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.
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 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 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 like carbon, sulfur, copper and gold. 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 in which the columns share recurring 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 potential compounds these unknown elements might form.
Description
The term " element" is used in two different but closely related meanings: it can mean a chemical substance consisting of a single kind of atom, or it can mean that kind of atom as a component of various chemical substances. For example, water consists of the elements hydrogen and oxygen even though it does not contain the chemical substances hydrogen and oxygen, as HO molecules are different from H and O 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 and corps simple ; Russian distinguishes химический элемент and простое вещество.Chemical elements can be organized by name, chemical symbol, and also by properties. 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. 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
The lightest elements are hydrogen and helium, both created by Big Bang nucleosynthesis in the first 20 minutes of the universe in a ratio of around 3:1 by mass, along with tiny traces of the next two elements, lithium and beryllium. Almost all other elements found in nature were made by various natural methods of nucleosynthesis. On Earth, small amounts of new atoms are naturally produced in nucleogenic reactions, or in cosmogenic processes, such as cosmic ray spallation. New atoms are also naturally produced on Earth as radiogenic daughter isotopes of ongoing radioactive decay processes such as alpha decay, beta decay, spontaneous fission, cluster decay, and other rarer modes of decay.There are now 118 known elements. "Known" here means observed well enough, even from just a few decay products, to have been differentiated from other elements. Most recently, the synthesis of element 118 was reported in October 2006, and the synthesis of element 117 was reported in April 2010. Of these 118 elements, the first 94 elements have been detected directly on Earth as primordial nuclides present from the formation of the Solar System, or as naturally occurring fission or transmutation products of uranium and thorium. Six of these occur in extreme trace amounts: technetium, atomic number 43; promethium, number 61; astatine, number 85; francium, number 87; neptunium, number 93; and plutonium, number 94. These 94 elements have been detected in the universe at large, in the spectra of stars, as well as neutron star mergers and supernovae, where short-lived radioactive elements are newly being made.
Two or more atoms can combine to form molecules. Some elements form molecules of atoms of said element only: e.g. atoms of hydrogen form diatomic molecules. Chemical compounds are substances made of atoms of different elements; they can have molecular or non-molecular structure. Mixtures are materials containing different chemical substances; that means that they contain different types of molecules. When different elements undergo chemical reactions, atoms are rearranged into new compounds held together by chemical bonds. Less than twenty elements, including the gold, platinum, iron group metals, can sometimes be found uncombined as relatively pure native element minerals. Nearly all other naturally occurring elements exist in the Earth as compounds or mixtures. Air is mostly a mixture of molecular nitrogen and oxygen, though it does contain compounds including carbon dioxide and water, as well as atomic argon, a noble gas which is chemically inert and therefore does not undergo chemical reactions.
Atomic nucleus properties
The standard model of an atom is of a dense nucleus of charged protons and electrically-neutral neutrons, surrounded by an electrically-bound cloud of low mass, negatively charged electrons. Despite the force of mutual repulsion between the protons, the nucleus is held together by the short-ranged strong nuclear force between the particles. The neutron–proton ratio determines the stability of a nucleus, as a proper balance of neutrons counteracts the mutual repulsion of the protons.Nuclide
A nuclide, or nuclear species, is a class of atoms characterized by their number of protons, Z, their number of neutrons, N, and their nuclear energy state. Atomic nuclei other than, a lone proton, consist of protons and neutrons bound together by the residual strong force, overcoming electrical repulsion between protons. For that reason, neutrons are required to bind protons together; as the number of protons increases, so does the neutron–proton ratio necessary for stability. For example, although light elements up through oxygen have stable nuclides with the same number of neutrons as protons, lead requires about 3 neutrons for 2 protons.The atomic number of an element is equal to the number of protons in each atom, and defines the element. For example, all carbon atoms contain 6 protons in their atomic nucleus; so the atomic number of carbon is 6. The number of protons in the nucleus determines its electric charge, which in turn determines the number of bound electrons of an atom in its non-ionized state. The electrons occupy atomic orbitals that determine the atom's chemical properties.
Isotopes are atoms of the same element, but having different numbers of neutrons. Thus, for example, there are three main isotopes of carbon. All carbon atoms have 6 protons, but they can have either 6, 7, or 8 neutrons. Since the mass numbers of these are 12, 13 and 14 respectively, said three isotopes are known as carbon-12, carbon-13, and carbon-14. Natural carbon is a mixture of C, C and about 1 atom per trillion of C. The number of neutrons in a nucleus usually has very little effect on an element's chemical properties. An exception is hydrogen, for which the kinetic isotope effect is significant. Thus, all carbon isotopes have nearly identical chemical properties because they all have six electrons, even though they may have 6 to 8 neutrons. That is why atomic number, rather than mass number or atomic weight, is considered the identifying characteristic of an element.
Stability
All elements have radioactive isotopes, but many of these radioisotopes are not found in nature due to a low half life. Radioisotopes typically decay into other elements via alpha decay, beta decay, or inverse beta decay; some isotopes of the heaviest elements also undergo spontaneous fission. Isotopes that are not radioactive, are termed "stable" isotopes. Isotopes with even numbers of protons, even numbers of neutrons, or both, tend to be more stable as like particle can pair up with like. This pairing effect allows the identical particles to align with opposite spins, increasing the binding energy.Most naturally occurring elements have more than one stable isotope. Only 26 elements are monoisotopic, having exactly one stable isotope; these have an odd atomic number of protons, with the exception of beryllium-9 which has an odd number of neutrons. The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element. The largest number of stable isotopes for a single element is 10.
Elements with atomic numbers 1 through 82 each have at least one stable isotope. However, observationally stable isotopes of some elements are predicted to be slightly radioactive with very long half-lives: for example, the half-lives predicted for the observationally stable lead isotopes range from 10 to 10 years. Isotopes are observationally stable when they are theoretically unstable but no radioactive decay has yet been observed. Out of the over 250 nuclides that are called stable, only 90 are considered theoretically stable, meaning they lack a known decay mode.
Elements with atomic numbers 83 through 94 are unstable enough that radioactive decay of all isotopes can be detected. Some of these elements, notably bismuth, thorium, and uranium, have one or more isotopes with half-lives long enough to survive from before the Solar System formed. The remaining longest-lived isotopes have half lives too short for them to have been present at the beginning of the Solar System, and are therefore "transient elements". Of these 11 transient elements, five are relatively common decay products of thorium and uranium. The remaining six transient elements occur only rarely, as products of rare decay modes or nuclear reaction processes involving uranium or other heavy elements.
The remaining 24 heaviest elements are radioactive, with half-lives so short that they are not found on Earth and must be synthesized. Five have been discovered in the spectrum of Przybylski's star, from element 95 to 99. These are thought to be neutron capture products of uranium and thorium. All 24 heavier elements are radioactive, with short half-lives; if any of these elements were present when the Earth formed, they are certain to have completely decayed, and if present in novae, are in quantities too small to have been noted. Technetium was the first purportedly non-naturally occurring element synthesized, in 1937, though traces of technetium have since been found in nature. This pattern of artificial production and later natural discovery has been repeated with several other radioactive naturally occurring rare elements.
The lightest radioactive isotope is tritium, which undergoes Beta decay with a half-life of 12.3 years. At 2 years, over 10 times the estimated age of the universe, bismuth-209 has the longest known alpha decay half-life of any nuclide, and is almost always considered on par with the 80 stable elements. The isotope tellurium-128 transmutes through double beta decay with a half life of 2.25 years, over 100,000 longer than bismuth-209. The primary source of radiation exposure from isotope decays in the human body come from carbon-14 and potassium-40 intake, which produce an annual effective dose of.