什麼是碳

化學元素，週期表中IVa族非金屬，化學符號C，原子序數6。碳在自然界中分佈雖廣，但儲量並不特別豐富，在地殼中僅佔0.2%，然而其化合物卻比其他元素所形成的化合物的總和還多，已知的就有好幾百萬種。一般穩定的同位素是碳－12；另一個穩定的同位素是碳－13，是天然碳的1%。碳－14是五種放射性同位素（參閱radioactivity）中最穩定和最有名的一種，半衰期為5,730年，在碳－14年代測定法和放射性追蹤研究中十分有用。元素碳以三種形式存在：金剛石、石墨和碳黑（包括煤炭、炭和焦炭），其物理特性各不相同。每個碳原子形成四種鍵合（4個單鍵、2單1雙鍵、2個雙鍵，或1單3雙鍵）和至多四個其他原子，可產生多種鍊形、分枝狀、圓形和三度空間的結構。對碳的化合物的研究已構成了一個特殊的化學領域，即有機化學。碳跟氫、氧、氮以及某些其他元素，形成約佔生命機體物質18%的化合物，如蛋白質、碳水化合物、類脂化合物和核酸。生物化學是研究這些化合物如何合成、分解，以及它們如何與其他有機體發生聯絡。生物體消耗碳的同時又將碳送回到它們的環境中的這一系列過程構成了碳迴圈。碳以二氧化碳的形式約佔地球大氣體積的3%。它溶解於所有天然水中。碳在地殼中以碳酸鹽的形式存在於大理石、石灰石、白堊等岩石中，以碳氫化合物的形式（化合物僅含碳和氫）存在於煤、石油和天然氣中。海洋中含有大量分解的二氧化碳和碳酸鹽。

carbon

Nonmetallic chemical element, chemical symbol C, atomic number 6. The usual stable isotope is carbon-12; carbon-13, another stable isotope, is 1% of natural carbon. Carbon-14 is the most stable and best known of five radioactive isotopes (see radioactivity); its half-life of approximately 5,730 years makes it useful in carbon-14 dating and radiolabeling of research compounds. Carbon occurs in three allotropes: diamond, graphite, and carbon black (amorphous carbon), including coal, coke, and charcoal. Carbon forms more compounds than all other elements combined; several million are known. Each carbon atom forms four bonds (four single bonds, two single and one double bond, two double bonds, or one single and one triple bond) with up to four other atoms. Multitudes of chain, branched, ring, and three-dimensional structures can occur. The study of these carbon compounds and their properties and reactions is organic chemistry. With hydrogen, oxygen, nitrogen, and a few other elements whose small amounts belie their important roles, carbon forms the compounds that make up all living things: proteins, carbohydrates, lipids, and nucleic acids. Biochemistry is the study of how those compounds are synthesized and broken down and how they associate with each other in living organisms. Organisms consume carbon and return it to the environment in the carbon cycle. Carbon dioxide, produced when carbon is burned, is about 0.03% of air, and carbon occurs in the earth's crust as carbonate rocks and the hydrocarbons in coal, petroleum, and natural gas. The oceans contain large amounts of dissolved carbon dioxide and carbonates.