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Pengguna:Agung.karjono/Bak pasir/Karbon

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6C
Karbon
Intan (kiri) dan grafit (kanan), 2 alotrop terkenal dari karbon
Garis spektrum karbon
Sifat umum
Nama, lambangkarbon, C
Pengucapan/karbon/[1]
Alotropgrafit, intan dan lainnya (lihat alotrop karbon)
Penampilan
  • intan: bening
  • grafit: hitam, tampak seperti metalik
Karbon dalam tabel periodik
Perbesar gambar

6C
Hidrogen Helium
Lithium Berilium Boron Karbon Nitrogen Oksigen Fluor Neon
Natrium Magnesium Aluminium Silikon Fosfor Sulfur Clor Argon
Potasium Kalsium Skandium Titanium Vanadium Chromium Mangan Besi Cobalt Nikel Tembaga Seng Gallium Germanium Arsen Selen Bromin Kripton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
–
↑
C
↓
Si
boron ← karbon → nitrogen
Lihat bagan navigasi yang diperbesar
Nomor atom (Z)6
Golongangolongan 14 (golongan karbon)
Periodeperiode 2
Blokblok-p
Kategori unsur  nonlogam poliatomik
Berat atom standar (Ar)
  • [12,0096, 12,0116]
  • 12,011±0,002 (diringkas)
Konfigurasi elektron1s2 2s2 2p2 atau [He] 2s2 2p2
Elektron per kelopak2,4
Sifat fisik
Fase pada STS (0 °C dan 101,325 kPa)padat
Titik sublimasi3915 K ​(3642 °C, ​6588 °F)
Kepadatan mendekati s.k.amorf:[2] 1,8–2,1 g/cm3
intan: 3,515 g/cm3
grafit: 2,267 g/cm3
Titik tripel4600 K, ​10.800 kPa[3][4]
Kalor peleburangrafit: 117 kJ/mol
Kapasitas kalor molarintan: 6,155 J/(mol·K)
grafit: 8,517 J/(mol·K)
Sifat atom
Bilangan oksidasi−4, −3, −2, −1, 0, +1,[5] +2, +3,[6] +4[7] (oksida agak asam)
ElektronegativitasSkala Pauling: 2,55
Energi ionisasike-1: 1086,5 kJ/mol
ke-2: 2352,6 kJ/mol
ke-3: 4620,5 kJ/mol
(artikel)
Jari-jari kovalensp3: 77 pm
sp2: 73 pm
sp: 69 pm
Jari-jari van der Waals170 pm
Lain-lain
Kelimpahan alamiprimordial
Struktur kristalintan: ​kubus-rombus acuan muka
Struktur kristal Diamond cubic untuk intan: karbon

(bening)
Struktur kristalgrafit: ​heksagon sederhana
Struktur kristal Simple Hexagonal untuk grafit: karbon

(hitam)
Kecepatan suara batang ringanintan: 18.350 m/s (suhu 20 °C)
Ekspansi kalorintan: 0,8 µm/(m·K) (suhu 25 °C)[8]
Konduktivitas termalintan: 900–2300 W/(m·K)
grafit: 119–165 W/(m·K)
Arah magnetdiamagnetik[9]
Modulus Youngintan: 1050 GPa[8]
Modulus Shearintan: 478 GPa[8]
Modulus curahintan: 442 GPa[8]
Rasio Poissonintan: 0,1[8]
Skala Mohsgrafit: 1–2
Nomor CAS7440-44-0
Sejarah
PenemuanMesir Kuno dan orang Sumeria[10] (3750 SM)
Diketahui sebagai unsur kimia olehA. Lavoisier[11] (1789)
Isotop karbon yang utama
Iso­top Kelim­pahan Waktu paruh (t1/2) Mode peluruhan Pro­duk
11C sintentis 20 mnt β+ 11B
12C 98,9% stabil
13C 1,1% stabil
14C renik 5730 thn β− 14N
| referensi | di Wikidata

Karbon adalah suatu unsur kimia dalam tabel periodik yang memiliki lambang C dan nomor atom 6. Karbon (dari bahasa Latin: carbo) adalah unsur pertama (baris 2) dari enam unsur yang berada di kolom (golongan) 14, yang memiliki kesamaan komposisi pada kulit elektron terluarnya. Karbon adalah nonlogam dan tetravalen dengan empat elektron tersedia untuk membentuk ikatan kovalen. Terdapat tiga isotop alami, dengan 12C dan 13C berupa isotop stabil, dan 14C bersifat radioaktif, yang meluruh dengan waktu paruh sekitar 5.730 tahun.[12] Karbon adalah salah satu dari sedikit unsur yang dikenal sejak zaman purba.[13]

Karbon adalah unsur yang paling melimpah dalam kerak bumi dan urutan 4 kelimpahan setelah hidrogen, helium, dan oksigen. Ia berada dalam semua bentuk kehidupan berbasis karbon, dan dalam tubuh manusia karbon adalah unsur dengan kelimpahan kedua berdasarkan massa (sekitar 18,5%) setelah oksigen.[14] Kelimpahan ini, ditambah keanekaragaman senyawa organik yang unik serta kemampuan anehnya membentuk polimer pada temperatur umumnya yang ada di bumi, menjadikannya sebagai dasar kimia seluruh bentuk kehidupan.

Atom karbon dapat berikatan bersama-sama dalam berbagai cara: alotrop karbon. Paling banyak dikenal adalah grafit, intan, dan karbon amorf.[15] Sifat fisik karbon sangat bervariasi tergantung pada bentuk alotropiknya. Sebagai contoh, grafit bersifat opak dan hitam, sementara intan sangat transparan. Grafit cukup lunak untuk membuat goresan pada kertas (oleh karena itu nama diambil dari Yunani: γράφω yang berarti "menulis"), sementara intan dikenal sebagai bahan paling keras yang terjadi secara alami.

Grafit adalah konduktor yang sangat baik, sementara intan memiliki konduktivitas listrik yang sangat rendah. Pada kondisi normal, intan, carbon nanotube, dan grafena memiliki konduktivitas panas tertinggi di antara semua materi yang diketahui. Seluruh alotrop karbon berbentuk padat pada kondisi normal, dengan grafit adalah bentuk yang paling stabil secara termodinamika. Mereka tahan bahan kimia dan memerlukan temperatur tinggi untuk bereaksi dengan oksigen sekalipun.

Tingkat oksidasi karbon yang paling umum dalam senyawa anorganik adalah +4, sementara +2 ditemukan dalam karbon monoksida dan kompleks karbonil logam transisi. Sumber terbesar karbon anorganik adalah batu gamping (limestone), dolomit dan karbon dioksida, tetapi jumlah yang signifikan terdapat dalam deposit organik batu bara, gambut, minyak dan metana klatrat. Karbon membentuk sangat banyak senyawa, melebihi unsur lainnya, dengan hampir sepuluh juta senyawa telah diketahui saat ini,[16] yang pada gilirannya hanya sejumlah kecil dari senyawa-senyawa tersebut yang secara teoritis memungkinkan dalam kondisi standar.

Karakteristik sunting

 
Diagram fasa teoritis karbon

Perbendaan bentuk atau alotropi karbon (lihat di bawah) meliputi salah satu bahan yang dikenal palilng lunak, grafit, dan juga bahan terkeras yang terbentuk secara alami, intan. Selain itu, karbon mempunyai afinitas membentuk ikatan dengan atom kecil lainnya, termasuk atom karbon lainnya, dan mampu membentuk ikatan kovalen multi-stabil dengan atom-atom ini. Alhasil, karbon dikenal membentuk hampir sepuluh juta senyawa yang berbeda; mayoritas dari seluruh senyawa kimia.[16] Karbon juga mempunyai titik sublimasi tertinggi di antara unsur-unsur kimia. Pada tekanan atmosfer, karbon tidak memiliki titik lebur karena titik tripelnya adalah 10,8 ± 0,2 MPa and 4.600 ± 300 K (~4.330 °C or 7.820 °F),[3][4] sehingga ia menyublim pada sekitar 3.900 K.[17][18]

Karbon menyublim dalam arc karbon dengan temperatur sekitar 5.800 K (5.530 °C; 9.980 °F). Oleh karena itu, apapun bentuk alotropinya, karbon tetap berbentuk padat pada temperatur yang lebih tinggi daripada titik leleh logam seperti wolfram atau renium. Meskipun secara termodinamika cenderung mengalami oksidasi, karbon dapat menahan oksidasi lebih efektif daripada besi dan tembaga yang merupakan reduktor lemah pada temperatur kamar.

Senyawa karbon membentuk dasar seluruh kehidupan di bumi, dan siklus karbon-nitrogen menyediakan beberapa energi yang dihasilkan oleh matahari dan bintang lainnya. Meskipun karbon memebentuk senyawa dengan ragam yang luar biasa, sebagian besar bentuk karbon relatif tidak reaktif pada kondisi normal. Pada temperatur dan tekanan standar, ia dapat bertahan dari seluruh oksidator kecuali oksidator terkuat. Ia tidak bereaksi dengan asam sulfat, asam klorida, klorin, atau alkalis apapun. Pada temperatur sedikit di atas temperatur kamar, karbon bereaksi dengan oksigen membentuk karbon oksida, dan akan mereduksi oksida logam seperti, besi oksida, menjadi bentuk logamnya. Reaksi eksotermal ini digunakan dalam industri besi dan baja untuk mengendalikan kandungan karbon dalam baja:

Fe + 4 C(s) → 3 Fe(s) + 4 CO(g)

dengan belerang untuk membentuk karbon disulfida dan dengan uap air panas dalam reaksi batubara-gas:

C(s) + H(g) → CO(g) + H(g).

Karbon bereaksi dengan beberapa logam pada temperatur tinggi untuk membentuk logam karbida, seperti besi karbida sementit dalam baja dan wolfram karbida, yang digunakan sebagai abrasif dan untuk mengasah alat potong.

Pada tahun 2009, grafena muncul sebagai bahan terkuat yang pernah diuji.[19] Proses pemisahannya dari grafit memerlukan beberapa pengembangan teknologi sebelum dicapai nilai keekonomian untuk diproses secara komersial.[20]

Sistem alotrop karbon mencakup rentang yang ekstrim:

Grafit adalah salah satu bahan terlunak yang diketahui. Intan nanokristal sintetis adalah bahan terkeras yang diketahui.[21]
Grafit adalah pelumas yang sangat baik, menunjukkan sifat superlubrisitas.[22] Intan adalah abrasif tertinggi.
Grafit adalah penghantar listrik.[23] Intan adalah isolator listrik yang baik[24], dan merupakan pemutus medan listrik tertinggi dibandingkan bahan lainnya.
Beberapa bentuk grafit digunakan sebagai isolator panas (yaitu pemecah api dan pelindung panas), tetapi beberapa bentuk lainnya adalah penghantar yang baik. Intan adalah konduktor termal terbaik yang terjadi secara alami.
Grafit bersifat opak. Intan sangat tembus pandang (transparan)
Grafit mengkristal dalam sistem heksagonal.[25] Intan mengkristal dalam sistem kubik.
Karbon amorf adalah isotrop total. Karbon nanotube adalah bahan paling anisotropi yang pernah diproduksi.

Alotropi sunting

Artikel utama: Alotrop karbon

Atom karbon adalah spesies dengan umur paling pendek dan, oleh sebab itu, karbon distabilkan dalam beragam struktur multi atom dengan konfigurasi molekul yang berbeda yang disebut alotrop. Tiga alotrop karbon yang relatif cukup dikenal adalah karbon amorf, grafit, dan intan. Setelah dianggap eksotis, fulerena yang saat ini sering disintesis dan digunakan dalam penelitian; mereka mulai mengungkap juga bulkyballs,[26][27] karbon nanotube,[28] karbon nanobud[29] dan nanofiber.[30][31] Beberapa alotrop eksotis lainnya juga telah ditemukan, seperti lonsdaleit,[32] glassy carbon,[33] karbon nanofoam[34] dan karbon asetilen linear (karbin) (Inggris: carbyne).[35]

 
Sampel karbon besar mengkilat

Bentuk amorf adalah campuran beragam atom karbon dalam bentuk non-kristal, iregular, kondisi glassy, yang secara esensi adalah grafit tetapi tidak mempertahankan struktur makro kristalnya. Karbon ini hadir dalam bentuk serbuk, dan merupakan konstituen utama dalam arang, jelaga, dan karbon aktif. Pada tekanan normal, karbon berada dalam bentuk grafit, di mana masing-masing atom terikat secara trigonal dengan tiga atom karbon lainnya pada satu bidang cincin heksagonal, seperti yang diperlihatkan oleh hidrokarbon aromatik.[36] Hasilnya adalah 2-dimensi datar yang tersusun dan terikat lemah melalui gaya van der Waals.Hal ini menyumbang sifat grafit yang lunak dan mudah patah.

Karena delokalisasi salah satu elektron terluar masing-masing atom untuk membentuk awan-π, grafit menghantarkan listrik, tetapi hanya pada bidang masing-masing lembar ikatan kovalen. Hal ini yang menyebabkan karbon mempunyai konduktivitas listrik massal yang rendah dibandingkan kebanyakan logam. Delokalisasi juga menyebabkan stabilitas energik grafit lebih besar daripada berlian pada suhu kamar.

Pada tekanan yang sangat tinggi karbon membentuk alotrop berlian yang lebih kompak, memiliki hampir dua kali lipat kepadatan grafit. Di sini, masing-masing atom berikatan tetrahedral dengan empat atom lain, sehingga membuat jaringan 3-dimensi dari cincin beranggotakan enam atom. Berlian memiliki struktur kubik yang sama dengan silikon dan germanium, dan karena kekuatan ikatan karbon-karbon menjadikannya zat paling keras yang terjadi secara alami dalam hal ketahanan terhadap goresan. Berlawanan dengan kepercayaan populer bahwa "diamonds are forever", mereka sebenarnya secara termodinamika tidak stabil dalam kondisi normal dan berubah menjadi grafit.[15] Karena penghalang energi aktivasi yang tinggi, transisi menjadi grafit begitu sangat lambat pada suhu kamar sehingga menjadi tidak kentara. Dalam beberapa kondisi, karbon mengkristal sebagai lonsdaleit. Bentuk ini memiliki kisi kristal heksagonal di mana semua atom berikatan secara kovalen. Oleh karena itu, semua sifat lonsdaleit mendekati dengan sifat-sifat berlian.[32]

 
Beberapa alotrop karbon: a) berlian; b) grafit; c) lonsdaleit; d–f) fulerena (C60, C540, C70); g) karbon amorf; h) karbon nanotube.

Fullerenes have a graphite-like structure, but instead of purely hexagonal packing, they also contain pentagons (or even heptagons) of carbon atoms, which bend the sheet into spheres, ellipses or cylinders. The properties of fullerenes (split into buckyballs, buckytubes and nanobuds) have not yet been fully analyzed and represent an intense area of research in nanomaterials. The names "fullerene" and "buckyball" are given after Richard Buckminster Fuller, popularizer of geodesic domes, which resemble the structure of fullerenes. The buckyballs are fairly large molecules formed completely of carbon bonded trigonally, forming spheroids (the best-known and simplest is the soccerball-shaped C60 buckminsterfullerene).[26] Carbon nanotubes are structurally similar to buckyballs, except that each atom is bonded trigonally in a curved sheet that forms a hollow cylinder.[27][28] Nanobuds were first reported in 2007 and are hybrid bucky tube/buckyball materials (buckyballs are covalently bonded to the outer wall of a nanotube) that combine the properties of both in a single structure.[29]

Of the other discovered allotropes, carbon nanofoam is a ferromagnetic allotrope discovered in 1997. It consists of a low-density cluster-assembly of carbon atoms strung together in a loose three-dimensional web, in which the atoms are bonded trigonally in six- and seven-membered rings. It is among the lightest known solids, with a density of about 2 kg/m3.[37] Similarly, glassy carbon contains a high proportion of closed porosity,[33] but contrary to normal graphite, the graphitic layers are not stacked like pages in a book, but have a more random arrangement. Linear acetylenic carbon[35] has the chemical structure[35] -(C:::C)n-. Carbon in this modification is linear with sp orbital hybridization, and is a polymer with alternating single and triple bonds. This type of carbyne is of considerable interest to nanotechnology as its Young's modulus is forty times that of the hardest known material – diamond.[38]

Occurrence sunting

 
Graphite ore
 
Raw diamond crystal.
 
"Present day" (1990s) sea surface dissolved inorganic carbon concentration (from the GLODAP climatology)

Carbon is the fourth most abundant chemical element in the universe by mass after hydrogen, helium, and oxygen. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets.[39] Some meteorites contain microscopic diamonds that were formed when the solar system was still a protoplanetary disk. Microscopic diamonds may also be formed by the intense pressure and high temperature at the sites of meteorite impacts.[40]

In 2014 NASA announced a greatly upgraded database for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. More than 20% of the carbon in the universe may be associated with PAHs, complex compounds of carbon and hydrogen without oxygen.[41] These compounds figure in the PAH world hypothesis where they are hypothesized to have a role in abiogenesis and formation of life. PAHs seem to have been formed "a couple of billion years" after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets.[39]

It has been estimated that the solid earth as a whole contains 730 ppm of carbon, with 2000 ppm in the core and 120 ppm in the combined mantle and crust.[42] Since the mass of the earth is 5,972×1024 kg, this would imply 4360 million gigatonnes of carbon. This is much more than the amounts in the oceans or atmosphere (below).

In combination with oxygen in carbon dioxide, carbon is found in the Earth's atmosphere (approximately 810 gigatonnes of carbon) and dissolved in all water bodies (approximately 36,000 gigatonnes of carbon). Around 1,900 gigatonnes of carbon are present in the biosphere. Hydrocarbons (such as coal, petroleum, and natural gas) contain carbon as well. Coal "reserves" (not "resources") amount to around 900 gigatonnes with perhaps 18 000 Gt of resources.[43] Oil reserves are around 150 gigatonnes. Proven sources of natural gas are about 175 1012 cubic metres (representing about 105 gigatonnes carbon), but it is estimated that there are also about 900 1012 cubic metres of "unconventional" gas such as shale gas, representing about 540 gigatonnes of carbon.[44] Carbon is also locked up as methane hydrates in polar regions and under the seas. Various estimates of the amount of carbon this represents have been made: 500 to 2500 Gt,[45] or 3000 Gt.[46] In the past, quantities of hydrocarbons were greater. According to one source, in the period from 1751 to 2008 about 347 gigatonnes of carbon were released as carbon dioxide to the atmosphere from burning of fossil fuels.[47] Another source puts the amount added to the atmosphere for the period since 1750 at 879 Gt, and the total going to the atmosphere, sea, and land (such as peat bogs) at almost 2000 Gt.[48]

Carbon is a major component in very large masses of carbonate rock (limestone, dolomite, marble and so on). Coal is the largest commercial source of mineral carbon, accounting for 4,000 gigatonnes or 80% of fossil carbon fuel.[49] It is also rich in carbon – for example, anthracite contains 92–98%.[50]

As for individual carbon allotropes, graphite is found in large quantities in the United States (mostly in New York and Texas), Russia, Mexico, Greenland, and India. Natural diamonds occur in the rock kimberlite, found in ancient volcanic "necks", or "pipes". Most diamond deposits are in Africa, notably in South Africa, Namibia, Botswana, the Republic of the Congo, and Sierra Leone. There are also deposits in Arkansas, Canada, the Russian Arctic, Brazil and in Northern and Western Australia. Diamonds are now also being recovered from the ocean floor off the Cape of Good Hope. Diamonds are found naturally, but about 30% of all industrial diamonds used in the U.S. are now made synthetically.

Carbon-14 is formed in upper layers of the troposphere and the stratosphere, at altitudes of 9–15 km, by a reaction that is precipitated by cosmic rays.[51] Thermal neutrons are produced that collide with the nuclei of nitrogen-14, forming carbon-14 and a proton.

Carbon-rich asteroids are relatively preponderant in the outer parts of the asteroid belt in our solar system. These asteroids have not yet been directly sampled by scientists. The asteroids can be used in hypothetical space-based carbon mining, which may be possible in the future, but is currently technologically impossible.[52]

Isotopes sunting

Artikel utama: Isotopes of carbon

Isotopes of carbon are atomic nuclei that contain six protons plus a number of neutrons (varying from 2 to 16). Carbon has two stable, naturally occurring isotopes.[12] The isotope carbon-12 (12C) forms 98.93% of the carbon on Earth, while carbon-13 (13C) forms the remaining 1.07%.[12] The concentration of 12C is further increased in biological materials because biochemical reactions discriminate against 13C.[53] In 1961, the International Union of Pure and Applied Chemistry (IUPAC) adopted the isotope carbon-12 as the basis for atomic weights.[54] Identification of carbon in NMR experiments is done with the isotope 13C.

Carbon-14 (14C) is a naturally occurring radioisotope which occurs in trace amounts on Earth of up to 1 part per trillion (0.0000000001%), mostly confined to the atmosphere and superficial deposits, particularly of peat and other organic materials.[55] This isotope decays by 0.158 MeV β− emission. Because of its relatively short half-life of 5730 years, 14C is virtually absent in ancient rocks, but is created in the upper atmosphere (lower stratosphere and upper troposphere) by interaction of nitrogen with cosmic rays.[56] The abundance of 14C in the atmosphere and in living organisms is almost constant, but decreases predictably in their bodies after death. This principle is used in radiocarbon dating, invented in 1949, which has been used extensively to determine the age of carbonaceous materials with ages up to about 40,000 years.[57][58]

There are 15 known isotopes of carbon and the shortest-lived of these is 8C which decays through proton emission and alpha decay and has a half-life of 1.98739x10−21 s.[59] The exotic 19C exhibits a nuclear halo, which means its radius is appreciably larger than would be expected if the nucleus were a sphere of constant density.[60]

Formation in stars sunting

Artikel utama: Triple-alpha process dan CNO cycle

Formation of the carbon atomic nucleus requires a nearly simultaneous triple collision of alpha particles (helium nuclei) within the core of a giant or supergiant star which is known as the triple-alpha process, as the products of further nuclear fusion reactions of helium with hydrogen or another helium nucleus produce lithium-5 and beryllium-8 respectively, both of which are highly unstable and decay almost instantly back into smaller nuclei.[61] This happens in conditions of temperatures over 100 megakelvin and helium concentration that the rapid expansion and cooling of the early universe prohibited, and therefore no significant carbon was created during the Big Bang. Instead, the interiors of stars in the horizontal branch transform three helium nuclei into carbon by means of this process.[62] In order to be available for formation of life as we know it, this carbon must then later be scattered into space as dust, in supernova explosions, as part of the material which later forms second, third-generation star systems which have planets accreted from such dust.[39][63] The Solar System is one such third-generation star system. Another of the fusion mechanisms powering stars is the CNO cycle, in which carbon acts as a catalyst to allow the reaction to proceed.

Rotational transitions of various isotopic forms of carbon monoxide (for example, 12CO, 13CO, and C18O) are detectable in the submillimeter wavelength range, and are used in the study of newly forming stars in molecular clouds.[64]

Carbon cycle sunting

Artikel utama: Carbon cycle
 
Diagram of the carbon cycle. The black numbers indicate how much carbon is stored in various reservoirs, in billions tonnes ("GtC" stands for gigatonnes of carbon; figures are circa 2004). The purple numbers indicate how much carbon moves between reservoirs each year. The sediments, as defined in this diagram, do not include the ~70 million GtC of carbonate rock and kerogen.

Under terrestrial conditions, conversion of one element to another is very rare. Therefore, the amount of carbon on Earth is effectively constant. Thus, processes that use carbon must obtain it somewhere and dispose of it somewhere else. The paths that carbon follows in the environment make up the carbon cycle. For example, plants draw carbon dioxide out of their environment and use it to build biomass, as in carbon respiration or the Calvin cycle, a process of carbon fixation. Some of this biomass is eaten by animals, whereas some carbon is exhaled by animals as carbon dioxide. The carbon cycle is considerably more complicated than this short loop; for example, some carbon dioxide is dissolved in the oceans; dead plant or animal matter may become petroleum or coal, which can burn with the release of carbon, should bacteria not consume it.[65][66]

Compounds sunting

Artikel utama: Compounds of carbon

Organic compounds sunting

 
Structural formula of methane, the simplest possible organic compound.
 
Correlation between the carbon cycle and formation of organic compounds. In plants, carbon dioxide formed by carbon fixation can join with water in photosynthesis (green) to form organic compounds, which can be used and further converted by both plants and animals.

Carbon has the ability to form very long chains of interconnecting C-C bonds. This property is called catenation. Carbon-carbon bonds are strong, and stable. This property allows carbon to form an almost infinite number of compounds; in fact, there are more known carbon-containing compounds than all the compounds of the other chemical elements combined except those of hydrogen (because almost all organic compounds contain hydrogen as well).

The simplest form of an organic molecule is the hydrocarbon—a large family of organic molecules that are composed of hydrogen atoms bonded to a chain of carbon atoms. Chain length, side chains and functional groups all affect the properties of organic molecules.

Carbon occurs in all known organic life and is the basis of organic chemistry. When united with hydrogen, it forms various hydrocarbons which are important to industry as refrigerants, lubricants, solvents, as chemical feedstock for the manufacture of plastics and petrochemicals and as fossil fuels.

When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds including sugars, lignans, chitins, alcohols, fats, and aromatic esters, carotenoids and terpenes. With nitrogen it forms alkaloids, and with the addition of sulfur also it forms antibiotics, amino acids, and rubber products. With the addition of phosphorus to these other elements, it forms DNA and RNA, the chemical-code carriers of life, and adenosine triphosphate (ATP), the most important energy-transfer molecule in all living cells.

Inorganic compounds sunting

Commonly carbon-containing compounds which are associated with minerals or which do not contain hydrogen or fluorine, are treated separately from classical organic compounds; the definition is not rigid (see reference articles above). Among these are the simple oxides of carbon. The most prominent oxide is carbon dioxide (CO2). This was once the principal constituent of the paleoatmosphere, but is a minor component of the Earth's atmosphere today.[67] Dissolved in water, it forms carbonic acid (H), but as most compounds with multiple single-bonded oxygens on a single carbon it is unstable.[68] Through this intermediate, though, resonance-stabilized carbonate ions are produced. Some important minerals are carbonates, notably calcite. Carbon disulfide (C) is similar.

The other common oxide is carbon monoxide (CO). It is formed by incomplete combustion, and is a colorless, odorless gas. The molecules each contain a triple bond and are fairly polar, resulting in a tendency to bind permanently to hemoglobin molecules, displacing oxygen, which has a lower binding affinity.[69][70] Cyanide (CN−), has a similar structure, but behaves much like a halide ion (pseudohalogen). For example, it can form the nitride cyanogen molecule ((CN)2), similar to diatomic halides. Other uncommon oxides are carbon suboxide (C),[71] the unstable dicarbon monoxide (C2O),[72][73] carbon trioxide (CO3),[74][75] cyclopentanepentone (C5O5)[76] cyclohexanehexone (C6O6),[76] and mellitic anhydride (C12O9).

With reactive metals, such as tungsten, carbon forms either carbides (C4−), or acetylides (C) to form alloys with high melting points. These anions are also associated with methane and acetylene, both very weak acids. With an electronegativity of 2.5,[77] carbon prefers to form covalent bonds. A few carbides are covalent lattices, like carborundum (SiC), which resembles diamond.

Organometallic compounds sunting

Artikel utama: Organometallic chemistry

Organometallic compounds by definition contain at least one carbon-metal bond. A wide range of such compounds exist; major classes include simple alkyl-metal compounds (for example, tetraethyllead), η2-alkene compounds (for example, Zeise's salt), and η3-allyl compounds (for example, allylpalladium chloride dimer); metallocenes containing cyclopentadienyl ligands (for example, ferrocene); and transition metal carbene complexes. Many metal carbonyls exist (for example, tetracarbonylnickel); some workers consider the carbon monoxide ligand to be purely inorganic, and not organometallic.

While carbon is understood to exclusively form four bonds, an interesting compound containing an octahedral hexacoordinated carbon atom has been reported. The cation of the compound is [(Ph3PAu)6C]2+. This phenomenon has been attributed to the aurophilicity of the gold ligands.[78]

History and etymology sunting

 
Antoine Lavoisier in his youth

The English name carbon comes from the Latin carbo for coal and charcoal,[79] whence also comes the French charbon, meaning charcoal. In German, Dutch and Danish, the names for carbon are Kohlenstoff, koolstof and kulstof respectively, all literally meaning coal-substance.

Carbon was discovered in prehistory and was known in the forms of soot and charcoal to the earliest human civilizations. Diamonds were known probably as early as 2500 BCE in China, while carbon in the form of charcoal was made around Roman times by the same chemistry as it is today, by heating wood in a pyramid covered with clay to exclude air.[80][81]

 
Carl Wilhelm Scheele

In 1722, René Antoine Ferchault de Réaumur demonstrated that iron was transformed into steel through the absorption of some substance, now known to be carbon.[82] In 1772, Antoine Lavoisier showed that diamonds are a form of carbon; when he burned samples of charcoal and diamond and found that neither produced any water and that both released the same amount of carbon dioxide per gram. In 1779,[83] Carl Wilhelm Scheele showed that graphite, which had been thought of as a form of lead, was instead identical with charcoal but with a small admixture of iron, and that it gave "aerial acid" (his name for carbon dioxide) when oxidized with nitric acid.[84] In 1786, the French scientists Claude Louis Berthollet, Gaspard Monge and C. A. Vandermonde confirmed that graphite was mostly carbon by oxidizing it in oxygen in much the same way Lavoisier had done with diamond.[85] Some iron again was left, which the French scientists thought was necessary to the graphite structure. In their publication they proposed the name carbone (Latin carbonum) for the element in graphite which was given off as a gas upon burning graphite. Antoine Lavoisier then listed carbon as an element in his 1789 textbook.[86]

A new allotrope of carbon, fullerene, that was discovered in 1985[87] includes nanostructured forms such as buckyballs and nanotubes.[26] Their discoverers – Robert Curl, Harold Kroto and Richard Smalley – received the Nobel Prize in Chemistry in 1996.[88] The resulting renewed interest in new forms lead to the discovery of further exotic allotropes, including glassy carbon, and the realization that "amorphous carbon" is not strictly amorphous.[33] The developement of carbon technology was very slow, but by the late 1960s it gave a boost.

Production sunting

Graphite sunting

Artikel utama: Graphite

Commercially viable natural deposits of graphite occur in many parts of the world, but the most important sources economically are in China, India, Brazil and North Korea. Graphite deposits are of metamorphic origin, found in association with quartz, mica and feldspars in schists, gneisses and metamorphosed sandstones and limestone as lenses or veins, sometimes of a meter or more in thickness. Deposits of graphite in Borrowdale, Cumberland, England were at first of sufficient size and purity that, until the 19th century, pencils were made simply by sawing blocks of natural graphite into strips before encasing the strips in wood. Today, smaller deposits of graphite are obtained by crushing the parent rock and floating the lighter graphite out on water.[89]

There are three types of natural graphite—amorphous, flake or crystalline flake, and vein or lump. Amorphous graphite is the lowest quality and most abundant. Contrary to science, in industry "amorphous" refers to very small crystal size rather than complete lack of crystal structure. Amorphous is used for lower value graphite products and is the lowest priced graphite. Large amorphous graphite deposits are found in China, Europe, Mexico and the United States. Flake graphite is less common and of higher quality than amorphous; it occurs as separate plates that crystallized in metamorphic rock. Flake graphite can be four times the price of amorphous. Good quality flakes can be processed into expandable graphite for many uses, such as flame retardants. The foremost deposits are found in Austria, Brazil, Canada, China, Germany and Madagascar. Vein or lump graphite is the rarest, most valuable, and highest quality type of natural graphite. It occurs in veins along intrusive contacts in solid lumps, and it is only commercially mined in Sri Lanka.[89]

According to the USGS, world production of natural graphite was 1.1 million tonnes in 2010, to which China contributed 800,000 t, India 130,000 t, Brazil 76,000 t, North Korea 30,000 t and Canada 25,000 t. No natural graphite was reported mined in the United States, but 118,000 t of synthetic graphite with an estimated value of $998 million was produced in 2009.[89]

Diamond sunting

Artikel utama: Diamond
 
Diamond output in 2005

The diamond supply chain is controlled by a limited number of powerful businesses, and is also highly concentrated in a small number of locations around the world (see figure).

Only a very small fraction of the diamond ore consists of actual diamonds. The ore is crushed, during which care has to be taken in order to prevent larger diamonds from being destroyed in this process and subsequently the particles are sorted by density. Today, diamonds are located in the diamond-rich density fraction with the help of X-ray fluorescence, after which the final sorting steps are done by hand. Before the use of X-rays became commonplace, the separation was done with grease belts; diamonds have a stronger tendency to stick to grease than the other minerals in the ore.[90]

Historically diamonds were known to be found only in alluvial deposits in southern India.[91] India led the world in diamond production from the time of their discovery in approximately the 9th century BCE[92] to the mid-18th century AD, but the commercial potential of these sources had been exhausted by the late 18th century and at that time India was eclipsed by Brazil where the first non-Indian diamonds were found in 1725.[93]

Diamond production of primary deposits (kimberlites and lamproites) only started in the 1870s after the discovery of the Diamond fields in South Africa. Production has increased over time and now an accumulated total of 4.5 billion carats have been mined since that date.[94] About 20% of that amount has been mined in the last 5 years alone, and during the last ten years 9 new mines have started production while 4 more are waiting to be opened soon. Most of these mines are located in Canada, Zimbabwe, Angola, and one in Russia.[94]

In the United States, diamonds have been found in Arkansas, Colorado and Montana.[95][96] In 2004, a startling discovery of a microscopic diamond in the United States[97] led to the January 2008 bulk-sampling of kimberlite pipes in a remote part of Montana.[98]

Today, most commercially viable diamond deposits are in Russia, Botswana, Australia and the Democratic Republic of Congo.[99] In 2005, Russia produced almost one-fifth of the global diamond output, reports the British Geological Survey. Australia has the richest diamantiferous pipe with production reaching peak levels of 42 ton metrik (41 ton panjang; 46 ton pendek) per year in the 1990s.[95] There are also commercial deposits being actively mined in the Northwest Territories of Canada, Siberia (mostly in Yakutia territory; for example, Mir pipe and Udachnaya pipe), Brazil, and in Northern and Western Australia.

Applications sunting

 
Pencil leads for mechanical pencils are made of graphite (often mixed with a clay or synthetic binder).
 
Sticks of vine and compressed charcoal.
 
A cloth of woven carbon fibres
 
Silicon carbide single crystal
 
The C60 fullerene in crystalline form
 
Tungsten carbide milling bits

Carbon is essential to all known living systems, and without it life as we know it could not exist (see alternative biochemistry). The major economic use of carbon other than food and wood is in the form of hydrocarbons, most notably the fossil fuel methane gas and crude oil (petroleum). Crude oil is used by the petrochemical industry to produce, amongst other things, gasoline and kerosene, through a distillation process, in refineries. Cellulose is a natural, carbon-containing polymer produced by plants in the form of cotton, linen, and hemp. Cellulose is mainly used for maintaining structure in plants. Commercially valuable carbon polymers of animal origin include wool, cashmere and silk. Plastics are made from synthetic carbon polymers, often with oxygen and nitrogen atoms included at regular intervals in the main polymer chain. The raw materials for many of these synthetic substances come from crude oil.

The uses of carbon and its compounds are extremely varied. It can form alloys with iron, of which the most common is carbon steel. Graphite is combined with clays to form the 'lead' used in pencils used for writing and drawing. It is also used as a lubricant and a pigment, as a molding material in glass manufacture, in electrodes for dry batteries and in electroplating and electroforming, in brushes for electric motors and as a neutron moderator in nuclear reactors.

Charcoal is used as a drawing material in artwork, for grilling, and in many other uses including iron smelting. Wood, coal and oil are used as fuel for production of energy and space heating. Gem quality diamond is used in jewelry, and industrial diamonds are used in drilling, cutting and polishing tools for machining metals and stone. Plastics are made from fossil hydrocarbons, and carbon fiber, made by pyrolysis of synthetic polyester fibers is used to reinforce plastics to form advanced, lightweight composite materials. Carbon fiber is made by pyrolysis of extruded and stretched filaments of polyacrylonitrile (PAN) and other organic substances. The crystallographic structure and mechanical properties of the fiber depend on the type of starting material, and on the subsequent processing. Carbon fibers made from PAN have structure resembling narrow filaments of graphite, but thermal processing may re-order the structure into a continuous rolled sheet. The result is fibers with higher specific tensile strength than steel.[100]

Carbon black is used as the black pigment in printing ink, artist's oil paint and water colours, carbon paper, automotive finishes, India ink and laser printer toner. Carbon black is also used as a filler in rubber products such as tyres and in plastic compounds. Activated charcoal is used as an absorbent and adsorbent in filter material in applications as diverse as gas masks, water purification and kitchen extractor hoods and in medicine to absorb toxins, poisons, or gases from the digestive system. Carbon is used in chemical reduction at high temperatures. Coke is used to reduce iron ore into iron. Case hardening of steel is achieved by heating finished steel components in carbon powder. Carbides of silicon, tungsten, boron and titanium, are among the hardest known materials, and are used as abrasives in cutting and grinding tools. Carbon compounds make up most of the materials used in clothing, such as natural and synthetic textiles and leather, and almost all of the interior surfaces in the built environment other than glass, stone and metal.

Diamonds sunting

The diamond industry can be broadly separated into two basically distinct categories: one dealing with gem-grade diamonds and another for industrial-grade diamonds. While a large trade in both types of diamonds exists, the two markets act in dramatically different ways.

A large trade in gem-grade diamonds exists. Unlike precious metals such as gold or platinum, gem diamonds do not trade as a commodity: there is a substantial mark-up in the sale of diamonds, and there is not a very active market for resale of diamonds.

The market for industrial-grade diamonds operates much differently from its gem-grade counterpart. Industrial diamonds are valued mostly for their hardness and heat conductivity, making many of the gemological characteristics of diamond, including clarity and color, mostly irrelevant. This helps explain why 80% of mined diamonds (equal to about 100 million carats or 20 tonnes annually), unsuitable for use as gemstones and known as bort, are destined for industrial use.[101] In addition to mined diamonds, synthetic diamonds found industrial applications almost immediately after their invention in the 1950s; another 3 billion carats (600 tonnes) of synthetic diamond is produced annually for industrial use.[102] The dominant industrial use of diamond is in cutting, drilling, grinding, and polishing. Most uses of diamonds in these technologies do not require large diamonds; in fact, most diamonds that are gem-quality except for their small size, can find an industrial use. Diamonds are embedded in drill tips or saw blades, or ground into a powder for use in grinding and polishing applications.[103] Specialized applications include use in laboratories as containment for high pressure experiments (see diamond anvil cell), high-performance bearings, and limited use in specialized windows.[104][105] With the continuing advances being made in the production of synthetic diamonds, future applications are beginning to become feasible. Garnering much excitement is the possible use of diamond as a semiconductor suitable to build microchips from, or the use of diamond as a heat sink in electronics.[106]

Precautions sunting

 
Worker at carbon black plant in Sunray, Texas (photo by John Vachon, 1942)

Pure carbon has extremely low toxicity to humans and can be handled and even ingested safely in the form of graphite or charcoal. It is resistant to dissolution or chemical attack, even in the acidic contents of the digestive tract, for example. Consequently, once it enters into the body's tissues it is likely to remain there indefinitely. Carbon black was probably one of the first pigments to be used for tattooing, and Ötzi the Iceman was found to have carbon tattoos that survived during his life and for 5200 years after his death.[107] Inhalation of coal dust or soot (carbon black) in large quantities can be dangerous, irritating lung tissues and causing the congestive lung disease coalworker's pneumoconiosis. Similarly, diamond dust used as an abrasive can do harm if ingested or inhaled. Microparticles of carbon are produced in diesel engine exhaust fumes, and may accumulate in the lungs.[108] In these examples, the harmful effects may result from contamination of the carbon particles, with organic chemicals or heavy metals for example, rather than from the carbon itself.

Carbon generally has low toxicity to almost all life on Earth; but carbon nanoparticles are deadly to Drosophila.[109]

Carbon may also burn vigorously and brightly in the presence of air at high temperatures. Large accumulations of coal, which have remained inert for hundreds of millions of years in the absence of oxygen, may spontaneously combust when exposed to air, for example in coal mine waste tips.

In nuclear applications where graphite is used as a neutron moderator, accumulation of Wigner energy followed by a sudden, spontaneous release may occur. Annealing to at least 250 °C can release the energy safely, although in the Windscale fire the procedure went wrong, causing other reactor materials to combust.

The great variety of carbon compounds include such lethal poisons as tetrodotoxin, the lectin ricin from seeds of the castor oil plant Ricinus communis, cyanide (CN−) and carbon monoxide; and such essentials to life as glucose and protein.

Bonding to carbon sunting

See also sunting

 
Buku: Period 2 elements

Buku: Chemical elements (sorted alphabetically)

Buku: Chemical elements (sorted by number)

Buku Wikipedia adalah koleksi artikel yang bisa diunduh atau dipesan dalam bentuk cetak.

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Tabel periodik
(besar)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1 H He
2 Li Be B C N O F Ne
3 Na Mg Al Si P S Cl Ar
4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
6 Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
7 Fr Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og
Logam alkali Logam alkali tanah Lan­tanida Aktinida Logam transisi Logam miskin Metaloid Nonlogam poliatomik Nonlogam diatomik Gas mulia Sifat kimia
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