Chemical element

A chemical element is a <a href="/facts/Chemical_substance/EWcXU41d">chemical substance</a> whose <a href="/facts/Atom/FoQ4kGN5">atoms</a> all have the same number of <a href="/facts/Proton/clCjepXP">protons</a>. The number of protons is called the <a href="/facts/Atomic_number/bgAr581S">atomic number</a> of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its <a href="/facts/Atomic_nucleus/orpIm5Xl">nucleus</a>. Atoms of the same element can have different numbers of <a href="/facts/Neutron/oRCM1geb">neutrons</a> in their nuclei, known as <a href="/facts/Isotope/KD9B1y6o">isotopes</a> of the element. Two or more atoms can combine to form <a href="/facts/Molecule/N9ljSfFq">molecules</a>. Some elements form <a href="/facts/Homonuclear_molecule/Pn0bshdv">molecules of atoms of said element only</a>: e.g. atoms of hydrogen (H) form diatomic molecules (H2). <a href="/facts/Chemical_compound/37FXqqtv">Chemical compounds</a> are substances made of atoms of different elements; they can have molecular or non-molecular structure. <a href="/facts/Mixture/tms8rbmn">Mixtures</a> are materials containing different chemical substances; that means (in case of molecular substances) that they contain different types of molecules. Atoms of one element can be transformed into atoms of a different element in <a href="/facts/Nuclear_reaction/202yClUr">nuclear reactions</a>, which change an atom's atomic number.
Historically, the term "chemical element" meant a substance that cannot be broken down into constituent substances by chemical reactions, and for most practical purposes this definition still has validity. There was some controversy in the 1920s over whether isotopes deserved to be recognised as separate elements if they could be separated by chemical means.
The term "(chemical) element" is used in two different but closely related meanings: it can mean a chemical substance consisting of a single kind of atom (a <a href="/facts/Free_element/dPTEISA9">free element</a>), or it can mean that kind of atom as a component of various chemical substances. For example, water (H2O) consists of the elements hydrogen (H) and oxygen (O) even though it does not contain the chemical substances (di)hydrogen (H2) and (di)oxygen (O2), as H2O molecules are different from H2 and O2 molecules. For the meaning "chemical substance consisting of a single kind of atom", the terms "elementary substance" and "simple substance" have been suggested, but they have not gained much acceptance in English chemical literature, whereas in some other languages their equivalent is widely used. For example, French distinguishes élément chimique (kind of atoms) and corps simple (chemical substance consisting of one kind of atom); Russian distinguishes химический элемент and простое вещество.
Almost all <a href="/facts/Baryonic_matter/QFwAUWK2">baryonic matter</a> in the universe is composed of elements (among rare exceptions are <a href="/facts/Neutron_star/4enfoxBA">neutron stars</a>). When different elements undergo chemical reactions, atoms are rearranged into new compounds held together by <a href="/facts/Chemical_bond/ArNUMnIB">chemical bonds</a>. Only a few elements, such as <a href="/facts/Silver/xXe41BGg">silver</a> and <a href="/facts/Gold/1znknojp">gold</a>, are found uncombined as relatively pure <a href="/facts/Native_element_mineral/kWjWAus5">native element minerals</a>. Nearly all other naturally occurring elements occur in the <a href="/facts/Earth/HLLmDfj0">Earth</a> as compounds or mixtures. <a href="/facts/Atmosphere_of_Earth/FV97Am25">Air</a> is mostly a mixture of molecular <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> and <a href="/facts/Oxygen/acyn6o8r">oxygen</a>, though it does contain compounds including <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a> and <a href="/facts/Water/0lkXnJPK">water</a>, as well as atomic <a href="/facts/Argon/QX6NRH6d">argon</a>, a <a href="/facts/Noble_gas/j0yHMEAE">noble gas</a> which is <a href="/facts/Chemically_inert/oglbxzgk">chemically inert</a> and therefore does not undergo chemical reactions.
The history of the discovery and use of elements began with early <a href="/facts/Society/PulvrYfa">human societies</a> that discovered native minerals like <a href="/facts/Carbon/ukrgQexo">carbon</a>, <a href="/facts/Sulfur/IK0amTfM">sulfur</a>, <a href="/facts/Copper/I8PEE8oX">copper</a> and gold (though the modern concept of an element was not yet understood). Attempts to classify materials such as these resulted in the concepts of <a href="/facts/Classical_element/LIzQ4aoX">classical elements</a>, <a href="/facts/Alchemy/CEx0U1Em">alchemy</a>, and similar theories throughout history. Much of the modern understanding of elements developed from the work of <a href="/facts/Dmitri_Mendeleev/xdouRrx8">Dmitri Mendeleev</a>, a Russian chemist who published the first recognizable <a href="/facts/Periodic_table/umdGIfUb">periodic table</a> in 1869. This table organizes the elements by increasing atomic number into rows ("<a href="/facts/Period_(periodic_table)/WpGUFh5V">periods</a>") in which the columns ("<a href="/facts/Group_(periodic_table)/YGBcUabv">groups</a>") share recurring ("periodic") <a href="/facts/Physical_property/ujcMUceW">physical</a> and <a href="/facts/Chemical_property/uMVxKspS">chemical properties</a>. The periodic table summarizes various properties of the elements, allowing chemists to derive relationships between them and to make predictions about elements not yet discovered, and potential new compounds.
By November 2016, the <a href="/facts/International_Union_of_Pure_and_Applied_Chemistry/2AV6myCK">International Union of Pure and Applied Chemistry</a> (IUPAC) recognized a total of 118 elements. The first 94 occur naturally on <a href="/facts/Earth/HLLmDfj0">Earth</a>, and the remaining 24 are <a href="/facts/Synthetic_element/azU9Nr1m">synthetic elements</a> produced in nuclear reactions. Save for unstable radioactive elements (radioelements) which <a href="/facts/Radioactive_decay/Ya6FVjt4">decay</a> quickly, nearly all elements are available industrially in varying amounts. The <a href="/facts/Timeline_of_chemical_element_discoveries/QY4X2RP3">discovery and synthesis of further new elements</a> is an ongoing area of scientific study.

Chemical element open-in-new

Chemical element