The periodic table is an arrangement of the chemical elements in order of increasing atomic number, structured to display recurring similarities in their properties. Its rows and columns connect an element’s position with its atomic structure and chemical behavior. A central framework of chemistry, the table provides both a classification system and a basis for predicting properties. The recognized elements extend from hydrogen, atomic number 1, to oganesson, atomic number 118, completing seven periods. (periodic-table.rsc.org)
Organizing principle
Atomic number, conventionally written Z, is the number of protons in the nucleus of an atom. It uniquely identifies an element. A neutral atom has an equal number of protons and electrons; gaining or losing electrons produces an ion without changing the element’s identity. Atoms of the same element may have different numbers of neutrons: these are its isotopes, which occupy the same position in the table. (openstax.org)
The modern periodic law states that elemental properties recur systematically with increasing atomic number. This differs from the nineteenth-century formulation, which emphasized atomic weight. Atomic-number order resolves cases in which mass order conflicts with chemical similarities, notably tellurium and iodine. Periodicity does not mean exact repetition: members of a column resemble one another while also displaying systematic differences. (periodic-table.rsc.org)
Rows, columns, and blocks
Horizontal rows are called periods, and vertical columns are called groups. The conventional long-form table has 18 groups. Elements in the same group often have related outer-electron arrangements and consequently similar chemical properties. The main-group elements occupy groups 1, 2, and 13–18; the central d-block occupies groups 3–12. (openstax.org)
Several groups have established family names. Group 1, excluding hydrogen, contains the alkali metals; group 2 contains the alkaline-earth metals. Group 17 contains the halogens, and group 18 contains the noble gases. Metals dominate the left and center, whereas nonmetals occur mainly toward the upper right. Metalloids occupy parts of the boundary between these regions. Hydrogen is conventionally placed above group 1, although it is a nonmetal with distinctive chemistry. (openstax.org)
The table is also divided into s-, p-, d-, and f-blocks, reflecting the subshells associated with its electronic organization. Their widths—2, 6, 10, and 14 positions—correspond to subshell capacities. The lanthanide and actinide series are usually displayed below the main table to save space, but belong within periods 6 and 7. Helium illustrates the difference between electronic and chemical classification: its configuration is 1s², yet it is placed with the noble gases in group 18. (openstax.org)
Electronic explanation
Quantum mechanics explains periodicity through the arrangement of electrons in shells and subshells. An electron configuration describes the occupation of atomic orbitals. The Pauli exclusion principle limits each orbital to two electrons with opposite spin projections, constraining how electronic structures develop as atomic number increases. (openstax.org)
Chemical similarities arise especially from valence electrons, which participate in chemical bonding and chemical reactions. Lithium and sodium, for example, each have one outer s electron, helping explain their related behavior. Transition and inner-transition elements have more complicated patterns because d and f electrons can participate in chemistry. Simple orbital-filling rules are useful approximations, but some observed configurations, including those of chromium and copper, depart from their straightforward predictions. (openstax.org)
Periodic trends
Several properties change systematically across the table. Atomic radius generally decreases from left to right across a period and increases down a group. Across a period, increasing nuclear attraction tends to draw electrons inward; down a group, occupied outer shells extend farther from the nucleus. Because atoms lack sharp boundaries, numerical radii depend on the definition and measurement used. (openstax.org)
First ionization energy, the energy needed to remove an electron from an isolated gaseous atom, generally increases across a period and decreases down a group. Electronegativity, an atom’s tendency to attract electrons in a bond, broadly follows the opposite direction to atomic size. Metallic character generally increases toward the lower left. These are trends rather than exceptionless laws; subshell structure and electron pairing produce local irregularities. (openstax.org)
Historical development
Early classification schemes included Johann Wolfgang Döbereiner’s triads of chemically similar elements and John Newlands’s recurring “octaves.” Lothar Meyer developed tables connecting atomic weight and chemical properties during the 1860s. In 1869, Dmitri Mendeleev published a periodic arrangement that preserved chemical relationships, left gaps for undiscovered elements, and supported predictions of their properties. Subsequent discoveries, including gallium, provided important tests of those predictions. (periodic-table.rsc.org)
In 1913, Henry Moseley’s measurements of characteristic X-rays established a physical basis for atomic-number ordering. The later understanding of nuclear charge and electronic structure explained why this ordering, rather than atomic weight alone, governs the periodic system. (rsc.org)
Information displayed in element entries
An element’s box typically gives its symbol, atomic number, name, and an atomic-weight value. Atomic weights reflect isotopic composition, rather than simply counting nuclear particles. Where natural isotopic abundances vary, standard atomic weights may be expressed as intervals. For elements lacking characteristic isotopic abundances in terrestrial samples, IUPAC tables instead show a selected isotope’s mass number in square brackets; this integer is not a standard atomic weight. (openstax.org)
The International Union of Pure and Applied Chemistry coordinates element naming and maintains standardized table information. On November 28, 2016, it approved the names nihonium, moscovium, tennessine, and oganesson for elements 113, 115, 117, and 118, respectively. (iupac.org)