Introduction

Every atom has, at its centre, a tiny, extremely dense core called the nucleus, in which almost the whole mass of the atom -- but only a positive charge, never the atom's electrons -- is concentrated. This part of the course looks at the nucleus itself: what it is made of, how big it is, and, most importantly, why some nuclei are permanently stable while others are unstable and spontaneously break apart, releasing particles and energy in the process called radioactivity.

The chapter builds up in a fixed order. Section 8.2 looks at what a nucleus is made of and how its size scales with the number of nucleons it contains. Section 8.3 defines the atomic mass unit used to measure nuclear masses, and the three families -- isotopes, isobars and isotones -- into which related nuclides are grouped. Section 8.4 studies radioactivity itself: the three kinds of radiation, alpha, beta and gamma, that an unstable nucleus can emit, and how their physical properties differ. Section 8.5 derives the radioactive decay law that describes HOW FAST a radioactive sample decays with time. Sections 8.6-8.7 turn to the energy locked up inside the nucleus itself: Einstein's mass-energy relation, the mass defect of a nucleus, and the binding energy per nucleon that measures how tightly a nucleus is held together -- and whose variation with mass number explains why nuclear reactions release such enormous amounts of energy. Finally, Sections 8.8-8.9 apply this idea to the two great nuclear energy-releasing processes: nuclear fission, the splitting of a heavy nucleus, and nuclear fusion, the joining of light nuclei -- the reactions that power nuclear reactors on Earth and every star, including the Sun, in the sky.