Can Atoms Touch Each Other?

A couple of weeks back I posted an answer to a question from a Twitter follower’s child, who asked “How Strong Is Space?” That was fun, so here’s another kid-question answered, this one from my own eight-year-old who goes by “The Pip” for Internet purposes. The other night, he asked “Why can’t atoms touch each other?”

I’m not sure the exact reason why he asked this, but the phrasing suggests it’s related to the observation that there’s almost always some microscopic empty space between things that appear to be touching on a macroscopic scale. Possibly it’s even connected to the “Atoms are mostly empty space” idea that Ethan talked about recently.

The short and simple version of the answer is that it’s not really correct to think of atoms as solid objects like little balls that can be forced into physical contact with one another. Most of the “size” of an atom is just the electron cloud that surrounds the nucleus, and that’s not a solid thing— it will shift around in response to electric and magnetic fields applied from the outside. These can’t be made to “touch” in the everyday sense because they’re not solid in the way that we think of everyday objects being solid.

Of course, when you start to dig into this a little more, it becomes trickier than it initially seems, because of course an everyday solid object is made up of atoms. So what’s happening to those atoms when solid objects “touch” that isn’t what we’d call “touching” in an everyday sense?

Our idea of two objects “touching” is basically that the outer boundary of one is at the same place as the outer boundary of the other, so you would pass between them without going through any intervening space. At the scale we experience the macroscopic world, this is a reasonably good approximation: there is no perceptible gap between two objects that are touching. The microscopic analogue of this would be to have a situation where you can move from the nucleus at the center of one atom to the nucleus at the center of the other atom without passing through any space that isn’t part of the electron cloud of one of the two.

This tends not to happen, because atoms are composed of charged particles that interact at a distance. When you try to bring two objects close together, the atoms in one begin to “see” the atoms in the other not as a perfectly neutral single object, but as a composite of a positive nucleus and negative electrons. Since the electrons are around the outside of the atom, those are the things that first interact, and as they have the same charge, they repel one another.

If you want to look at what happens in extreme detail, this gets very complicated, since the electron clouds in the individual atoms can re-arrange themselves in complicated ways. The end result, though, is that as the separation between the atoms at the contact point decreases, the energy of each pair of atoms increases, until it matches the energy of whatever’s pushing them together. That generally happens in a way that keeps the two distorted electron clouds from overlapping, so technically there’s some tiny amount of space between the atoms in neighboring objects. They’re held apart by electromagnetic interactions, so it wouldn’t be too horribly wrong to imagine that gap as being crisscrossed by innumerable photons carrying the force back and forth between the two.

(It should be noted that even though there’s technically empty space between the atoms, you can still do all the things we associate with “touching” an object. A scanning tunneling microscope (STM) can be used to move atoms around and arrange them into tiny patterns even though there’s technically always a bit of space between the tip and the atom in question.)

So, in most cases atoms aren’t technically “touching” since there’s some space between them that isn’t occupied by the electron cloud of either. If this is just a question of energy, though, it’s not unreasonable to ask how strong a prohibition that is. Is it truly forbidden, or is it possible under the right circumstances for the electron cloud of one atom and the electron cloud of another to come into contact?

The answer here is “sort of.” You can, in fact, have a situation where you don’t cross any space that isn’t part of an electron cloud as you move from the nucleus of one atom to the nucleus of a neighboring atom. It happens all the time— it’s just not clear it makes sense to call it “touching.”

The situation I’m talking about is the formation of an interatomic bond, which in most cases involves having electrons “shared” between atoms within the same molecule or crystal. In a sense, these electrons aren’t orbiting one atom or the other, but both at the same time. There’s no empty space between the atoms, but that’s because they’ve ceased to be entirely independent objects, but are instead components of a large molecule. You can talk about this or that atom within the molecule, in that there are specific regions of space where the interactions are dominated by one particular nucleus, but it’s no longer quite as sensible to talk about them as discrete objects with their own electron clouds.

So, as with basically all kid-questions, the answer to “Can atoms touch each other?” turns out to be more complicated that you might think. In the end, I’d probably go with “Yes, but when they do, they stop being independent atoms.”