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78

Hardness and toughness are not the same Hardness and toughness are very different qualities in materials and are weakly related. Hardness is strongly related to the more well-defined quantity of stiffness which measures how easily a compound can be deformed under stress. Glass and diamond are very stiff materials, for example. If you try to poke them with ...


41

how long would it take for this super-material to convert to the stuff I scribble with? No, despite the fact that James Bond said "Diamonds are Forever", that is not exactly the case. Although Bond's statement is a fair approximation of reality it is not a scientifically accurate description of reality. As we'll soon see, even though diamond is slightly ...


32

Aromaticity is not binary, but rather there are degrees of aromaticity. The degree of aromaticity in benzene is large, whereas the spiro-aromaticity in [4.4]nonatetraene is relatively small. The aromaticity in naphthalene is not twice that of benzene. Aromaticity has come to mean a stabilization resulting from p-orbital (although other orbitals can also be ...


26

Liquid carbon does indeed exist, but perhaps surprisingly, relatively little is known about it. It only exists above around $4000\ \mathrm{K}$ and $100\ \mathrm{atm}$, which are not trivial conditions to sustain and probe. There certainly are many theoretical studies into the properties of liquid carbon, though. You can find a phase diagram for carbon here, ...


25

Diamond is one of the best thermal conductors known, in fact diamond is a better thermal conductor than many metals (thermal conductivity (W/m-K): aluminum=237, copper=401, diamond=895). The carbon atoms in diamond are $\ce{sp^3}$ hybridized and every carbon is bonded to 4 other carbon atoms located at the vertices of a tetrahedron. Hence the bonding in ...


25

Diamond is a covalent network solid, like a number of other common materials (quartz, graphite, glass, and a whole bunch of stuff). Because they are not discrete molecules - there is no 'diamond' molecule the same way there are molecules of caffeine, benzoic acid, citric acid, N,N-dimethylaminopyridine, etc. - network solids form one of the two main classes ...


23

The first thing to say is that I'm not sure where that image is taken from; it's neither in the original article nor in the supporting information to the article. Therefore, it appears to be more of an "artist's impression" rather than an actual atomic force microscopy (AFM) image, which is what was reported in the paper. Nevertheless, the actual AFM images ...


21

Chemical structures are a tradeoff of several factors, including the conditions on how they were formed. The stability of any given chemical structure depends on the ease with which any specific reaction can turn it into something else. Both graphite and diamond are very stable structures which basically means they are hard to easily convert into something ...


20

Good catch - you are absolutely correct. This is a mistake in the Wikipedia page, as a quick check with the reference listed there confirms. The values are the wrong way around. Changing it now... Edited to add: this of course raises the question of why it should be this way around. A somewhat hand-wavy answer is that, just like in benzene, the $p_z$ ...


20

Diamond has dangling bonds on the outer surface of the crystal for pretty much the same reason as graphite. If you understood graphite differently, then you understood it wrong. See, a molecule of oxygen contains 2 atoms, a molecule of sulfur has 8; but how many atoms are there in a "molecule" of diamond or graphite? Try drawing one to the end, so as to ...


18

In the hexagonal graphite structure the carbon atoms are $\ce{sp^2}$ hybridized, just like in benzene. In graphite, this p-orbital is used for bonding just as it is in benzene, resulting in an extended pi system in the graphite structure. Graphite is really just a large number of benzene rings annelated together to form a continuous, ring structure. Just ...


17

Ron's answer is great, but I'd just like to touch on the mechanisms behind thermal conductivity so we can rationalize the differences between the behaviour of diamond, graphite, and metals: There are two ways in which heat is transmitted through solids: phonons and electronic conductivity. The latter occurs in electrically conductive solids, where ...


17

Graphite is definitely aromatic and boron nitride is at least partially aromatic. For instance, in this paper, the authors calculate the percent resonance energy (%RE) of graphite as a comparison to other known aromatic systems. The %RE is a measure of the total resonance stabilization per carbon atom relative to some reference system. They find the %RE of ...


16

There are two key factors that account for the ubiquity of carbon compounds. Bond strengths: Look at the following table of bond strengths and notice how the strength of both carbon-carbon single and double bonds is much greater than the bond strengths found in other molecules. \begin{array}\hline Typical~Single ~Bond~ Dissociation ~Energies~ (kcal/mole)\\ ...


16

TL;DR: Isolated derivatives of cyclo-$\ce{C18}$ most likely have the smallest isolated cumulenic 18-membered ring, though upon direct complexation of transition metal in/outside the ring the strain as well as its size can be reduced further. In the past two decades a research group of Prof. Dr. François Diederich performed numerous studies on cyclo[n]...


16

Yes, diamond will combust in air. Regardless of the ambient air temperature, e.g. your example of $21\ \mathrm{^\circ C}$, you of course have to heat it to it's ignition temperature somehow, whether in a furnace, a flame, etc. The autoignition temperature for diamond is around $900\ \mathrm{^\circ C}$ (source 1, source 2), compared to about $730\ \mathrm{^\...


15

There are quite a few conjugated $\pi$-systems out there; some of them are pretty stable, some are less so. The ultimate way to find out is to go and solve the eigenvalues problem for the corresponding matrix; in some cases this can be done manually, with pen and paper. There are simple cases with analytical solutions. One of them is the case of cyclic ...


13

I am sorry to say, but your question contains several false assumptions, most importantly: Nanotubes are good conductors: No, not all of them. Certain types have metallic/semi-metallic conductance, others are semiconductors. Single wall carbon nanotubes are generally indexed with a so-called chirality index (m,n) that tells you how the carbon sheet is ...


13

Diamond has cleavage planes. If you want something nearly unbreakable, try nephrite, which is a tough form of jade used by the ancient Aztecs to make axe heads! Actinolite is another related "tough as steel" mineral. These minerals are made up of interlocking strands (actinolite) or microscopic fibers (nephrite). But diamond is a regular geometric lattice, ...


12

Well, let's see. This is not quite a chemical question, though I wouldn't recommend moving it to Math.SE either, because they might have hard time recognizing what's $\ce{C60}$. Now, let's imagine we're considering locally planar conjugated carbon structures. Each carbon has to reserve one $p$ orbital for $\pi$-bonding, which leaves it with 3 bonds. Now, ...


11

You are on the right track - diamond is not the thermodynamically stable carbon phase at STP. Taking two figures from A.T. Dinsdale, 'SGTE Data for Pure Elements', CALPHAD 15(4) 317-425 (1991) one sees: and Since graphite is the thermodynamically stable phase of carbon at STP, it is usually selected as the reference phase so it has $\Delta H^0_f = 0$. In ...


11

The trick with graphene is that a lot of its amazing properties only work when you have continuous perfect sheets of it, and making graphene like this is currently beyond us, for large scales anyways. It is true that graphene has very high electron mobility $\approx10^{5}~\mathrm{cm^2/Vs}$ at room temperature, which works out to on the order of $10~\mathrm{n\...


11

At normal room temperature and pressure graphite is (slightly) more stable than diamond. But the melting point is not a good indicator of this. Melting points are determined by the bonding structure of the solid and any potential liquid (indeed there may not be a liquid phase at some pressures). They don't necessarily tell you whether a different bonding ...


10

Yes, both graphite and borazine are aromatic in nature as each ring of a plane have six π-electrons (similar to benzene). This aromaticity explains why graphite and borazine is unusually thermodynamically more stable than diamond. That's why graphite have more meting point than diamond and on pyrolysis of diamond (unstable), it turns to graphite (more ...


10

It might be scientifically correct but it is linguistically misleading The sentence "diamond is an element" can be seen to be misleading when compared to the sentence "diamond is an allotrope of the element carbon". Or even "diamond consists of the element carbon". The issue is that clear language should distinguish between the form and the composition of ...


9

In short, graphite is several graphene sheets piled one above another. Graphene is made up of one single sheet of carbon atoms arranged in hexagonal pattern (like a honeycomb), and graphite are several such sheets, each sheet linked to another by weak intermolecular forces, which gives the graphite its lubricative properties. EDIT: Similarly, graphene ...


9

First off, take a look at the Wikipedia article: Synthetic diamond. Basically, there are two ways to artificially generate the heat and pressure needed. First is the "conventional" high-temperature, high-pressure method, which uses a press to put a sample of pure carbon graphite under pressure of about 5 gigapascals (1GPa = about 10,000 atmospheres or about ...


9

This questions calls for an answer from thermodynamics. The figure provided above, lifted from here, is what we call a phase diagram. On the abscissa is temperature in units of Kelvin and on the ordinate is the pressure given in units of Gigapascals. (For your own reference, 1 GPa is nearly 10,000 times the pressure we live under, the earth's core is ...


9

It has been done since 1954 and has been a commercial success for some time. Many products now have diamond-like coatings, produced comparatively inexpensively through vapor deposition without need for high pressure. BTW, in English "burning" means to set on fire, oxidize; I believe you mean "heating graphite", and, as your graph shows, pressure is needed ...


8

It does appear that the graphene sheets fold back on themselves. This paper reports that the graphene sheets that make up both natural and synthetic graphite, double back over each other with "nano-arches". This is an from the paper. There are also SEM and TEM micrographs of a graphite crystal that are pretty interesting.


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