element

In chemistry, an element is a substance 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. Elements are the fundamental constituents of all materials.

Elements are organized in the periodic table by increasing atomic number and recurring properties. They are typically classified as metals, nonmetals, and metalloids; specific groups include alkali metals, alkaline earth metals, halogens, noble gases, and transition metals.

Atoms of the same element may have different numbers of neutrons, yielding isotopes. The chemical behavior generally depends on electron configuration, not the exact isotopic mass. Some elements exist as diatomic molecules (e.g., H2, O2) or as elemental solids.

Elements occur naturally in the Earth's crust and in the universe; many must be synthesized in laboratories or produced by processing minerals. Isolated elements can be extracted through mining, electrolysis, or reduction of compounds.

Common examples include hydrogen, carbon, nitrogen, oxygen, iron, copper, and uranium. Elements vary widely in abundance and applications across science, medicine, technology, and industry.