Strange though electrons are, we have known about them significantly longer than we have the other components of the atom. By the late 1890s, electrons had been identified as particles, taking over the name that had been devised in 1891 for the charge in a chemical bond. But the names of the other component particles that make you up – quarks and gluons – would not enter the language until the 1960s. You might be wondering what happened to protons and neutrons. These are the familiar particles that make up the nucleus of an atom, starting with a single proton that is all that there is in the hydrogen nucleus and adding extra protons and neutrons to make up the heavier elements. So, for example, there are 79 protons and 118 neutrons in the nucleus of the most common isotope of gold. But neither the proton nor the neutron is a fundamental particle – each has sub-components.
Every proton is made up of two up quarks and one down quark, held together by a flow of gluons, while each neutron contains one up quark and two down quarks, again linked by gluons. The ‘up’ and ‘down’ part of the names simply comes from the way their mathematical representation is written out. The ‘quark’ part is more interesting. Murray Gell-Mann, the physicist who came up with the name, said that the sound (which is intended to be more like ‘kwork’ than ‘kwark’) came to him out of the blue. But later, seeing the line in James Joyce’s book Finnegans Wake ‘Three quarks for Muster Mark!’, he adopted that spelling of a word that is still, technically, pronounced ‘kwork’. Gell-Mann was inspired to do so as quarks happen to come in threes in protons and neutrons.
Like electrons, quarks are very light particles, with masses of 3.9 × 10–30 kg (up) and 8.4 × 10–30 kg (down), while gluons have no mass at all. This might seem to imply those more familiar protons and neutrons should have masses of 16.2 × 10–30 kg and 20.7 × 10–30 kg respectively, but that would give us two problems. First, protons and neutrons are significantly more similar in mass – and secondly it would make the particles around 100 times too light. The reason for this brings in the world’s most familiar equation, Einstein’s E = mc2, which makes energy (E) equal to mass (m) times the square of the speed of light (c). The more energy in something, the bigger the mass it has – and the majority of the mass of a proton or a neutron comes not from its component particles, but from the energy that holds those particles together.
BRUTE FORCE WITHOUT IGNORANCE
Thinking of particles being held together makes this a useful point to bring in the other part of what makes up your atoms. As well as the four particles, I mentioned ‘a few forces’. Specifically, there are two we need to consider. The more familiar is electromagnetism, responsible for the forces between magnets and between electrically charged objects. Electrons have a negative electrical charge and the nucleus of an atom has a positive electrical charge so there is attraction between them, just as there is a pull between the wires of an electric motor, or an attraction between a statically charged balloon and bits of paper.
This attraction between electrons and the nucleus prevents the electrons flying off and doing their own thing (though in good conductors, such as metals, some electrons do manage to escape and do just that). But it might seem at first glance that the relationship between a negatively charged electron and positively charged nucleus would be destructive. Why is it that the electrons don’t just zoom into the nucleus, collapsing the atom and bringing about the end of all matter?†
The problem of why atoms don’t collapse concerned physicists as a gradual understanding of the structure of atoms was built up. Initially it was thought that the positive part of the atom was a bit like a positively charged jelly with the negative electrons suspended within it. (The actual image used at the time was of a Christmas pudding, known then as a plum pudding, with the electrons playing the part of raisins.) But experimental evidence showed that the positive charge was concentrated in a tiny nucleus‡ at the heart of the atom.
With this realisation, the common-sense parallel was the solar system (somehow, this seemed more scientific than a plum pudding). After all, gravity ensures that all the planets are attracted to the Sun and do, indeed, fall towards it. But no catastrophe ensues because the planets are also moving sideways, at 90 degrees to that inward motion. They do so at just the right speed to miss the Sun, keeping a relatively constant distance away – this is what is involved in being in orbit. How convenient, then, if electrons were in orbit around the nucleus too. It made for a beautiful, elegant image of consistency between the vast scales of space and the sub-microscopic scale of the atomic structure. So much so that the standard graphic representation of an atom still usually involves electrons merrily orbiting. (Take a look, for instance, at the logo of the International Atomic Energy Agency.) Which makes it a shame that the idea is a total non-starter.
IAEA logo.
Unfortunately, electrons have a habit of giving off energy in the form of electromagnetic waves when they are accelerated. That’s how radios and mobile phones work. The transmitter accelerates electrons back and forth in the aerial, giving off electromagnetic radio waves which travel through the air (or through space) to the receiver. If the electrons around an atom were in orbit, they would be constantly accelerating. This may seem counterintuitive as they are not getting faster and faster, but acceleration can be a change of speed or of direction of travel – and to stay in orbit requires a constant change of direction. So orbiting electrons would give off energy and plummet inwards. Once again, atoms would self-destruct, and you wouldn’t exist.
The solution to this was radical and slightly bonkers. Led by Danish physicist Niels Bohr, the physics community decided that, in essence, electrons would have to be restricted to particular ranges from the nucleus, not able to move inward or outward smoothly, but only in jumps, known as quantum leaps. This was one of the earliest aspects of quantum physics, which came to be the standard way to understand the (sometimes very strange) behaviour of anything very small. By restricting the electrons to these regions, as if they ran on tracks around the nucleus, disaster was averted. No one could say why this happened, it was just the way things were.
As quantum theory was developed, it became clear that the structure of atoms was more convoluted than just having electrons running along fixed tracks. Left to their own devices, particles such as electrons could not be pinned down to clear positions and trajectories. Instead, a better picture of the electron’s orbital (the word devised to get away from the fixed image of an orbit) was a fuzzy cloud of probability around the atom. The electron would be somewhere in that three-dimensional space, but its exact location at any one time could not be predicted.
With electromagnetism keeping the electrons in place, another force was needed in the nucleus. Something else has to keep those quarks bound in place (and for that matter to ensure that the positively charged protons made from them don’t fly away from each other due to electromagnetic repulsion). We don’t see quarks at all in the wild for the simple reason that a very strong force holds them together. Unlike the familiar forces of gravity and electromagnetism, this force only operates over very small distances, but when it does apply it has the weird behaviour that as you separate two objects, the attraction between them gets stronger. In this respect, it acts more like a taut elastic band than a force like electromagnetism.