Nuclear Structure in Physics

The nucleus is smaller than its atom by a factor of about a hundred thousand, yet it holds almost all the mass and a great deal of stored energy. That energy shows up as a mass defect, since a bound nucleus weighs slightly less than its separate nucleons would, and the difference reappears as the energy that would be needed to pull it apart. Plot that binding energy per nucleon against mass number and the curve rises steeply, flattens near iron and falls slowly after it, which tells you at a glance that light nuclei can gain energy by fusing and heavy ones by splitting. The blocks give the size and binding relations, the Fermi picture of the nucleus, and the decay laws that let an unstable nucleus be dated, since a quantity that falls by a fixed fraction in a fixed time can be used as a clock.