Uranium hexafluoride, UF₆, has a sublimation point of 56.5 degrees Celsius. Which statement best accounts for this behaviour? A) Uranium hexafluoride is an ionic compound, with U⁶⁺ and F⁻ ions with a lattice arrangement. B) Uranium hexafluoride is a simple molecular compound, with covalent bonds. C) The bonding in uranium hexafluoride is metallic, with a sea of electrons being shared equally between all particles in a sample. D) Uranium hexafluoride has a network covalent structure, with a continuous lattice of covalently bonded U and F atoms. Or E) Uranium hexafluoride is a mixture of uranium and fluorine atoms, which are not bonded together.
Uranium hexafluoride is a key chemical in the process of enriching uranium. The output from this process is used as fuel in nuclear reactors to generate energy. The only piece of information we’ve been given in the question about uranium hexafluoride is its sublimation point, which is the temperature at which it can transition from a solid to a gas, without becoming a liquid in between. 56.5 degrees Celsius seems unexpectedly low. After all, uranium hexafluoride is a combination of uranium and fluorine. So it’s a metal-nonmetal compound. We’d normally expect such substances to be ionic. And as such, we’d expect them to have high melting, boiling, or sublimation points.
So we’re left in a bit of a pickle. But fortunately, we have five statements which might guide us to the correct reason for this anomalous behaviour. The first suggestion is that uranium hexafluoride is simply an ionic compound, with U⁶⁺ and F⁻ ions in a lattice arrangement. The first thing that doesn’t look quite right is just how big of a charge there is on the uranium ions. Such a high concentration of charge is generally not stable. And often, other atoms or ions will interact with it and spread the charge out.
For instance, in the permanganate anion, the manganese doesn’t actually have a charge of seven plus. It just has a formal oxidation state of plus seven. There are covalent bonds between the manganese and the oxygens, helping to distribute the charge. If uranium hexafluoride were ionic, we would expect much higher transition temperatures. Uranium six plus and F⁻ ions would attract each other strongly. And it wouldn’t take such a low temperature to break those bonds. So we’re forced to admit that uranium hexafluoride cannot be a simple ionic compound.
The next suggestion is that uranium hexafluoride is a simple molecular compound, with covalent bonds. If you look up uranium on a periodic table, you’ll see that its atomic number is 92 and that it’s an actinide. This makes the geometry of compounds of uranium a little harder to predict because the way electrons interact when they have so many is much more complicated than for smaller elements. But we can take a guess that if uranium hexafluoride is molecular, a molecule of uranium hexafluoride would look something like this. An octahedron with the uranium in the middle and the six fluorines trying to get us far away from one another as they can.
To have such a low sublimation point, a simple molecular compound would have to have very weak intermolecular forces. Fluorine is the most electron negative element. So in our molecule of uranium hexafluoride, all the bonds would be polarised, with the fluorines being Δ negative and the uranium Δ positive. However, because of the symmetry of the octahedron, there will be no net dipole. So we wouldn’t get dipole-dipole interactions between the molecules. What we would get would be London dispersion forces.
The molar mass of uranium hexafluoride is about 352 grammes per mole although it does depend on the isotope of uranium you’re using. The molar mass of uranium hexafluoride just happens to be the same as a long chain alkane containing 25 carbon atoms as you might find in paraffin wax, which has a melting point between 47 and 64 degrees Celsius. Alkanes similarly only have London dispersion forces between their molecules. So they have relatively low melting, boiling, and sublimation points. So it seems like we’re on the right track. If uranium hexafluoride is a simple molecular compound, then its sublimation point, being so low, would make sense.
However, before taking this off, let’s have a look at the other three suggestions to see if any of those are better. The third suggestion is that bonding in uranium hexafluoride is metallic, with a sea of electrons being shared equally between all particles in a sample. The bonding in metallic substances is generally pretty strong. So this doesn’t make a great deal of sense with a low sublimation point. Even mercury, the only metal that’s a liquid at room temperature, has a boiling point over 350 degrees Celsius. Besides, fluorine is a nonmetal. It holds onto its electrons so strongly that they can’t be delocalised in a sea of electrons. So it seems highly unlikely that uranium hexafluoride would have metallic bonding.
The next suggestion is that uranium hexafluoride has a network covalent structure, with a continuous lattice of covalently bonded uranium and fluorine atoms. Network covalent structures are known for their strong, continuous lattice of bonds. It tends to take a lot of energy to disrupt these continuous lattices. For example, glass, which is a lattice of silicon-oxygen atoms, melts between 1400 and 1600 degrees Celsius. And diamond, which is a continuous lattice of carbon, sublimes at around 4000 degrees Celsius. The low sublimation point of uranium hexafluoride suggests that such a network covalent structure just isn’t realistic.
And here we are at the last suggestion, which is uranium hexafluoride is a mixture of uranium and fluorine atoms, which are not bonded together. This looks suspicious. Fluorine is one of the most reactive substances known to man. If we had atoms of fluorine just sitting around, uranium wouldn’t stop them reacting. They pair up and fly away as a gas. With nothing to hold the structure together, there’s no way uranium hexafluoride could form this structure, which means that the best description that accounts for the very low sublimation point of uranium hexafluoride is that it is a simple molecular compound with covalent bonds.