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39

First, a note: while oxygen has fewer allotropes than sulfur, it sure has more than two! These include $\ce{O}$, $\ce{O_2}$, $\ce{O_3}$, $\ce{O_4}$, $\ce{O_8}$, metallic $\ce{O}$ and four other solid phases. Many of these actually have a corresponding sulfur variant. However, you are right in a sense that sulfur has more tendency to catenate… let's try to ...

23

Interestingly, nobody addressed the reason why diamonds are hard in the first place. The pressure (and temperature) are not the reason why they're hard, only the reason why they are formed. The diamonds are hard because the carbon atoms are bonded together by sigma ($sp^3$) bonds, which are the strongest chemical bonds. Other materials exhibiting the same ...

21

Here are some compounds that have other structures, followed by their hardest structure (based on Moh's Scale). Titanium dioxide: Rutile structure or Cotunnite structure Aluminum oxide: Corundum Silicon Oxide: Stishovite Boron Nitride: Wurtzite Boron Nitride There are many, many more.

16

There has been recent research on mercury dimers in its ground state. Bond energies, transition energies, band spectrum and other spectroscopic parameters has been calculated. Here are the abstracts of two papers on mercury dimer: The potential energy curve of the ground electronic state of the $\ce{Hg}$ dimer has been calculated using the CCSD(T) ...

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 ...

8

I searched CCDC for the infinite structures with the following building block: where where each vertex contains any non-metal atom linked by any type of bond. The only known crystal structure that would meet these criteria is $\ce{[B12]2[CBC][C2]Mg_{1.42}}$ where $\ce{B12}$ icosahedra are linked via $\ce{B–C}$ and $\ce{B–B}$ bonds forming the moieties of ...

8

It's possible, and it's already been done! The compound is called Silicene. However, silicene generally isn't perfectly flat. (Then again, as Karl Ratzsch points out in the comments, graphene isn't perfectly flat either). The Wikipedia article notes that it usually takes a puckered shape. I don't know the reasons for the puckered shape, but I recall being ...

7

Bismuth certainly has other known crystal structures at elevated pressures and temperatures, at least 4 others in addition to the rhombohedral structure stable at room temperature and pressure. One place to start would be an article from NIF on shock physics of bismuth. For a scholarly article, you might start with 'Phase Diagrams of Arsenic, Antimony, and ...

7

I think the key to this question lies in the understanding of the difference between thermodynamic and kinetic stability. In this post, I find Thomij's answer the most rigorous and enlightening. From this you should learn, that the thermodynamically most stable allotrope corresponds to the global minimum on the potential energy surface, while a kinetically ...

7

Allotropy is a special case of polymorphism. Polymorphism is the phenomenon of a substance exhibiting different crystal structures. Allotropism is the same phenomenon limited to the subset of all substances that contains only the chemical elements. According to the Wikipedia article on polymorphism: Polymorphism ... is the ability of a solid material to ...

7

Yes, $\ce{Hg2}$ has a bond length of $\pu{0.334nm}$ and a dissociation energy of $\pu{7.5 kJ/mol}$. See Mercury Handbook: Chemistry, Applications and Environmental Impact at page 10. and Mass spectrometric equilibrium study of the molecule $\ce{Hg2}$ J. Chem. Phys. 1982, 77(3), 1425-1427 (https://doi.org/10.1063/1.443968).

7

To quote my answer to Is a diamond a single molecule?: 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-...

5

The nature of mercury associates was studied. Quick googling gave paper with words "$\ce{Hg_x}$ cluster transition from VdW to metallic bahavior between 20 and 70 atoms", suggesting that $\ce{Hg_2}$ associate, if exists, have VdW bonding (i.e. not a chemical, but physical bond) only.

5

Some peer review journal articles and an India high school textbook and various study guides do refer to the spin isomers as allotropes. There are physical differences such as different heat capacities. See Orthohydrogen, Parahydrogen and Heavy Hydrogen for comprehensive information. However, aside from the spin isomers, there are genuine allotropes ...

5

You might want to read up on this Wikipedia article My answer is a bit oversimplified, but I think it would serve your purpose. When you refer to a chemical element, as it is, you'd be referring to one of the 118 elements in the Periodic Table. Something's a chemical element if it can't be broken down into more atoms that can be located on the periodic ...

5

Does a neutral dimercury molecule ($\ce{Hg2}$) exist? Simple answer is yes. But when I look at the answers given so far, a simple answer might not be good enough without giving some evidence. So, I decided to dig into the question a little deeper. I'm not a physical chemist, but can't resist the data I have seen in review and research articles for this ...

5

Liquid phosphorus is white phosphorus. It melts at 44.1 °C. Red phosphorus does not melt. It burns at 200 °C and sublimes in an argon atmosphere at 280 °C, without melting.

4

$\alpha$Sn and $\beta$Sn are the two solid allotropes of Sn. As you not, below 13C the stable phase is $\alpha$Sn, which has a diamond cubic crystal structure (like diamond, Si, and Ge) and is a semi-metal. Above 13C, the thermodynamically stable phase is $\beta$Sn, a body-centered tetragonal crystal. So, cycling back and forth around 13C varies the ...

4

$\ce{O3}$ is not a chemical element, but is a molecular form or allotrope of an element. Sulfur, for example, exists in different allotropes: puckered rings of $\ce{S8}$, a red triatomic ($\ce{S3}$) gaseous form, similar to ozone, etc. Oxygen can exist temporarily in monatomic form, as ordinary diatomic oxygen in air, as triatomic ozone, even as $\ce{O8}$, ...

3

White phosphorus has P4 molecules packed into a crystal,these dissolve readily in Carbon Disulfide. Whereas Red phosphorus is polymeric in nature. It is a derivative of white phosphorus where one P-P bond is broken and an addtional bond is formed with neighboring tetrahedron molecule resulting in a chain like structure . Like all long chain polymers this ...

3

In phosphorus’ case, the connection between stability of the allotrope and its colour is only accidental. White phosphorus, i.e. $\ce{P4}$ molecules, has very strained bonds because of the very small tetrahedral angles the atoms need to accomodate for. Going by the edges of a regular tetrahedron, the bond angles are $60^\circ$ which is very strained. This ...

2

In my own work, I've never used the term polymorph for anything but molecular crystals in a highly but differently ordered solid state. In the case of crystal solids, it seems clear that allotropy is a particular case of polymorphism. Yes, allotropy is the polymorphism of elements in the same state. This does not mean that each solid allotrope has to be ...

2

Phase changes are fickle. Much like supersaturated solutions. Add a crystal to a supersaturated solution and it will precipitate rapidly. Bring a crystal of the thermodynamically favorable phase into contact with a "supersaturated" phase and it may just do this: https://www.youtube.com/watch?v=sXB83Heh3_c I am not sure how exactly the proceedure in the ...

2

I suspect that it does not. As far as I know, all trivalent phosphorous is highly pyramidal--or at least not planar. This would rule out any graphene-like structure. The reasoning that I have seen is based on hybridization. In the second row (third period) the 3s and 3p atomic orbitals are well separated, implying that s and p can not mix and that the ...

2

Diamond is an Allotrope of Carbon. As cleared by @Ben Norris, Allotropy is property of any element to exist in two or more different forms. Whereas the term Polymorphism meant the ability of a solid material to exist in more than one form or crystal structure. In diamond, each carbon (an element) is bonded to four Carbon atoms forming a rigid 3-...

2

Given proper conditions, any element could have multiple allotropes For example, under sufficient pressure, hydrogen has a metallic phase, which may have been observed at Harvard recently. The extreme example is degenerate matter, formed under the pressure of a collapsed star, though it could be argued that the elements are no longer the same, since ...

2

Solid allotropes of pretty much anything are different phases. They have different crystal structures, and you can't have a single phase with two different crystal structures inside it. As for the other criteria, some of them are sufficient but not necessary, and some are difficult to check. The distinct boundary must be there, but that's a knowledge that ...

2

"Exists" is an awfully tricky word: it can be understood in a thermodynamic sense or in a kinetic (=everyday) sense, and the meanings are sometimes quite different. Ditto for "stable". Thermodynamically, there exists only one stable allotrope of any element at any given conditions. All other allotropes are not allowed to exist and must eventually transform ...

1

Technically Yes, but Scientifically No. Diamond is made of pure Carbon, so yes. However, it is not on the periodic table nor is it classified as an element by most scientists, so on that case, no. It really depends what you think.

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