For an object to be transparent, most of the light that hits it must pass through it; the light can be neither reflected or absorbed. For a diamond to absorb light, a photon would need to move an electron from an low energy bonding molecular orbital to a high energy anti-bonding orbital. Based on this thinking, we immediately conclude that there is something different between bonds holding C atoms together in diamond from the bonds holding C atoms together in graphite, even though we do not know, at this point, what it could be.
An important point to consider here is that we have described the bonding in carbon using two different models hybridization and molecular orbital. While this may be a bit! Typically we use the simplest model that will allow us to explain and predict the phenomenon we are interested in. Usually the bonding in carbon is described using the hybrid orbital model, because it is highly predictive and easier to use in practice.
Different allotropes that is different forms of the same element can have quite different properties. Diamond is hard, transparent, and does not conduct electricity. How can this be possible if both are pure carbon? The answer lies in how the carbon atoms are organized with respect to one another. While the carbon atoms in diamond form a three-dimensional network, in graphite, the atoms are organized in two-dimensional sheets that stack one on top of the other.
Within each two dimensional sheet, the carbon atoms are linked via covalent bonds in an extended array of six-membered rings. This means that the carbon sheets are very strongly bonded, but the interactions between sheets are much weaker.
While there are no covalent bonds between the sheets, the atoms of the sheets do interact via London dispersion forces, very much like the interactions that hold He atoms together.
Because the sheets interact over very much larger surface areas, however, these interactions are much stronger than those in helium. Yet another allotrope of carbon, graphene, consists of a single sheet of carbon atoms, these sheets can be rolled into tubes to form nanotubes, which are the subject of a great deal of interest because of their inherently high tensile strength.
Carbon atoms can also form spherical molecules, known as buckminsterfullerenes or buckyballs. Why are the patterns of covalent bonding so different, three-dimensional tetrahedral in diamond, with each carbon bonded to four others, and two-dimensional planar in graphite and graphene, with each carbon atom bonded to only three others?
As we discussed above, to form the four bonds attached to each carbon atom in diamond, we needed to hybridize four atomic orbitals to form four bonding orbitals. This is not exactly true. We do indeed use a model in which we consider that only three atomic orbitals are hybridized — an s and 2 p orbitals in order to form three sp2 bonding orbitals. Perhaps surprisingly, there is no good answer for why carbon takes up different forms — except maybe because it can.
But in fact, carbon does form four bonds in graphite carbon almost always forms four bonds - a central principle of organic chemistry. The trick is that the four bonds are not always equivalent; in graphite, the fourth bond is not formed by the sp2 bonding orbitals, but rather involves an unhybridized 2p atomic orbital. These p orbitals stick out at right angles to the sheet and can overlap with p orbitals from adjacent carbons in the same sheet. Remember that p orbitals have two regions of electron density.
So before we delve further into the properties associated with graphite, let us take a look at bonding in metals. General Intellingence and Reasoning. Current Affairs. Elementary Mathematics. English Literature. A Silver.
C Aluminium. D Sodium. Login to Bookmark. Previous Question. Next Question. Report Error. Add Bookmark View My Bookmarks. Gobar gas contains mainly : A. Match the following : A. Aluminium 1. Monazite B. Uranium 2. Pith blende C. Thorium 3. Relative supply risk 4. Relative supply risk 6. Relative supply risk 8. Young's modulus A measure of the stiffness of a substance. Shear modulus A measure of how difficult it is to deform a material. Bulk modulus A measure of how difficult it is to compress a substance.
Vapour pressure A measure of the propensity of a substance to evaporate. Pressure and temperature data — advanced. Listen to Carbon Podcast Transcript :. You're listening to Chemistry in its element brought to you by Chemistry World , the magazine of the Royal Society of Chemistry. Hello, this week to the element that unites weddings, wars, conflicts and cremations and to explain how, here's Katherine Holt. Any chemist could talk for days about carbon.
It is after all an everyday, run-of-the-mill, found-in-pretty-much-everything, ubiquitous element for us carbon-based life forms. An entire branch of chemistry is devoted to its reactions. In its elemental form it throws up some surprises in the contrasting and fascinating forms of its allotropes. It seems that every few years a new form of carbon comes into fashion - A few years ago carbon nanotubes were the new black or should I say 'the new bucky-ball' - but graphene is oh-so-now!
But today I'm going to talk about the most glamorous form that carbon can take - diamond. For millennia diamond has been associated with wealth and riches, as it can be cut to form gemstones of high clarity, brilliance and permanence. Diamonds truly are forever! Unfortunately, diamond also has a dark side - the greed that diamond induces leads to the trade of so-called 'conflict diamonds' that support and fund civil wars. Mans desire for diamond has led alchemists and chemists over many centuries to attempt to synthesise the material.
After many fraudulous early claims diamond was finally synthesised artificially in the s. Scientists took their inspiration from nature by noting the conditions under which diamond is formed naturally, deep under the earth's crust. This was an impressive feat, but the extreme conditions required made it prohibitively expensive as a commercial process. Since then the process has been refined and the use of metal catalysts means that lower temperatures and pressures are required.
Crystals of a few micron diameter can be formed in a few minutes, but a 2-carat gem quality crystal may takes several weeks. These techniques mean its now possible to artificially synthesise gemstone quality diamonds which, without the help of specialist equipment, cannot be distinguished from natural diamond. It goes without saying that this could cause headaches among the companies that trade in natural diamond! It is possible to turn any carbon based material into a diamond - including hair and even cremating remains!
Yes - you can turn your dearly departed pet into a diamond to keep forever if you want to! Artificial diamonds are chemically and physical identical to the natural stones and come without the ethical baggage.
However, psychologically their remains a barrier - if he really loves you he'd buy you real diamond - wouldn't he? From the perspective of a chemist, materials scientist or engineer we soon run out of superlatives while describing the amazing physical, electronic and chemical properties of diamond. It is the hardest material known to man and more or less inert - able to withstand the strongest and most corrosive of acids.
It has the highest thermal conductivity of any material, so is excellent at dissipating heat. That is why diamonds are always cold to the touch. Having a wide band gap, it is the text book example of an insulating material and for the same reason has amazing transparency and optical properties over the widest range of wavelengths of any solid material.
You can see then why diamond is exciting to scientists. Its hardness and inert nature suggest applications as protective coatings against abrasion, chemical corrosion and radiation damage.
Its high thermal conductivity and electrical insulation cry out for uses in high powered electronics. Its optical properties are ideal for windows and lenses and its biocompatibility could be exploited in coatings for implants. These properties have been known for centuries - so why then is the use of diamond not more widespread? The reason is that natural diamond and diamonds formed by high pressure high temperature synthesis are of limited size - usually a few millimeters at most, and can only be cut and shaped along specific crystal faces.
This prevents the use of diamond in most of the suggested applications. However, about 20 years ago scientists discovered a new way to synthesise diamond this time under low pressure, high temperature conditions, using chemical vapour deposition. If one were to consider the thermodynamic stability of carbon, we would find that at room temperature and pressure the most stable form of carbon is actually graphite, not diamond.
Strictly speaking, from a purely energetic or thermodynamic point of view, diamond should spontaneously turn into graphite under ambient conditions! Clearly this doesn't happen and that is because the energy required to break the strong bonds in diamond and rearrange them to form graphite requires a large input of energy and so the whole process is so slow that on the scale of millennia the reaction does not take place.
It is this metastability of diamond that is exploited in chemical vapour deposition. The carbon-based molecules then deposit on a surface to form a coating or thin film of diamond.
Actually both graphite and diamond are initially formed, but under these highly reactive conditions, the graphitic deposits are etched off the surface, leaving only the diamond.
The films are polycrystalline, consisting of crystallites in the micron size range so lack the clarity and brilliance of gemstone diamond.
While they may not be as pretty, these diamond films can be deposited on a range of surfaces of different size and shapes and so hugely increase the potential applications of diamond.
Challenges still remain to understand the complex chemistry of the intercrystalline boundaries and surface chemistry of the films and to learn how best to exploit them. This material will be keeping chemists, materials scientists, physicists and engineers busy for many years to come. However, at present we can all agree that there is more to diamond than just a pretty face!
Katherine Holt extolling the virtues of the jewel in carbon's crown. Next week we're heading to the top of group one to hear the story of the metal that revolutionised the treatment of manic depression. Its calming effect on the brain was first noted in , by an Australian doctor, John Cade, of the Victoria Department of Mental Hygiene.
He had injected guinea pigs with a 0. Cade then gave his most mentally disturbed patient an injection of the same solution. The man responded so well that within days he was transferred to a normal hospital ward and was soon back at work.
And it's still used today although despite 50 years of medical progress we still don't know how it works. That was Matt Wilkinson who will be here with the story of Lithium on next week's Chemistry in its Element, I do hope you can join us.
I'm Chris Smith, thank you for listening and goodbye. Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.
There's more information and other episodes of Chemistry in its element on our website at chemistryworld. Click here to view videos about Carbon. View videos about. Help Text. Learn Chemistry : Your single route to hundreds of free-to-access chemistry teaching resources. We hope that you enjoy your visit to this Site. We welcome your feedback. Data W. Haynes, ed. Version 1. Coursey, D. Schwab, J. Tsai, and R. Dragoset, Atomic Weights and Isotopic Compositions version 4.
Periodic Table of Videos , accessed December Podcasts Produced by The Naked Scientists. Download our free Periodic Table app for mobile phones and tablets. Explore all elements. D Dysprosium Dubnium Darmstadtium. E Europium Erbium Einsteinium. F Fluorine Francium Fermium Flerovium. G Gallium Germanium Gadolinium Gold. I Iron Indium Iodine Iridium. K Krypton. O Oxygen Osmium Oganesson. U Uranium. V Vanadium. X Xenon. Y Yttrium Ytterbium.
0コメント