Atoms touch all the time! But to understand why we first have to decide what we mean by the word “touch.”
Our normal conception of touching is grounded in the macroscopic world. I put a cup on the table – the cup is touching the table. You dip your toes in the water – you are touching the water, and so on. In all these cases, one solid boundary or surface (the bottom of the cup, the edge of your toe) touches another solid boundary or surface (the top of the table, the surface of the ocean). But our macroscopic conceptions break down at the microscopic level, which is where the confusion over “touching” comes in.
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Atoms and Boundaries
If we could zoom into atomic scales, we would see a madhouse. Atoms and molecules are constantly flying around, bumping into each other, twisting, spinning and overall making a mess. But one thing quickly becomes apparent: Atoms do not have a strict boundary.
There’s the nucleus, a bundle of protons and neutrons in the center, surrounded by clouds of probability of where orbiting electrons might be located the next time we go looking for them. The theory of quantum mechanics tells us how to calculate those probabilities, and the range of those probabilities covers the entire universe. However, almost every single time we look at an atom, the electrons are safely bound near the nucleus, so unless we’re doing high-energy collider experiments, we don’t have to worry about it too much.
Since atoms don’t have a solid surface, in one sense there’s nothing to “touch,” because there’s never a situation where one boundary meets another boundary. But “touch” also conveys a sense of up-close-and-personal influence, and in that sense atoms touch all the time.
Macroscopic v. Microscopic
Atoms interact with each other through the electromagnetic force, because the electrons and protons in the atoms are electrically charged. Technically this force has infinite range, but it only becomes significant when atoms are close enough. Sometimes atoms literally bounce off of each other because of the repulsive electromagnetic force between their electrons – in that brief instant, it’s hard to describe their interaction as anything but touching.
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Even if the interaction isn’t brief, it still counts as touching. When it comes to that cup on the table, if we zoom in to the individual atoms and molecules, we would see the electrons at the very outer edge of the cup repulsed by the electrons on the very outer edge of the table. Even if there’s a gap between the two layers of electrons (and there almost always is), the atoms are close enough to influence each other in a significant way. This is evident by the fact that the cup doesn’t just slip through the table, and that the electromagnetic forces between the atoms is enough to counteract the force of gravity pulling the cup towards the Earth.
When we say that two objects are touching at macroscopic scales, this is exactly what it means at microscopic scales.
Nuclear Fusion
Atoms can touch in other ways too, because the electromagnetic force isn’t always repulsive. When atoms get close enough, a manifestation of the electromagnetic force, called the Van der Walls force, emerges, that can cause atoms to bond together. This is precisely how molecules form, and the atoms inside molecules are definitely touching.
Lastly, even the nuclei inside of atoms can touch. This is incredibly hard to do, because of the extremely strong electromagnetic repulsion between the positively charged protons in each nucleus. But once again, quantum mechanics comes into the picture. If two atoms are brought close enough for long enough, then occasionally, completely randomly, the nuclei will find themselves intermingling.
The result is nuclear fusion, where the two separate atoms are replaced with a single, larger atom. If the atoms are lighter than the element iron, the resulting fusion will release energy. It’s through this process that all stars, including our Sun, power themselves.
