Periodic table

The periodic table arranges elements in order of atomic number, which is the number of protons in an atom’s nucleus. In the widely used long form, the horizontal rows are called periods and the vertical columns groups. The table contains 118 named elements, from hydrogen (atomic number 1) to oganesson (118).

The periodic law describes the recurrence of chemical and physical properties as atomic number increases. Elements in the same group often have related chemical behavior, although the pattern is not an assertion that every member behaves identically. The two rows usually printed below the main body contain the lanthanoids and actinoids; they belong to periods 6 and 7 and are placed there to make the chart more compact. The familiar 18-column layout is conventional, not the only possible form: IUPAC notes that it does not recommend one specific layout over alternatives such as an 18- or 32-column table.

The table’s pattern reflects the arrangement of electrons in atoms. As atomic number rises, electrons fill available states in recurring sequences. Atoms in many of the same groups therefore have related outer-electron arrangements, which helps explain similarities in bonding and reactions. The structure also makes broad trends—such as changes in atomic size and tendency to attract shared electrons—visible across rows and down columns.

These patterns are useful for organizing knowledge and forming expectations about an element’s behavior, but they do not erase exceptions or replace measurements. Hydrogen, for example, occupies a distinctive position because its chemistry does not closely match that of the other members of its displayed group. The table is thus both a classification system and a guide to recurring tendencies, not a complete account of every element or compound.

The modern table emerged from a series of efforts to classify elements, rather than from a single sudden invention. Antoine Lavoisier published an early list of substances regarded as elements in 1789. In the 19th century, chemists including Johann Döbereiner, John Newlands, Julius Lothar Meyer and William Odling identified different kinds of recurring relationships among elements.

In 1869, Dmitri Mendeleev presented a table that arranged known elements largely by atomic weight and grouped those with similar properties. He left gaps where he judged that undiscovered elements belonged and used the pattern to predict properties; subsequent discoveries, including gallium, scandium and germanium, supported important parts of the scheme. The ordering principle was refined in 1913, when Henry Moseley’s work on characteristic X-rays showed how atomic number could provide a more fundamental sequence than atomic weight. The contemporary table incorporates elements identified or synthesized since those early versions.

Chemists use the table to identify elements, compare their structures and properties, and reason about likely patterns in reactions. A position can provide a useful first clue about an element’s broad chemical family, while reference tables supply precise values such as atomic weights and other measured properties. The periodic arrangement also connects laboratory chemistry with questions about materials, biological processes and the behavior of matter.

The chart is periodically updated as element names, measurements and standard atomic-weight evaluations change. IUPAC publishes a conventional long-form table and its Commission on Isotopic Abundances and Atomic Weights reviews standard atomic-weight values; NIST separately compiles critically evaluated atomic-property data. This distinction matters: the table’s ordering principle is stable, but some numerical values are evaluated and revised as measurements improve.