Periodic Table: Structure, Law, and Quantum Basis

Periodic table as ordered elements by atomic number

The periodic table is an ordered arrangement of chemical elements into rows (periods) and columns (groups). It is based on the periodic law: when elements are arranged in order of increasing atomic number, their properties show an approximate recurrence. In the standard (modern) form, elements are listed by increasing atomic number, where atomic number corresponds to the number of protons in the nucleus and uniquely identifies the element. The table’s structure reflects electron configuration. A new period begins when a new electron shell starts, and groups are determined by the number of electrons in specific outer subshells (valence electrons). Elements in the same group tend to have similar chemical characteristics, and trends across the table (vertical, horizontal, and diagonal) describe how properties change—such as increasing metallic character down a group and from right to left across a period, and increasing nonmetallic character from the bottom left toward the top right. The table is divided into blocks (s, p, d, and f), with the f-block commonly displayed separately at the bottom to save space, even though it fits between groups 2 and 3. Historically, the periodic table became widely accepted after Dmitri Mendeleev (1869), who used atomic mass to formulate the periodic law and predicted properties of missing elements. Later, the discovery of atomic numbers and advances in quantum mechanics explained why periodicity occurs. The modern placement of the actinides as part of the f-block was established in 1945 (Glenn T. Seaborg). Today, 118 elements are known, with the first 94 occurring naturally and the rest synthesized, and the table continues to evolve as new elements and placements are investigated.

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Periodic law and recurrence of properties

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