74 votes
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Why does F replace the axial bond in PCl5?

Recently, there has been a lot of discussion of Bent's rule (see for example "What is Bent's rule?") here in SE Chem. Simply stated, the rule suggests that $\mathrm{p}$-character tends to ...
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48 votes
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Do molecules with bridges through rings exist?

I'm not sure about the existence of molecules with bridges through rings. However, there are several publications of synthesis of molecules mimicking wheels and axles ([2]rotaxanes; The “[2]” refers ...
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47 votes
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What is the molecular structure of xenon hexafluoride?

TL;DR Xenon hexafluoride has a fluxional structure in the gas phase, with multiple rapidly interconverting conformers. The three most important conformers have $C_\mathrm{3v}$, $O_\mathrm{h}$, and $C_\...
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44 votes
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Has anyone even taken a picture of a molecule to confirm the geometry predicted by theory?

Yes. Researchers have been using atomic force microscopy (AFM) and scanning tunneling microscopy (STM) for some time for this purpose. Do note that these images are not photographs in the sense that ...
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42 votes
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Why does bond angle decrease in the order H2O, H2S, H2Se?

Here are the $\ce{H-X-H}$ bond angles and the $\ce{H-X}$ bond lengths: \begin{array}{lcc} \text{molecule} & \text{bond angle}/^\circ & \text{bond length}/\pu{pm}\\ \hline \ce{H2O} & 104.5 &...
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  • 82k
30 votes

Do molecules with bridges through rings exist?

A variation on this theme is Ice VII, in which two cubic ice structures are intertwined with hydrogen bonds from each component structure passing through the hydrogen-bonded rings formed by the other ...
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27 votes
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How can you build a model of tetrahedral coordination from objects found at home?

Inflate balloons, and tie them «at their stem» like a bouquet of flowers. If you take four of them, not too much inflated, you well demonstrate a situation close to $sp^3$ hybridization. These models ...
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26 votes
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Why do the atoms in molecular models have the colors they have?

There's an interesting article here - Illustrating Atoms and Molecules - that discusses atom colors: In 1865, the chemist August Hoffman gave a Friday Evening Discourse at London’s Royal ...
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  • 1,647
25 votes

Does benzene structure stand for a single resonance form or the whole molecule?

The answer is you are referring to neither of them. That is because resonance structures don't actually exist in reality. We only use them to give us a rough idea what the actually molecule and bonds ...
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  • 7,076
24 votes

Has anyone even taken a picture of a molecule to confirm the geometry predicted by theory?

In addition to the answer provided by Todd, there is also the very established technique of single crystal X-ray diffraction. The basic principle is sending X-rays through a single crystal of a ...
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24 votes
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Why does the image of cyclo[18]carbon look like a nonagon?

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 ...
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22 votes
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What are the important implications to the field of chemistry, if any, of the recently confirmed hexavalent carbon species?

The carbon is not hexavalent, it is hexacoordinated. A covalent bond does not equal to a total of two electrons between the bonding partners and the nature of the chemical bond may lie somewhere ...
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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 ...
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22 votes
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Why is cyclooctateraene shaped like a "tub" rather than a "chair"?

It's all about the 3D structure of double bonds. If we look at the tub form we see that all dihedral C-C=C-C angles are 0°. the C=C-C angles are 125°, also pretty close to the optimal value. ...
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  • 5,652
21 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 ...
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21 votes

What is this fluorinated organic substance?

The name of the compound is 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (CAS #: 140681-55-6), which is commonly known as Selectfluor, a trademark of Air Products ...
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20 votes
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PBP vs TBP geometry?

General Rule #1: Most elements use only s and p orbitals to form bonds, only transition elements and heavier elements use d, f, etc. orbitals in bonding. General Rule #2: The more s-character in a ...
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20 votes
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Basicity Alteration Due To Ortho Effect In Methoxy Aniline System

Take home message: Just because you have an ortho substituent does not mean you will have an ortho effect. To have an ortho effect two conditions must be met: you need an ortho substituent and, ...
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20 votes
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Structure of Br3O8

The electron didn't go anywhere. It's in an unhybridized p orbital on the central bromine, and yes, $\ce{Br3O8}$ is a free radical. That is why it decomposes above -80ºC.$^{[1]}$ $^{[1]}$ Cotton, F. ...
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19 votes

Why does bond angle decrease in the order H2O, H2S, H2Se?

The question asks why water has a larger angle than other hydrides of the form $\ce{XH2}$ in particular $\ce{H2S}$ and $\ce{H2Se}$. There have been other similar questions, so an attempt at a general ...
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  • 26.7k
19 votes
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What is the molecular geometry of NHF₂?

You can look up the molecule on chemspider, where you have a little applet for the 3D structure. Or you can download a coordinate file from NIST and view it in a molecular viewer, like Avogadro. Or ...
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19 votes
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How to rationalise the planar structre of I2Cl6?

The bonding situation in the compound $\ce{(ICl3)2}$ is by far more complex than what is depicted in this book. The molecule itself has very high symmetry, i.e. $D_\mathrm{2h}$, that needs to be ...
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19 votes
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Why are the two CH2 groups in allene perpendicular to each other?

No, VSEPR theory does not allow for a coplanar arrangement. Let us consider allene and its orbitals: Source. Green indicates $\mathrm{p}$ orbitals; blue $\mathrm{sp^2}$; and red $\mathrm{s}$. ...
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  • 23.3k
19 votes

Has anyone even taken a picture of a molecule to confirm the geometry predicted by theory?

In 2005, a field ion microscope captured an image of a very sharp tungsten needle. The small round features are individual atoms. At even smaller scale than molecules, a quantum microscope has ...
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19 votes
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Can Fe(CO)5 adopt both square pyramidal and trigonal bipyramidal geometries?

Your book was correct that a five coordinate metal complex is able to adopt both square pyramidal and trigonal bipyramidal geometries, and in both cases the sp3d hybridisation scheme applies (if you ...
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  • 16.7k
19 votes

Is a compound composed out of gold, manganese, xenon, krypton, and oxalate possible?

No, this is not possible. Actually, if I would have to think of the most unlikely chemical conceivable, that would be it. Let's see why: Krypton is a noble gas that doesn't bond to anything. All of ...
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  • 4,994
19 votes
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Why do we not generally have "enneane"?

Perhaps not a complete answer, but this might put you on the right track. It looks as though a lot of the system we have today grew out of recommendations made by August Wilhelm von Hofmann in a paper ...
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  • 11.3k
18 votes
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Why is tetraamminecopper(II) a square planar and not a tetrahedral species?

It is very convenient to use crystal field theory to discuss this. It is usually assumed that in octahedral coordination the energy levels of the five d-orbitals are split, with two orbitals ($d_{z^...
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  • 20.7k
18 votes
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Complex organic molecules

tl;dr: two different definitions. Astronomy: multiple carbon atoms in molecule. Chemistry: polymer Interestingly enough, after reading about COMs here, as well as reading the Wikipedia page and the ...
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