Periodic Table | Definition, Elements, Groups, Charges, Trends, & Facts

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periodic table, in chemistry, the organized array of all the chemical elements in order of increasing atomic number—i.e., the total number of protons in the atomic nucleus. When the chemical elements are thus arranged, there is a recurring pattern called the “periodic law” in their properties, in which elements in the same column (group) have similar properties. The initial discovery of this pattern by Dmitri I. Mendeleev in the mid-19th century has been of inestimable value in the development of chemistry.

In full: periodic table of the elements (Show more) Key People: Dmitri Mendeleev Lothar Meyer Paul-Émile Lecoq de Boisbaudran (Show more) Related Topics: atom When Was the Periodic Table Invented? group period periodic law (Show more) On the Web: Chemistry LibreTexts - The Periodic Table (Dec. 10, 2025) (Show more) See all related content

It was not recognized until the 1910s that the order of elements in the periodic system is that of their atomic numbers, which are equal to the positive electrical charges of the atomic nuclei expressed in electronic units. In subsequent years great progress was made in explaining the periodic law in terms of the electronic structure of atoms and molecules. This clarification has increased the value of the law, which is used as much today as it was at the beginning of the 20th century, when it expressed the only known relationship among the elements.

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