What constitutes an element? The ancient Greeks believed in four elements: earth, wind, fire and water. By the 1920s, however, physicists were defining elements according to the number of protons within the atomic nucleus. Meanwhile, pioneering work by Frederick Soddy and Kasimir Fajans demonstrated the existence of different isotopes - atoms that have similar chemical properties but different masses. This work culminated in 1932 with James Chadwick's discovery of the neutron, the uncharged particle that effectively damps out the Coulomb repulsion between the charged protons.
Today, we know that there are over 260 stable nuclei that do not undergo radioactive decay. However, a total of approximately 7000 different unstable nuclei are thought to exist, and almost 3000 of these have been created and studied in the laboratory. These include unstable "exotic" nuclei that are very rich in either protons or neutrons. Time and again, these experiments have highlighted that many of the theories and models that apply to stable nuclei are not valid for exotic nuclei.
Unstable nuclei are also of fundamental interest because it is thought that they were created in violent stellar reactions, and later decayed to form the heavier, stable elements now found on Earth. Thus one of the main thrusts in nuclear physics today is to synthesize and study the most exotic nuclear species at the very boundaries of the nuclear domain - at the so-called proton and neutron driplines.
In the January issue of Physics World magazine, Paddy Regan from the University of Surrey in the UK and Bertram Blank at the CEN Bordeaux-Gardignan in France describe the latest research into the structure of exotic nuclei.