Chemical element

Chemical element is the collective name for all nuclides with the same atomic number. Thus all the atoms of a chemical element have the same number of protons in the nucleus. An element is identified by an element symbol, an abbreviation that is mostly derived from the Latin name of the element (eg Pb from plumbum, Fe from ferrum ). The elements are arranged in the periodic table according to increasing atomic number. Overall, one knows to this day 118 members ( as of 2012).

• 7.1 Frequency of the chemical elements

History

Conceptual history

Robert Boyle defined in 1661 a chemical element as a pure substance that can not be broken down further by chemical methods ( Boyle, The Sceptical Chymist ), and thus used the term quite differently than before the four- element theory (fire, water, air and earth ).

Today's element - term liability for the materials a classification according to their constituents, the atoms ago. It goes back to John Dalton and his atomic hypothesis, is more abstract, but also more precise. Its practical significance is that it summarizes atoms with the same chemical behavior in chemical reactions. These are atoms with the same number of protons. (Also known as a chemical reaction ) with the transformations of the chemical elements is concerned chemistry.

Discovery history

In ancient times and until well into the Middle Ages it was believed that the world is made up of the four elements earth, water, air and fire.

Of the elements in the modern sense in ancient times were few in pure form known which occurred either pure or could be melted from ore: carbon, sulfur, iron, copper, zinc, silver, tin, gold, mercury and lead. During the medieval history of mining is then, especially in the Ore Mountains, discovered in ores small amounts of impurities of unknown metals and named after mountain spirits ( cobalt, nickel, tungsten). The discovery of phosphorus in 1669 by Hennig Brand finally ushered in the era of discovery of most elements, including uranium from pitchblende by Martin Heinrich Klaproth in 1789.

Prior to 1751, the following subgroup elements were known: iron, cobalt, nickel, copper, zinc, silver, platinum, gold and mercury, and also the main group elements carbon, phosphorus, sulfur, arsenic, tin, antimony, lead and bismuth.

From 1751 to the year 1800 were still hydrogen, titanium, chromium, manganese, yttrium, zirconium, molybdenum, tungsten, uranium added, further nitrogen, oxygen, chlorine, tellurium.

In the period from 1800 to 1830 a total of twenty-two new elements were discovered, the subgroup elements vanadium, tantalum, rhodium, palladium, cadmium, osmium, iridium, and the rare earth thorium, the main group elements lithium, beryllium, sodium, magnesium, potassium, calcium, strontium, barium, boron, aluminum, silicon, selenium, iodine, and bromine.

Eleven other elements were added between the years 1830-1869. They were also a marker for the technical and scientific development state, because even hard to find and rare elements have been discovered and described. It was helium, rubidium, cesium, indium, thallium, niobium, ruthenium. As well as lanthanum, cerium, terbium, erbium.

During the 19th century, the rare earth metals were discovered, with which almost all naturally occurring elements were known. During this time, many hypothetical elements have been postulated, which were later discarded as about the nebulium. The transuranic elements - - In the 20th and the 21st century began, many not found in nature occurring elements have been artificially created, partly in nuclear reactors, partly in particle accelerators. All of these elements have in common that they are unstable, ie that they are different to quickly convert into other elements. With the discovery of other such short-lived elements is to be expected, they arise in each case only in extremely small amounts. Your name received each of the elements of their discoverer, which led to a Elementnamensgebungskontroverse in the 20th century. Elements that have not yet been created or named, bear systematic element name.

Order system

The elements are arranged according to their atomic number ( atomic number ) and the electron configuration of their atoms in the Periodic Table of the Elements ( PSE) in groups and periods. This system was the same time as the German physician and chemist Lothar Meyer in 1869 founded by Dmitri Ivanovich Mendeleev Russian scholars.

Properties

Many basic properties of chemical elements can be derived from the structure of their atoms. Diverse, historically developed atomic models, such as the successful Bohr's shell model to provide the theoretical foundations.

All atoms of an element have in the uncharged state, the number of protons corresponding to the same number of electrons in the electron shells. In the arrangement of the elements according to increasing atomic number or atomic number in the periodic table the elements are known also for related or recurring characteristics ( shell completion ) grouped.

Only in chemical reactions, the electrons are re-arranged on the outer shell of the reaction partners, the nucleus will remain unchanged. The effort to complete shells by release or absorption of electrons and thus stabilize relative, dominates over the electrical charge state of an atom. This endeavor is described by the electronegativity. Cup final state and charge state are thus directly linked to the chemical reactivity of an element. Noble gases, elements with closed shell in the neutral state are non-reactive, they form only under drastic conditions connections. Atoms are primarily looking for the so-called noble gas configuration ( shell stability) to achieve, even if it comes at the expense of electrical neutrality, and strive secondary to balance the charge of the overall configuration. An even finer discrimination scheme to uniquely identify the electrons an element gives the quantum number Quartet: principal quantum number, azimuthal quantum number, the magnetic quantum number, spin quantum number, ie, quantum- theoretical element properties.

Other properties of the elements resulting from the observance of the core configurations of an element atom. Cores of one and the same element can be equipped with a different number of neutrons. These different according to the number of neutrons atoms of an element are called isotopes derived from Gr. isos topos, which, mutatis mutandis, the same place ( in the periodic table ) means. Isotopes differ in mass and show nuclear reactions different behavior.

Identified chemical elements through detection reactions in analytical chemistry.

Mass

The atomic mass of elements is not exactly a multiple of the mass of the hydrogen atom; Explanations for this are:

• Protons and neutrons that make up the bulk of the mass, are almost, but not exactly the same weight.
• For very precise measurements, the binding energy shows a mass defect, so that the core mass is always minimally smaller than the sum of the masses of protons and neutrons.

Pure and mixing elements

Chemical elements, which have only one kind of atom are called pure elements, whereas if it consist of two or more isotopes, they are called mixing elements. Most elements are mixing elements, only 19 elements are pure elements.

From hydrogen, for example, there are three isotopes: protium (0 neutrons), deuterium (1 neutron), tritium (2 neutrons). The core of the most common Wasserstoffnuklids ( 99.9851 % protium ) consists of a proton and neutrons. Hydrogen with one proton and one neutron in the nucleus (deuterium ) occurs in natural hydrogen in a proportion of 0.0149 % to tritium with < 10-10 %.

The helium nucleus consisting of two protons and two neutrons. But there are also helium atoms, which contain two protons, but only one neutron. These occur in natural helium, however, only a proportion of 0.000137 %.

Chlorine (17 protons) consists of a mixture of atoms with neutrons 18 ( 75.8 %) and 20 neutrons ( 24.2 %).

In the periodic table stands for mixing elements, the average atomic mass according to relative abundance of the isotope, which is further corrected for the mass defect. The natural mixing ratio is usually equal to an element. For some elements, the isotopic composition can also vary locally. Lead for example can have different average atomic masses, depending on the deposit from which it is derived. 2010 therefore decided to IUPAC that the future for the elements hydrogen, boron, lithium, carbon, nitrogen, oxygen, silicon, sulfur, chlorine and thallium a mass range must be specified in the periodic table.

The terms pure substance and pure element, as well as mixture and mixing element can be distinguished strictly.

Chemical compounds

Chemical elements can, to some noble gases undergo chemical compounds. Several of the elementary atoms are joined together to form molecules or ionic crystals.

Elements can form a bond with other elements or with itself: Many gases such as chlorine or fluorine F, Cl two atoms of the same element combine with each other to form a molecule, in this case Cl2 and F2. Oxygen forms in addition to O2 also less stable triatomic molecules O3, sulfur forms ringfömige from 6 to 8 atoms. Ordinary water ( Molecular Formula: H 2 O), however, is a combination of the elements hydrogen H (2 atoms per molecule) and oxygen ( 1 atom per molecule).

Basically there are three types of chemical bonds between the atoms of the elements:

• Molecular compounds are formed from non-metal and non-metal - they are non-conductive ( electrically non- conductive), usually with relatively low boiling point ( diamond-like or plastic-like compounds with giant molecules excluded). Examples of molecular compounds besides water are methane, sugar.
• Ionic compounds are formed from metal ( cation ) and nonmetal ( anion ). They are salt-like: brittle, often of a high melting point and melt or solution electrically conductive. Examples of ionic compounds include iron (II ) oxide and salt (sodium chloride).
• Metallic compounds are formed of two or more metals. The metal atoms are here connected by metallic bonding, and often by additional ionic or covalent bonding. They are not to be confused with alloys.

The emergence of elements

Already during the Big Bang, the light elements hydrogen ( about 75%) and helium formed (about 25%), together with small amounts of lithium and beryllium. At the beginning of cosmochemistry is therefore of hydrogen with a relative atomic mass of about 1.0 u ( a proton ). Heavier elements are produced in the universe by nuclear reactions in the stars. In main-sequence stars like our sun, melt under high temperature (several million degrees Celsius ) and high pressure, for example, four hydrogen nuclei via several intermediates to form a helium nucleus ( atomic weight about 4.0 u). This is a little lighter than the four protons together, the difference in mass is released as energy.

This fusion (atoms of lower atomic number higher merge ) proceeds in most stars up to the formation of carbon in the mass range up to the formation of iron, the most densely packed nucleus. This is always done with the release of energy, the energy yield with increasing atomic number of the elements formed by the iron is always low. The fusion reactions to form heavier nuclei would require a supply of energy.

Therefore Heavier elements than iron are produced not by nuclear fusion, but by neutron capture existing atoms are converted to elements of higher atomic number. This occurs at the low-mass stars in the so-called s- process in massive at the end of the lifetime of stars during a supernova in the r-process.

The resulting elements reach (scrolling by solar wind or explosive in a supernova ) in the interstellar medium and are available for creating the next generation of stars and other astronomical objects. Therefore younger stellar systems already included from the very beginning small amounts of heavier elements that can form planets like our solar system.

Statistics of the chemical elements

Of the 118 known elements (2012 ), 80 stable. All stable elements occur naturally on Earth, as well as 14 radioactive ( see element abundance). Other radioactive elements were produced artificially, their number is expected to rise further.

The elements can be divided according to different criteria. The most common is the division into those elements which form metals and account for the majority of the elements, as well as in non-metals and the intermediate semi-metals.

The group of non-metals include only 17 of all the elements, they form at standard conditions no metals. Of these, the six noble gases before monatomic, because their atoms do not form molecules, ie not react with each other. In contrast, others combine with atoms of the same element to form molecules. These include the other five under normal conditions gaseous elements: hydrogen ( H2 ), nitrogen ( N2 ), oxygen ( O2), fluorine (F2) and chlorine (Cl2 ) and the liquid bromine ( Br2 ).

Frequency of the chemical elements

The frequency of the chemical elements differs depending on the region considered.

In the universe, it is closely linked to the development processes in cosmological time frame ( nucleosynthesis ). There is by far the most abundant element, hydrogen, followed by his simplest fusion product helium, both of which emerged soon after the Big Bang. The next most abundant elements are carbon and oxygen. Lithium, beryllium and boron also emerged during the Big Bang, but in much smaller quantities.

Helium, carbon and oxygen, as well as all other types of atoms were formed by nuclear fusion in stars or other astrophysical processes. This frequently caused atoms with a straight atomic number such as oxygen, neon, iron, or sulfur, while elements with an odd number of protons are more rare. This rule is named after the American chemist William Draper Harkins (1873-1951) Harkinssche rule. Striking feature is the very large concentration of iron as the end point of the possible nuclear fusion in stars.

The distribution on Earth is different from that which prevails in the whole universe. In particular, relatively small amounts of hydrogen and helium are present on the earth, because these gases from the Earth's gravitational field can not be held; in the solar system, they are mainly located in the gaseous planets like Jupiter and Neptune. On rocky planets like the Earth outweigh the heavier elements, especially oxygen, silicon, aluminum and iron.

Organisms are mainly composed of hydrogen, oxygen, carbon and nitrogen.

In the considered range of very frequently occurring elements are known as elements quantity, very rare as trace elements.