down into simpler chemical components, and all its atoms have the same number of protons. Elements in the table are ordered by atomic number (the number of protons in the nucleus of an atom), beginning with hydrogen, which has an atomic number of 1.

The structure of the table is not merely a convenient design for viewing. The rows (periods) and columns (groups) show how electrons are distributed in orbital shells around the nucleus. Elements in the same column typically have the same number of electrons in their outermost shell, which causes them to exhibit similar chemical behavior.

History

The periodic table as known today is credited to the Russian scientist Dmitri Mendeleev, who in the 19th century presented an arrangement based on atomic mass and recurring patterns of chemical properties. Mendeleev left gaps for elements not yet discovered and predicted their properties. Later, following the discovery of atomic structure and protons, the table was revised so that atomic number, not atomic mass, determines the ordering.

Categories of the table

The table is divided into major categories: metals, which conduct heat and electricity; non-metals; and metalloids, which display properties of both. There are also named groups, such as alkali metals, alkaline earth metals, halogens, and noble gases (which do not readily combine with other elements). The two lower rows, the lanthanides and actinides, are often displayed separately to reduce the width of the table.

Number of elements

Currently 118 elements are recognized, a figure reflected in the designation "118 elements." Those with higher atomic numbers are synthesized in laboratories, are highly unstable, and decay within minutes or fractions of a second. New elements and their names are verified by the International Union of Pure and Applied Chemistry, IUPAC.