[Johulus]

Which one has higher boiling point, phosphorus pentachloride or sulphur hexafluoride?

So, phosphorus pentachloride is shaped as a trigonal bipyramid. It is not polar and the forces that act between molecules are London dispersion forces. Sulphur hexafluoride is shaped as octahedral, it is not polar and forces acting between molecules should also be London dispersion forces. So, which then has the higher boiling point? Maybe it has hot something with molecule size? Sulphur is smaller atom than phosphorus. And fluoride is also smaller atom than chloride. So, sulphur hexafluoride should be a smaller molecule. Sulphur hexafluoride has the smaller boiling point, but how would you explain that in words? It is smaller, but what does that imply? Does it have something with the strength of intermolecular forces?

[Corribus]

Boiling points are ONLY affected by intermolecular/van der Waals forces. So let's start there. You've noted that both of these are nonpolar. Therefore the types of intermolecular forces you must consider primarily are London forces.

Facts: SF₆ is a gas at room temperature. PCl₅ is a solid.

This tells you obviously that SF₆ has a far lower boiling point than PCl₅. Why?

Van der Waals forces between nonpolar molecules (those with no permanent dipoles) are mostly forces between what are known as "induced dipoles", called London dispersion forces. This basically refers to the way that a dipole (charge imbalance) in one molecule can create weak dipole in a neighboring molecule by temporarily pulling at its electrons. For instance, if you have a molecule where the electrons are evenly spread in a sphere, and you put a positive charge near one side of the molecule, then the electrons will tend to be attracted to that positive charge and spend a little more time on the side of the molecule close to the positive charge, and a little less time on the opposite side. This creates an "induced" dipole in the molecule, which disappears as soon as the external force is removed.  These induced dipoles can cause molecules to stick together a little bit, and it is why completely nonpolar molecules like hexane are liquid even though there doesn't seem like there should be anything holding them together. Instantaneous fluctuations in electron density create temporary dipoles, hence weak forces that hold molecules together.

The strength of these induced dipole forces is dependent on two basic things you may consider: how frequently momentary charge imbalances are formed and how easy it is for a charge imbalance in one molecule to induce a charge imbalance in a neighboring molecule. The former accounts in part for why bigger molecules tend to have higher boiling points. As to the latter... we may introduce a concept called "polarizability". If a molecule holds onto its electrons very tightly, it will be harder to form a charge imbalance or induce a dipole. These molecules are said to have "low polarizability". On the other hand, if electrons are held weakly, then it is easy to pull electrons away and induce dipoles - high polarizability. All things being equal, molecules with low polarizability should have weaker intermolecular forces and lower boiling points. Molecules with high polarizability should have stronger intermolecular forces and higher boiling points.

Now, all that said. What do you think about the polarizability of fluorine versus chlorine?

[Enthalpy]

I like the explanation because fluorides do boil easily, more so than chlorides. At alkanes they are similar to the unsubsituted homologues. With heavy metals they often make the only gaseous molecules.

There could have been an "AND". PCl₅ is not a completely symmetric molecule like SF₆ is. The two Cl at the "poles" are not completely identical with the three ones at the equator: they're surrounded by 3 Cl instead of 4 at different angles and distances. Maybe the bond with P differs too, I've no idea. Anyway, at PCl₅, the Cl at the poles can have a slightly different charge from the equatorial ones, which strengthens the intermolecular forces.

To feed the thoughts, here is PF₅ too:
PCl₅ Bp=+161°C https://en.wikipedia.org/wiki/Phosphorus_pentafluoride
SF₆ Bp=-64°C https://en.wikipedia.org/wiki/Sulfur_hexafluoride
PF₅ Bp=-85°C with the same shape https://en.wikipedia.org/wiki/Phosphorus_pentafluoride
which clearly shows that fluorine vs chlorine is the cause, not molecular symmetry.

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Fluorides have a low optical index too, and for instance fluosilicates serve to coat fibres - pure silica core is a must, and few materials have a lower index and low losses too. Up to now I believe that the optical index relates with the polarisability of the molecular orbitals (while at lower frequencies, atoms movements contribute too). Opinions please?

I also noticed that fluorinated compounds have a low bulk modulus, with Ptfe being more easily compressed than even Pe. This holds for compounds that have similar melting or boiling points too: perfluoroalkanes are more compressible than alkanes and chloroalkanes. Up to now I believe it results from smaller intermolecular forces. Opinions please?

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"Instantaneous fluctuations in electron density create temporary dipoles"... It's hard to find a proper wording for a static effect, isn't it? When I try it takes me several lines every time. Maybe human languages just don't have the words for the ψ common to several particles, as this is abstract to humans.

[Corribus]

Fluorides have a low optical index too, and for instance fluosilicates serve to coat fibres - pure silica core is a must, and few materials have a lower index and low losses too. Up to now I believe that the optical index relates with the polarisability of the molecular orbitals (while at lower frequencies, atoms movements contribute too). Opinions please?

I also noticed that fluorinated compounds have a low bulk modulus, with Ptfe being more easily compressed than even Pe. This holds for compounds that have similar melting or boiling points too: perfluoroalkanes are more compressible than alkanes and chloroalkanes. Up to now I believe it results from smaller intermolecular forces. Opinions please?

Could you provide some representative data for each of these cases? Also, with respect to bulk moduli of polymers, it is sometimes difficult to make comparisons, because polymer properties depend so much on the relative molecular weight and degree of branching (density). E.g., I'd expect the compressibility of polyethylene to change dramatically depending on whether it is HDPE, LDPE, etc.

"Instantaneous fluctuations in electron density create temporary dipoles"... It's hard to find a proper wording for a static effect, isn't it? When I try it takes me several lines every time. Maybe human languages just don't have the words for the ψ common to several particles, as this is abstract to humans.

Particularly for students, I think a classical approach is easiest. It's easy to visualize momentary charge asymmetry when picturing electrons as having absolute trajectories. Who knows what "quantum fluctuations" really means?

[Enthalpy]

[...] with respect to bulk moduli of polymers, it is sometimes difficult to make comparisons, because polymer properties depend so much on the relative molecular weight and degree of branching (density). E.g., I'd expect the compressibility of polyethylene to change dramatically depending on whether it is HDPE, LDPE, etc.

Sadly I didn't take my measures with me when leaving this employer because my work belongs to him. I do remember that polyolefins are more compressible than polyamides, polyesters... but clearly less than Ptfe, with silicone oils and rubbers being the most compressible, with K<1GPa near room pressure.

Between Hdpe and Ldpe, I didn't notice an important difference. The E modulus changes measurably, the K modulus little. Generally for soft materials, E drops more quickly than K - for the extreme case of rubbers, K sticks around 1GPa while E drops without limit. I haven't measured Pmp as it was too expensive. I can't exclude neither that after a few kilobar, all polyolefins get the same properties.

At that time I couldn't find a qualitative way to estimate how compressible a compound would be. Liquids tend to be more compressible and solids get a bit more compressible before melting, which suggests that "vacuum between the molecules" (whatever this means) contributes - but long paraffins far from melting are quite compressible as well, and I saw no overwhelming relation with the density at identical gross formula.

What I believe to have seen is that some atoms are more compressible, and that the stiffness of hydrogen depends fundamentally on the polarity of its bond: water and acids stiff, alkanes compressible.

Too little data is available about volume compressibility, and is often at 1atm, hence my measures then. The best tables are for sound velocity, since v² = K/ρ, but then at 1atm only.

Particularly for students, I think a classical approach is easiest. It's easy to visualize momentary charge asymmetry when picturing electrons as having absolute trajectories. Who knows what "quantum fluctuations" really means?

Sure. QM is abstract on this point. While the orbital of a single electron is reasonably concrete, a wavefunction of several particles is not. Personally, I prefer a wording like "if one electron is in this vicinity, the other is probably not because they repel an other" and "this is adequately represented by Ψ(r₁, r₂) ≠ Ψ(r₁) * Ψ(r₂)" since at least, it does not suggest the wrong idea of evolution over time, and sticks better to the math formulation. But then, I don't have an intuitive representation of it neither.