Why the empty atom picture misunderstands quantum theory

By | August 24, 2023

Mario Barbatti writes:

The camera zooms in on the person’s arm to reveal the cells, then a cell nucleus. A DNA strand grows on the screen. The camera focuses on a single atom within the strand, dives into a frenetic cloud of rocketing particles, crosses it, and leaves us in oppressive darkness. An initially imperceptible tiny dot grows smoothly, revealing the atomic nucleus. The narrator lectures that the nucleus of an atom is tens of thousands of times smaller than the atom itself, and poetically concludes that we are made from emptiness.

How often have you seen such a scene or read something equivalent to it in popular science? I am sure plenty, if you are fans of this genre like me. However, the narrative is wrong. Atomic nuclei in a molecule are not tiny dots, and there are no empty spaces within the atom.

The empty atom picture is likely the most repeated mistake in popular science. It is unclear who created this myth, but it is sure that Carl Sagan, in his classic TV series Cosmos (1980), was crucial in popularising it. After wondering how small the nuclei are compared with the atom, Sagan concluded that

[M]ost of the mass of an atom is in its nucleus; the electrons are by comparison just clouds of moving fluff. Atoms are mainly empty space. Matter is composed chiefly of nothing.

I still remember how deeply these words spoke to me when I heard them as a kid in the early 1980s. Today, as a professional theoretical chemist, I know that Sagan’s statements failed to recognise some fundamental features of atoms and molecules.

Yet his reasoning is still influential. While preparing this essay, I ran a poll on Twitter asking whether people agreed with Sagan’s quote above. Of the 180 voters, 43 per cent answered that they mostly agreed, and 27 per cent fully agreed. Google ‘atoms empty space’, and you will find tens of essays, blog posts and YouTube videos concluding that atoms are 99.9 per cent empty space. To be fair, you will also find a reasonable share of articles debunking the idea.

Misconceptions feeding the idea of the empty atom can be dismantled by carefully interpreting quantum theory, which describes the physics of molecules, atoms and subatomic particles. According to quantum theory, the building blocks of matter – like electrons, nuclei and the molecules they form – can be portrayed either as waves or particles. Leave them to evolve by themselves without human interference, and they act like delocalised waves in the shape of continuous clouds. On the other hand, when we attempt to observe these systems, they appear to be localised particles, something like bullets in the classical realm. But accepting the quantum predictions that nuclei and electrons fill space as continuous clouds has a daring conceptual price: it implies that these particles do not vibrate, spin or orbit. They inhabit a motionless microcosmos where time only occasionally plays a role. [Continue reading…]

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