22
votes
Aside from carbon, what other substances can be made "superhard"?
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 ...
- 339
22
votes
Aside from carbon, what other substances can be made "superhard"?
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
...
- 4,314
18
votes
Accepted
Melting point of sulfur
Maurice has a plausible explanation: solid-solid phase transformations are generally sluggish or require a strong nonequilibrium condition to force them because atoms and molecules in a solid are not ...
- 62.6k
17
votes
Does a neutral dimercury molecule exist?
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 ...
- 26.8k
12
votes
Calling Diamond an element?
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 ...
- 37.8k
9
votes
Why does the stratosphere have a lot of ozone, but not the Earth's surface?
Ozone is produced naturally when highly energetic UV radiation breaks down oxygen molecules and cause them to split apart in a process called photolysis. If a free oxygen atom collides with another ...
- 26.8k
9
votes
Accepted
Why does diamond have no molecular formula?
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 ...
- 43.2k
8
votes
Accepted
Are nuclear spin isomers "allotropes"?
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 ...
- 41k
8
votes
Polycyclic nitrogen network?
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 ...
- 38.5k
7
votes
Does a neutral dimercury molecule exist?
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 ...
- 41k
6
votes
Se+P White or red P?
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.
- 30k
6
votes
Accepted
Does a neutral dimercury molecule exist?
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 ...
- 42.1k
6
votes
Does a neutral dimercury molecule exist?
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 $\ce{Hg_2}$ ...
- 21.9k
6
votes
Accepted
Why is it hard to make red and black phosphorous in labs?
Maybe the issue here is consistency.
Red phosphorus is polymeric, so the conditions for creating red phosphorus may result in variable mixtures, i.e., chains with variable length and connectivity. ...
- 17.6k
5
votes
Are nuclear spin isomers "allotropes"?
Apart from the sources that DavePhD listed, there is also the 1932 Nobel Prize in Physics, awarded to Werner Heisenberg:
"for the creation of quantum mechanics, the application of which has, ...
- 71.9k
4
votes
Accepted
Solubility of white phosphorus
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 ...
- 2,391
4
votes
Is the acceleration of the low-temperature allotropic conversion of β-form white tin by the presence of α-form grey tin really considered catalysis?
There are elements of catalysis present.
Let's presume $\ce{A -> P}$ proceeds by a single elementary step. The two states of $\ce{A}$ and of $\ce{P}$ can be compared with each other by a change of ...
- 31.4k
3
votes
Are homoatomic molecules like ozone, graphite, diamond considered as elements
The definitions you provide were good enough for 18-19th centuries. Unfortunately, many school books in different parts of world have not changed. The modern understanding of an element is that its ...
- 42.6k
2
votes
Calling Diamond an element?
The IUPAC definition of element is as follows:
A pure chemical substance composed of atoms with the same number of protons in the atomic nucleus. IUPAC Goldbook definition of element
This seems to ...
- 12.3k
2
votes
Calling Diamond an element?
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 ...
- 29
2
votes
Does sulfur have various allotropes at room temperature?
"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".
...
- 31.6k
2
votes
Aside from carbon, what other substances can be made "superhard"?
Don't know if this is exactly what you want, but Titanium metal has two allotropes ($\alpha$ and $\beta$). $\beta$ phase is stable at high temperatures, but you can quench $\beta$ phase titanium so ...
- 911
2
votes
Accepted
Is Changing between Allotropic forms a Physical or Chemical Process?
Well, it's a matter of choice and tradition. We traditionally classify phase transitions (such as melting of ice or evaporation of water) as physical changes. The conversion of ferrite into austenite ...
- 36
2
votes
Are allotropes of sulfur two different phases?
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 ...
- 31.6k
2
votes
Accepted
Stability of α, β, γ allotropes at different temperatures
A German speaking mineral atlas states:
«Man unterscheidet die verschiedenen polymorphen Modifikationen durch
Voranstellen der griechischen Buchstaben alpha, beta, gamma usw. Die
Buchstaben ...
- 31.4k
2
votes
Stability of α, β, γ allotropes at different temperatures
I think most of the time, they sort according to the order which temperature the allotrope/polymorph is more stable. Let's say element Z has two allotropes A and B. A is stable below 100 °C and B is ...
- 492
2
votes
Which allotropes of sulfur exist naturally?
As described by Nilay Ghosh, the alpha and gamma $\ce{S_8}$ allotropes are the only ones found naturally — on Earth. On the volcanically active Jovian moon Io, however, allotropes having three or four ...
- 62.6k
2
votes
Accepted
Predicting sign of enthalpy of allotropic transition of phosphorus
You can use the fundamental relation between energy and enthalpy. At constant temperature and pressure:
$$\Delta H_m = \Delta U_m + P\Delta V_m = \Delta U_m + PM (\frac{1}{\rho_{red}}-\frac{1}{\rho_{...
- 22.9k
1
vote
Is this a correct Lewis Dot structure for ozone?
Dear OP, by the way, that 'trioxygen' is dubbed ozone.
For starters, the Lewis dot structure for ozone does not look like that. It is not a ring.
This is explained in this link: Can ozone have a ...
- 830
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