The concept of the atomic void is one of the most repeated mistakes in popular science. Molecules are packed with stuff
Electronic and nuclear quantum clouds in an ammonia molecule. The molecule is approximately 400,000 femtometres wide. There are approximately a trillion femtometres in a millimetre. Image supplied by the author
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.
Most problems surrounding the description of the submolecular world come from frustrated attempts to reconcile conflicting pictures of waves and particles, leaving us with inconsistent chimeras such as particle-like nuclei surrounded by wave-like electrons. This image doesn’t capture quantum theory’s predictions. To compensate, our conceptual reconstruction of matter at the submolecular level should consistently describe how nuclei and electrons behave when not observed – like the proverbial sound of a tree falling in the forest without anyone around.
Here’s a primer on how to think of the fundamental components of matter: a molecule is a stable collection of nuclei and electrons. If the collection contains a single nucleus, it is called an atom. Electrons are elementary particles with no internal structure and a negative electric charge. On the other hand, each nucleus is a combined system composed of several protons and a roughly equal number of neutrons. Each proton and neutron is 1,836 times more massive than an electron. The proton has a positive charge of the same magnitude as an electron’s negative charge, while neutrons, as their name hints, have no electric charge. Usually, but not necessarily, the total number of protons in a molecule equals the number of electrons, making molecules electrically neutral.
The interior of the protons and neutrons is likely the most complex place in the Universe. I like to consider each of them a hot soup of three permanent elementary particles known as quarks boiling along inside, with an uncountable number of virtual quarks popping into existence and disappearing almost immediately. Other elementary particles called gluons hold the soup within a pot of 0.9 femtometres radius. (A femtometre, abbreviated fm, is a convenient scale that measures systems tens of thousands of times smaller than an atom. Corresponding to 10‑15 m, we must juxtapose 1 trillion femtometres to make one millimetre.)
Instead of localised bullets in empty space, matter delocalises into continuous quantum clouds
Particles with the same electric charge sign repel each other. So additional interactions are required to hold protons close-packed in the nucleus. These interactions arise from quark and antiquark pairs called pions that constantly spill out of each proton and neutron to be absorbed by another such particle nearby. The energy exchanged in this transfer is big enough to compensate for the electric repulsion between protons and, thus, bind together protons and neutrons, storing the immense energy that may be released in nuclear fission processes.
However, the extremely short lifetime of the pions limits how far protons and neutrons may be from each other, curbing the nucleus size to a 1 to 10 fm radius. Thus, from a particle perspective, the nucleus is tiny compared with an atom. A nitrogen nucleus, composed of seven protons and seven neutrons, has a radius of about 3 fm. In contrast, nitrogen’s atomic radius is 179,000 fm. At the scale of atoms and molecules, nuclei are no more than heavy, point-like positive charges without any apparent internal structure. So are the electrons: they are just light, point-like negative charges.
If atoms and molecules remained a collection of point-like particles, they would be mostly empty space. But at their size scale, they must be described by quantum theory. And this theory predicts that the wave-like picture predominates until a measurement disturbs it. Instead of localised bullets in empty space, matter delocalises into continuous quantum clouds.
Matter is fundamentally quantum. Molecules cannot be assembled under the rules of classical physics. The classical electrical interactions between nuclei and electrons are insufficient to build a stable molecule. Due to the electric attraction of charges of opposite signs, the negatively charged electrons would quickly spiral toward the positively charged nuclei and glue to them. The resulting combined particles with no net charge would fly apart, preventing any molecule from forming.
Two quantum properties avoid this bleak fate.
The first property arises from the Heisenberg uncertainty principle, which holds that a quantum particle cannot simultaneously be at a precise position and also have zero speed. This implies that an electron cannot glue to a nucleus because both particles would be in a well-defined place and at rest to each other – defying a central rule of the quantum world.
The second quantum property is the Pauli exclusion principle. The fundamental components of matter are split into two types, bosons and fermions. The gluons inside the proton are examples of bosons. We can have as many of them as we want, sharing the same position simultaneously. On the other hand, fermions – such as electrons, quarks, protons and neutrons – obey a much more restrictive rule named the Pauli exclusion principle: no two identical fermions can simultaneously occupy the same space and have the same spin (a quantum property analogous to a classical rotation of a particle about its axis).
In the quantum world, the wave function represents more than a mere lack of knowledge
With all those effects encoded into the Schrödinger equation, the master equation of quantum theory, it predicts that our point-like nuclei and electrons must, in fact, behave like waves. They delocalise in quantum clouds much bigger than their particle-picture size to satisfy the Heisenberg uncertainty principle, with electrons shaped into different clouds to satisfy the Pauli exclusion principle. The lighter the particles are, the bigger the delocalisation. Thus, a single electron cloud may spread over multiple nuclei, forming a chemical bond and stabilising the molecule.[…]
Continue reading: Why the empty atom picture misunderstands quantum theory | Aeon Essays
is a theoretical chemist and physicist researching light and molecule interactions. He is professor of chemistry at Aix Marseille University in France and a senior member of Institut Universitaire de France.
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