Introduction

Nuclear chemistry studies reactions and changes that take place in atomic nuclei, as opposed to ordinary chemistry, which concerns the electrons around them. The field opened in 1896 when the physicist Antoine Henri Becquerel discovered that certain elements spontaneously give off radiation -- natural radioactivity -- and it now underpins work as varied as geologists tracing how the Earth's crust has changed over time, astronomers explaining how stars generate energy, and doctors using radiation both to see inside the body and to treat disease. This chapter looks briefly at four kinds of nuclear reaction in particular: radioactive decay, artificial transmutation, nuclear fission, and nuclear fusion.

It helps to first picture the atom the way this chapter keeps returning to it: much as the Sun sits at the centre of the solar system with the planets held in orbit around it by gravity, an atom has a tiny, dense central nucleus -- built from protons and neutrons -- with fast-moving electrons occupying the much larger volume of space around it, held there by electrostatic attraction rather than gravity.

A nucleus is described using the shorthand $^{A}_{Z}X$, where $X$ is the element's chemical symbol, $Z$ is the atomic number (the number of protons, which fixes which element it is), $A$ is the mass number (the total count of nucleons, i.e. protons plus neutrons), and the neutron number is simply $N = A - Z$. So $A = Z + N$. Carbon-12, written 12-6-C, is the standard example: it has 6 protons and 6 neutrons, for 12 nucleons in total.

An atom's nucleus is astonishingly small and dense compared with the rest of the atom: the nuclear radius is of order $10^{-15}$ m, while the radius of the surrounding electron cloud is of order $10^{-10}$ m -- about 100,000 times larger. If an atom were scaled up to the size of a football stadium, its nucleus would be about the size of a single pea at the centre. Despite this tiny volume, essentially the entire mass of the atom sits in the nucleus, because an electron's mass (roughly 1/1837th of a proton's) is negligible next to a proton's or neutron's mass -- so nuclear density works out to roughly 100,000 times the density of ordinary matter. The nuclear charge, $+Ze$, is exactly balanced by the $-Ze$ spread across the surrounding electron cloud, so a neutral atom carries no net charge overall.

An empirical formula links the nuclear radius to the number of nucleons it contains: $R = R_0 A^{1/3}$, where $R_0$ is a constant common to essentially all nuclei, with measured value $R_0 = 1.33 \times 10^{-15}$ m. Since nuclear volume $V$ scales as $R^3$, this formula also tells us $V \propto A$ -- in other words, adding nucleons adds volume roughly proportionally, consistent with nuclear matter having a roughly constant density regardless of which nuclide is being considered.