For helium and neon,(monatomic)molecular volume is just These results explain the difference with hydrogen (which is, after all, H,) by showing that a hydrogen molecule will be rather larger than a neon atom.(In fact, hydrogens b constant is much smaller than one might expect, given its molecular volume. Of that, more anon.) However, the neon atom will be slightly larger than a helium atom, so that volume cannot be the whole story. le must again be reminded that the van der Waals constants are empirical. Thus they reflect many real-world variables, such as "compressibility. Compressibility ought to be affected by how well the molecules can interact with each other the better the interactions, the higher the compressibility. You see, the stronger the non-bonded intermolecular (that is, the van der Waals forces, the more closely the molecules will be able to approach each other and the lower the value of the b constant. one source of van der Waals interactions is thought to be induced-dipole/induced-dipole" interactions, in which a temporary dipole in one molecule induces an opposite dipole in a neighbor. The two temporary dipoles then attract each other. However, the more tightly electrons are held within a molecule, the harder it will be to induce a dipole (this is called polarizability) and the weaker the van der Waals interactions. Indeed, the volume occupied by the molecules will go up because of electron-electron repulsions. Therefore, i think a clue to the van der Waals b constant may be found in ionization potentials, which measure how tightly electrons are held Looking at the three elements (and the same periodic table), we find Ionization Element Potential Hydroge 13.6 Helium 24.6 21.6 Here, at last, we find an explanation not only for neon but also for hydrogen. Both neon and hydrogen are more polarizable than helium - hydrogen very much so and thus their van der Waals b constants are lower than one would volumes alone I am afraid that I have been carried away Dan Berger Bluffton College http://cs.blufftonedu/berger Current Queue I Current Queue for Chemistry I Chemistry archivesFor helium and neon, (monatomic) molecular volume is just . These results explain the difference with hydrogen (which is, after all, H2 ) by showing that a hydrogen molecule will be rather larger than a neon atom. (In fact, hydrogen's b constant is much smaller than one might expect, given its molecular volume. Of that, more anon.) However, the neon atom will be slightly larger than a helium atom, so that volume cannot be the whole story. We must again be reminded that the van der Waals constants are empirical. Thus, they reflect many real-world variables, such as "compressibility." Compressibility ought to be affected by how well the molecules can interact with each other; the better the interactions, the higher the compressibility. You see, the stronger the non-bonded intermolecular (that is, the van der Waals) forces, the more closely the molecules will be able to approach each other and the lower the value of the b constant. One source of van der Waals interactions is thought to be "induced-dipole/induced-dipole" interactions, in which a temporary dipole in one molecule induces an opposite dipole in a neighbor. The two temporary dipoles then attract each other. However, the more tightly electrons are held within a molecule, the harder it will be to induce a dipole (this is called polarizability) and the weaker the van der Waals interactions. Indeed, the volume occupied by the molecules will go up because of electron-electron repulsions. Therefore, I think a clue to the van der Waals b constant may be found in ionization potentials, which measure how tightly electrons are held. Looking at the three elements (and the same periodic table), we find: Here, at last, we find an explanation not only for neon but also for hydrogen. Both neon and hydrogen are more polarizable than helium -- hydrogen very much so -- and thus their van der Waals b constants are lower than one would expect from volumes alone. I am afraid that I have been carried away! Current Queue | Current Queue for Chemistry | Chemistry archives Element Ionization Potential Hydrogen 13.6 Helium 24.6 Neon 21.6 ? Dan Berger ? Bluffton College ? http://cs.bluffton.edu/~berger