Humble_Cook212 t1_jcwmtr8 wrote on March 20, 2023 at 3:00 AM

been a while since high school chem but i'll give it a shot - at STP you can have approximately 602,214,076,000,000,000,000,000 hydrogen atoms in 22.4 liters. (molar mass ~1, ideal gas law, avogadro's constant)

throwing the above into some division and algebra, so a single atom of hydrogen would be about a 3.7 nanometers3, at stp.

i'm not sure where I went wrong with the above though, as it doesn't really pass the sniff test - several legit websites refer to the diameter of H being about 0.1nm, and semiconductor manufacturing is currently making silicon features around 20-30nm. so, I'm probably incorrect by an order of magnitude, more than i can explain from my sloppy rounding. you'll get a better answer at /r/chemistry or /r/physics probably.

lawblawg t1_jcxxbd3 wrote on March 20, 2023 at 12:30 PM

You're right to do a sanity check; the size of a single atom of hydrogen is definitely not 3.7 cubic nanometers.

Your problem is that you used STP -- standard temperature and pressure -- above. At sea level pressures and temperatures, the gaseous hydrogen molecules (they're molecules, not individual atoms, but that's the least of our problems at this point) have so much kinetic energy that they are zipping around at hundreds of meters per second, constantly bouncing into one another. And so those constant collisions force them to spread out, until they occupy (on average) 3.7 cubic nanometers, even though they are physically infinitesimally smaller.

Humble_Cook212 t1_jcxyve9 wrote on March 20, 2023 at 12:44 PM

That makes sense, thanks! I figured the mass/avogadro approach, while extremely useful for lots of stuff, proved not effective here at what I now read to be "what are the electron valence shell diameters". So it seems maybe I answered a question but the wrong question. :) Any thoughts on a different method that would get closer to the published 0.1nm?

lawblawg t1_jcya1wq wrote on March 20, 2023 at 2:13 PM

You were on to the right track; you just want to think about it in terms of what's happening at an atomic level. To get as close as possible to the actual physical diameter of a hydrogen atom, you'd want to imagine a situation where the hydrogen atoms were all physically in contact with each other. This doesn't happen in a gas, but it does happen (more or less) in a liquid.

The density of liquid hydrogen is going to vary based on external pressure, but let's start with sea level pressure just to give ourselves a benchmark. The density of liquid hydrogen is 70.85 g/L. I don't even bother doing the calculation with Avogadro's number here; I just asked Google "70.85 g/L = ? amu/nm^3" and it converted grams to atomic mass units and it converted liters to cubic nanometers just fine. The result? The density of liquid hydrogen is 42.67 amu/nm^2. We know that a single hydrogen molecule has a mass of 2 amu, so this suggests that a single hydrogen molecule in a liquid occupies a space of 0.047 cubic nanometers. Solving gives us a radius of 0.26 nanometers. So we can say confidently that a single diatomic hydrogen molecule MUST be small enough to fit within a sphere that has a diameter of 0.52 nanometers.

So how small does that make a single hydrogen atom? Well, you can look up the bond length of diatomic hydrogen and find that it is 74 picometers, or 0.074 nanometers. Covalent bond length is the distance between bonded nuclei, given intersecting electron clouds. So the diameter of a single hydrogen atom must be less than 0.45 nanometers. We're now well within an order of magnitude of the actual size.

Can we do better? Yes, we can, by looking once more at what's actually happening at an atomic level. Think about a ball pit: there's a lot of empty space between the actual individual balls. Being (essentially) spheres, hydrogen molecules don't pack into each other perfectly; they leave space in between each other. How tightly can you pack spheres together? This is a well-studied problem. In a perfectly hexagonal offset lattice, spheres can be packed with an average density (relative to the space between them) of π/3*2^0.5 or ~0.7405. That would be if the hydrogen molecules were arranged in a perfect crystalline structure. Unfortunately, hydrogen molecules are nonpolar so they don't have any intermolecular electromagnetic forces to align them that way; they will be packed randomly. This is called a random close pack and mathematicians have found that such irregular packings will produce a density of 0.64 or thereabouts.

What does this tell us? Well, if the density of liquid hydrogen is measured at 42.67 amu/nm^2, but the molecules are only packed to a volumetric density of 0.64, then 36% of that volume is going to be empty space. So the actual density of an individual hydrogen molecule is 66.7 amu/nm^2 or 0.03 cubic nanometers per molecule. This gives me a radius of 0.193 nanometers. Subtract the bond length and I get a diameter of 0.238 nanometers for a single hydrogen atom.

The actual size of an atom is known as the van der Waals radius. The van der Waals radius of a hydrogen atom is 1.09 angstroms or .109 nanometers. So the actual physical diameter of a hydrogen atom is 0.218 nanometers.

So our approach got us within 10% of the real number. Not bad!

Traditional_Cat_60 t1_jd0qpxy wrote on March 21, 2023 at 12:08 AM

Most of that 22.4 L of space is empty space, not atoms. The 22.4L/mol at STP is the volume the gas takes up, including all the empty space in between them . It is not the volume of the atoms themselves. Compared to the 22.4L, the atoms themselves take up a very small amount of the volume.