In chemistry, an element is a pure substance which cannot be broken down by chemical means, consisting of atoms which have identical numbers of protons in their atomic nuclei. The number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number (represented by the symbol Z).[1] Chemical elements constitute all of the baryonic matter of the universe.

In total, 118 elements have been identified. The first 94 occur naturally on Earth, and the remaining 24 are synthetic elements produced in nuclear reactions. Save for unstable radioactive elements (radionuclides) which decay quickly, nearly all of the elements are available industrially in varying amounts.

When different elements are combined, they may produce a chemical reaction and form into compounds due to chemical bonds holding the constituent atoms together. Only a minority of elements are found uncombined as relatively pure native element minerals. Nearly all other naturally-occurring elements appear as compounds or mixtures; for example, atmospheric air is primarily a mixture of the elements nitrogen, oxygen, and argon.

The history of the discovery and use of the elements began with primitive human societies that discovered native minerals like carbon, sulfur, copper and gold (though the concept of a chemical element was not yet understood). Attempts to classify materials such as these resulted in the concepts of classical elements, alchemy, and various similar theories throughout human history.

Much of the modern understanding of elements is attributed to Dmitri Mendeleev, a Russian chemist who published the first recognizable periodic table in 1869. The properties of the chemical elements are summarized in this table, which organizes them by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties. The use of the periodic table allows chemists to derive relationships between various elements and predict the behavior of theoretical but undiscovered new ones; the discovery and synthesis of further new elements is an ongoing area of scientific study.