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Carbon (from (ラテン語:carbo) "coal") is a chemical element with symbol C and atomic number 6. On the Periodic table, it is the first (row 2) of six elements in column (group) 14, which have in common the composition of their outer electron shell. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. There are three naturally occurring isotopes, with and being stable, while is radioactive, decaying with a half-life of about 5,730 years.〔(【引用サイトリンク】title=Carbon – Naturally occurring isotopes )〕 Carbon is one of the few elements known since antiquity.〔(【引用サイトリンク】title=History of Carbon )〕 Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is present in all forms of carbon-based life, and in the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen.〔(【引用サイトリンク】title=Biological Abundance of Elements )〕 This abundance, together with the unique diversity of organic compounds and their unusual polymer-forming ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life. The atoms of carbon can be bonded together in different ways: allotropes of carbon. The best known are graphite, diamond, and amorphous carbon.〔(【引用サイトリンク】title=World of Carbon – Interactive Nano-visulisation in Science & Engineering Education (IN-VSEE) )〕 The physical properties of carbon vary widely with the allotropic form. For example, graphite is opaque and black, while diamond is highly transparent. Graphite is soft enough to form a streak on paper (hence its name, from the Greek word "γράφω" which means "to write"), while diamond is the hardest naturally-occurring material known. Graphite is a very good conductor, while diamond has a very low electrical conductivity. Under normal conditions, diamond, carbon nanotubes, and graphene have the highest thermal conductivities of all known materials. All carbon allotropes are solids under normal conditions, with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen. The most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and other transition metal carbonyl complexes. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil and methane clathrates. Carbon forms a vast number of compounds, more than any other element, with almost ten million compounds described to date, which in turn are a tiny fraction of such compounds that are theoretically possible under standard conditions. ==Characteristics== The different forms or ''allotropes'' of carbon (see below) include one of the softest known substances, graphite, and also the hardest naturally occurring substance, diamond. Moreover, it has an affinity for bonding with other small atoms, including other carbon atoms, and is capable of forming multiple stable covalent bonds with such atoms. As a result, carbon is known to form almost ten million different compounds; the large majority of all chemical compounds.〔 Carbon also has the highest sublimation point of all elements. At atmospheric pressure it has no melting point as its triple point is at 10.8 ± 0.2 MPa and 4,600 ± 300 K (~4,330 °C or 7,820 °F),〔〔 so it sublimes at about 3,900 K. Carbon sublimes in a carbon arc which has a temperature of about 5,800 K (5,530 °C; 9,980 °F). Thus, irrespective of its allotropic form, carbon remains solid at higher temperatures than the highest melting point metals such as tungsten or rhenium. Although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper that are weaker reducing agents at room temperature. Carbon compounds form the basis of all known life on Earth, and the carbon-nitrogen cycle provides some of the energy produced by the Sun and other stars. Although it forms an extraordinary variety of compounds, most forms of carbon are comparatively unreactive under normal conditions. At standard temperature and pressure, it resists all but the strongest oxidizers. It does not react with sulfuric acid, hydrochloric acid, chlorine or any alkalis. At elevated temperatures carbon reacts with oxygen to form carbon oxides, and will reduce such metal oxides as iron oxide to the metal. This exothermic reaction is used in the iron and steel industry to control the carbon content of steel: : + 4 C → 3 Fe + 4 CO with sulfur to form carbon disulfide and with steam in the coal-gas reaction: :C + HO → CO + H. Carbon combines with some metals at high temperatures to form metallic carbides, such as the iron carbide cementite in steel, and tungsten carbide, widely used as an abrasive and for making hard tips for cutting tools. As of 2009, graphene appears to be the strongest material ever tested. However, the process of separating it from graphite will require some technological development before it is economical enough to be used in industrial processes. The system of carbon allotropes spans a range of extremes: 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「carbon」の詳細全文を読む スポンサード リンク
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