The Periodic Table of Elements
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The periodic table is a systematic arrangement of chemical elements. A chemical element is a substance composed of a single type of atom identified by the number of protons in its nucleus. The periodic table is organized by the number of protons, known as the atomic number.
The properties of elements repeat in a regular pattern as the atomic number increases. This phenomenon is called periodicity, which is why the table is named the 'periodic table'. The periodic table is used to predict the behaviour of elements, their interactions with other elements, and their structure.
Structure of the table
The table is divided into horizontal rows called periods and vertical columns called groups. Each period represents a new electron shell. A group contains elements with the same number of electrons in their outermost electron shell, which gives them similar chemical properties. Notable groups include the alkali metals (group 1), the alkaline earth metals (group 2), the halogens, and the noble gases.
Major categories
Elements are generally classified as metals, nonmetals, and metalloids. They are also classified by 'blocks' based on which electron subshell was last filled: the s-block, p-block, d-block (transition metals), and f-block. The two rows of the f-block shown below the main table are called the lanthanides and actinides.
History
The modern form of the table is primarily attributed to the Russian scientist Dmitri Mendeleev, who in the 19th century presented a classification of elements organized by atomic mass and recurring properties. He left gaps for elements not yet discovered, predicting their properties; these gaps were later filled by newly found elements. In the 20th century, research into atomic structure revealed that the number of protons was the correct basis for classification, rather than atomic mass.
Uses and development
The periodic table is a fundamental tool used in teaching chemistry, research, and industry. The heaviest elements on the table are created in special laboratories by combining atomic nuclei; most of these are extremely unstable and decay rapidly. The IUPAC confirms the recognition of new elements and their official names.