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Why don't atoms collapse if they are mostly empty space?
Category: Physics
Published: January 12, 2013
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Atoms are not mostly empty space because there is no such thing as purely empty space. Rather, space is filled with a wide variety of particles and fields. Sucking all the particles and fields out of a certain volume won't make the space completely empty because new particles will still flash into existence due to vacuum energy. Additionally, the Higgs field can't be removed. Even if we ignore every kind of field and particle except electrons, protons and neutrons, we find that atoms are still not empty. Atoms are filled with electrons. It's true that a large percentage of the atom's mass is concentrated in its tiny nucleus, but that does not imply that the rest of the atom is empty. Rather, it implies that the rest of the atom has relatively low density.
The misconception of an empty atom is taught by incorrect elementary-level science books and is based on the false picture of electrons as balls. In this view, the atom consists of electron balls whizzing around the atomic nucleus which is itself a ball. In this picture, the space between the electrons and the nucleus is therefore empty space. While this picture (the Bohr model) is simple to imagine, it was shown to be wrong almost a century ago. Electrons (as well as all particles) are partially particle-like and partially wave-like, depending on the situation. When bound in atoms in an undisturbed state, electrons act like waves. These waves are three-dimensional probability density waves that spread out to fill the entire atom. The electrons do not spread out uniformly, but rather follow specific distribution patterns called "orbitals". The shape of the orbitals underpin all chemical reactions. As an example of some orbitals, the single-electron density distribution is shown on the right for hydrogen in the first few lowest states. The lighter points indicate regions where the electron has a higher density. Note that each image represents a single electron. The different light spots and bands in a single image are all part of a single electron's wave state. Because bound electrons spread out into fuzzy density waves, there is no definite "edge" to an atom. The electron actually spreads out to fill all space, although far away from the atom it is thin enough to be negligible. Interestingly, electrons in the atom even spread out so as to overlap with the nucleus itself. This electron-nucleus overlap makes possible the effect of electron capture, where a proton in the nucleus can react with an electron and turn into a neutron. If atoms were mostly empty space, we could remove this space and shrink atoms. In reality, atoms do not contain any empty space. Rather, they are filled completely with spread-out electrons, making the shrinking of atoms impossible.