Read more: “Instant Expert: The atomic nucleus“
The idea of atoms as the ultimate, indivisible particles of matter dates back to the pre-Socratic philosophers of Ancient Greece. It worked amazingly well for many hundreds of years, and was the bedrock on which our burgeoning understanding of the elements – the new science of chemistry – was built from the 18th century onwards.
All that changed 100 years ago this year, with the first glimpse of something nestling at the heart of the atom, something vastly smaller than it, yet containing almost all its mass: the atomic nucleus.
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The impact of this discovery was so profound that the past century has sometimes been called “the nuclear age”. Well into the 21st century, however, the interior workings of the nucleus are still far from perfectly understood
Firing shells at tissue paper
In 1897, the British physicist J. J. Thomson was investigating streams of particles given off by metal electrodes placed under high voltage in a vacuum. These particles turned out to be much smaller than atoms and, unlike neutral atoms, negatively charged.
The discovery of these “electrons” put paid to the idea that the atom was uniform and indivisible. To maintain the atom’s overall electrical neutrality, Thomson suggested that electrons were embedded inside it of positive charge.
By 1908, New Zealander Ernest Rutherford, working with his assistant Hans Geiger at the University of Manchester, UK, had revealed a different picture. When fired from a radioactive source, positively charged “alpha particles” – themselves later revealed to be the atomic nuclei of helium – largely passed through metallic foils placed in their way, deflected by just a few degrees. The atom, it seemed, incorporated a large amount of empty space.
Follow-up experiments by Geiger and a research student, Ernest Marsden, delivered an even greater surprise. Some alpha particles bounced straight back, turned by up to 180 degrees. It was, as Rutherford later said, “as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you”.
, first delivered publicly in February 1911, was that the mass of the atom, itself less than a billionth of a metre (10-9 m) across, was concentrated in a tiny central volume just 10-14 m across. That is something akin to a fly buzzing around inside a cathedral – except the fly accounts for 99.9 per cent of the cathedral’s mass. The atomic nucleus was born.
Keeping it together
Long after the nucleus was discovered (see “Firing shells at tissue paper”, above), its basic structure remained a puzzle. By the early 1920s, Rutherford had isolated a positively charged constituent, the proton, while working at the University of Cambridge. Only in 1932, though, did his colleague James Chadwick isolate the other component of the nucleus: .
Neither protons nor neutrons, collectively called nucleons, are themselves elementary particles. They are made up of smaller constituents, quarks, plus gluons that hold them together. Slightly different compositions mean that the proton is lighter by a whisker. It weighs in at 938.3 megaelectronvolts (MeV) – still more than 1800 times the electron’s mass.
The neutron, meanwhile, tips the scales at 939.6 MeV. While a proton left on its own is stable, or at least has never been observed to decay, a neutron changes into a proton through the process of beta decay (see “The atomic nucleus: Nuclear technology“), with a half-life of just 10 minutes.
Combine this with the fact that their common positive charge makes all protons repel each other, and it seems a miracle that nuclei stay together at all. That they do is down to the trumping effect of the strong nuclear force, which binds together protons and neutrons over very small distances, albeit in constellations of varying stability.
Shells and liquid drops
Two very different models have helped researchers visualise the atomic nucleus in the century since its discovery. The way neutrons and protons appear to stick together, rather like molecules in a liquid, gave rise in the 1930s to the ““, which accurately predicts the binding energies of nuclei and the amount of energy fission or fusion processes will release, once factors such as charge repulsion between protons are taken into account.
The quantum-mechanical Pauli exclusion principle, meanwhile, teaches us that nucleons – protons and neutrons together – cannot all occupy the same energy states. In this picture they orbit in concentric energy ““, much as electrons are ordered into shells around the nucleus to complete our picture of the atom.
Just as a full electron shell makes an element peculiarly unreactive – a noble gas – a nucleus with just the right “magic” number of neutrons or protons to fill a shell gets a stability boost. If both proton and neutron shells are full, then the nucleus is “doubly magic”. Examples of these favoured nuclei are oxygen-16 (8 protons and 8 neutrons), lead-208 (82 and 126) and helium-4 (2, 2) – this last being better known as the alpha particle.
Both the liquid drop and the shell model remain popular for their different purposes. Nowadays, though, it’s possible to build a consistent quantum-mechanical picture of the nucleus – albeit only with substantial supercomputing power to take account of all its complexities.
On the origins of elements
Though many chemical elements are described as stable, their nuclei are not all as tightly corralled as each other. Different amounts of “binding energy” are needed to break them down into their constituent protons and neutrons (see diagram).
A few minutes after the big bang, the only elements present in any abundance in the cosmos were the two lightest, hydrogen and helium. The binding energy curve gives us a clue where the rest came from. For elements lighter than iron, the strong nuclear force reigns supreme; binding energy per nucleon depends largely on the ratio of a nucleus’s volume to its surface area, and varies starkly between nuclei. This is the realm of nuclear fusion, where rearranging nucleons into larger nuclei liberates vast amounts of energy. The elements up to iron, we think, were first forged through nuclear fusion in the cores of stars.
Beyond iron with its 26 protons, however, the charge (Coulomb) repulsion between protons comes to dominate nuclear structure. Energy is won not by joining nuclei together, but by splitting them apart in the process of nuclear fission – the reason why commercial nuclear reactors use very heavy elements such as uranium and plutonium as fuel.
So what made elements heavier than iron? A full complement of nuclei probably came about only when the first stars exhausted their fusion fuel and collapsed in on themselves, detonating supernovae. These massive explosions are thought to liberate a huge number of neutrons. According to the theory of nucleosynthesis by rapid neutron capture, also known as the r-process, these neutrons bombard and stick to already existing nuclei faster than these can cast them off, making elements all the way up to uranium and beyond. Exactly how this works, however, is still a matter of conjecture.
Next article: “Nuclear stability“
“A full complement of nuclei probably came about only when the first stars exhausted their fusion fuel and detonated supernovae”
