Alkuaineet

Alkuaineet (chemical elements) are the fundamental substances that cannot be broken down into simpler substances by chemical means. Each element is defined by its atomic number, the number of protons in its nucleus, and is represented by a symbol on the periodic table. The periodic table arranges elements by increasing atomic number and shows recurring chemical properties. There are 118 known elements; about 94 occur naturally, with the remainder having been synthesized in laboratories. Elements are broadly categorized as metals, metalloids, and nonmetals, with several well-known groups such as alkali metals, alkaline earth metals, halogens, and noble gases occupying characteristic regions of the table.

Most elements occur in Earth's crust in combined form, but elemental forms and compounds are encountered in many environments. Common extraction methods include mining, smelting, and electrochemical reduction, followed by refining to obtain pure metal or gas forms. Elements differ in physical properties such as state, density, and conductivity, and in chemical reactivity. They also form isotopes, some of which are stable and others radioactive; radioisotopes find use in medicine, dating, and research.

Historically, the concept of basic elements predates modern chemistry, but the periodic table was developed in the 19th century by Dmitri Mendeleev and later refined. Discoveries of new elements continued into the 20th century, including many synthetic elements produced in particle accelerators and reactors.

Elements are essential to life and technology. A small set—carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur—constitutes the majority of biological matter, while others enable materials like steel, electronics, and catalysts. Safety considerations vary by element, with particular attention to toxicity and radioactivity.