Chemical element

A chemical element is a species of atom defined by its number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its nucleus. Atoms of the same element can have different numbers of neutrons in their nuclei, known as isotopes of the element. Atoms of one element can be transformed into atoms of a different element in nuclear reactions, which change an atom's atomic number. Almost all baryonic matter in the universe is composed of elements (among rare exceptions are neutron stars). The term "chemical element" is also widely used to mean a pure chemical substance consisting of a single element. For example, oxygen gas consists only of atoms of oxygen. 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. By November 2016, the International Union of Pure and Applied Chemistry (IUPAC) recognized a total of 118 elements. The first 94 occur naturally on Earth, and the remaining 24 are synthetic elements produced in nuclear reactions. Save for unstable radioactive elements (radioelements) which decay quickly, nearly all elements are available industrially in varying amounts. The discovery and synthesis of further new elements is an ongoing area of scientific study. The history of the discovery and use of elements began with early human societies that discovered native minerals such as carbon, sulfur, copper, 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 classical elements, alchemy, and similar theories throughout history. Much of the modern understanding of elements developed from the work of Dmitri Mendeleev, a Russian chemist who published the first recognizable periodic table in 1869. This table organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties. The periodic table summarizes various properties of the elements, allowing chemists to derive relationships between them and to make predictions about exceedingly transient nuclides not yet observed, and the potential compounds these unknown elements might form.

Description 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 free element), 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.

Chemical elements can be organized by name, chemical symbol, and also by properties (as atoms or as substances). The properties of chemical elements as kinds of atom include the atomic number, atomic weight, isotopes, abundance in nature, ionization energy, electron affinity, oxidation states, and electronegativity. The radioactive nuclides can be arranged by length of half-life. As substances, the properties of chemical elements include their density, melting point, boiling point, electrical conductance, thermal conductivity. One of the most convenient, and certainly the most traditional presentation of the elements, is in the form of the periodic table, which groups together elements with similar chemical properties (and usually also similar electronic structures). Chemical elements can be categorised by their origin on Earth, with the first 94 considered naturally occurring, while those with atomic numbers beyond 94 have only been produced artificially via human-made nuclear reactions.

Occurrence

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