Elementary particles are the building blocks of all types of matter, more fundamental even than the neutrons and protons that form the nuclei of atoms. They are also the entities that convey the four basic forces of nature: electromagnetism, gravity, and the strong and weak forces that act within nuclei. By developing theories based on the interactions of elementary particles, theoreticians hope to form a single model—the Theory of Everything, or TOE—that explains the four forces as different facets of a single force.
Particle physics is the study of elementary particles. To date, experimental particle physicists have found evidence for more than 200 elementary particles, most of which are combinations of a much smaller set of the most fundamental elementary particles. Proof of the existence of particles predicted by theoretical physicists plays an important role in the quest for the TOE.
Protons, neutrons, and electrons
Until the very end of the 19th century, atoms were thought to be the most fundamental form of matter, since they are immune to attempts to split them into simpler substances by chemical means. This view changed in 1897, when the British physicist Joseph Thomson announced his discovery that cathode "rays" were in fact streams of negatively charged particles: fragments of atoms, each with around 1/2000 the mass of a hydrogen atom, that would become known as electrons.
In 1911, the New Zealand-born British physicist Ernest Rutherford discovered by experiment that each atom consists of an extremely dense positive core—the nucleus—surrounded by a diffuse cloud of negatively charged electrons. In 1919, Rutherford converted nitrogen atoms into oxygen atoms by bombarding them with alpha radiation, now known to consist of helium nuclei. The reaction released hydrogen nuclei, which Rutherford identified as components of all types of nuclei. He named them protons.
As early as 1920, Rutherford realized that a third type of subatomic particle must exist: one with no electrical charge but with a mass almost identical to that of the proton. Such a particle—the neutron—would account for the difference between the total mass of the electrons and protons in an atom and the actual atomic mass.
The neutron eluded detection for many years, however, since the particle detectors used at that time depended on the charge of a particle to produce a visible trace. Then, in 1932, the British physicist James Chadwick developed an ingenious technique for detecting neutrons. Chadwick bombarded a beryllium target with alpha particles to release neutrons. These neutrons then passed through paraffin (hydrocarbon) wax, where some of them knocked hydrogen nuclei out of the hydrocarbon molecules. The wax had the second function of blocking the passage of alpha particles, so the traces left by positively charged protons in a particle detector behind the paraffin could be indirectly attributed to free neutrons.
Quantum mechanics and structure
Chadwick’s confirmation of the existence of the neutron completed a triad of particles—proton, neutron, and electron—that is sufficient to describe the chemical behavior and some of the physical properties of the elements. Theoretical physicists used these components to build an atomic model based on the wavelike motion of electrons around a nucleus of protons and neutrons. In that model, the electrons in an atom can exist only in certain states, called wave functions, that are defined by three quantum numbers.
The principal quantum number, n, is related to the total energy of an electron in a given state and has whole-number values greater than zero. A second quantum number, l, refers to the total angular momentum of an electron in a given state and may be zero or a positive whole number up to n – 1. The third quantum number, m, refers to the component of angular momentum along an arbitrary axis. The permitted values of m for a given state are whole numbers from –l to l and zero.
