Into the nucleus

A simple model of the atom visualizes it as having a central nucleus surrounded by orbiting electrons.

Near the beginning of the 20th century, the New Zealand scientist Ernest Rutherford carried out one of the most significant experiments in physics. He directed a beam of alpha particles (helium nuclei) at a thin sheet of gold foil. Most of the particles passed straight through, some were deflected slightly from a straight-line path, and a few were scattered through quite large angles. Rutherford deduced that the positively-charged alpha particles were scattered by near collisions with the positively-charged nuclei at the center of the gold atoms. Such scattering techniques are .now central to the study of atomic and subatomic structures.

It is now known that every atom contains a nucleus, whose size is about 10,000 times less than that of the atom itself. Most of an atom is empty space. With the exception of hydrogen, every nucleus contains neutrons and protons (collectively called nucleons). It is the so-called strong nuclear force that holds the nucleus together; if the nucleons could not exert this force, the nucleus would fly apart because of the electrical repulsion between the closely packed protons. It turns out that this force is the strongest in nature, being 100 times more intense than the electrostatic force and 1045 times stronger than gravity.

The electrons found surrounding the nuclei of atoms appear to be point-like particles. That is, there is no experimental evidence which indicates that electrons have any size at all. Because of this, and because electrons are stable particles, never decaying into any other particles, it is generally believed that electrons are fundamental particles, not being made out of any other smaller particles.

Electrons are grouped into the family of fundamental particles called leptons. There are six different leptons in all: electrons, muons, taus, electron neutrinos, muon neutrinos, and tau neutrinos. Of these leptons, only electrons are found in ordinary matter. The other members of the family are usually produced in the radioactive decay of other particles. Electrons, muons, and taus all have an electric charge of —1. The neutrinos have no electric charge.

The protons and neutrons found in the nuclei of atoms are not fundamental particles. They are made from more basic elements called quarks. A total of six kinds of quarks are known to exist, the so-called up, down, charm, strange, top, and bottom quarks. Protons are made out of two up quarks and one down quark. Neutrons are made out of two down quarks and one up quark. Thus, all ordinary matter is made out of only up and down quarks and electrons.

Quarks appear to be point-like particles, as are the leptons. It is therefore likely, but not definite, that quarks make up a family of fundamental particles and are not themselves made out of anything else. Quarks can combine together in many combinations to make up larger particles, and over a hundred of these have been detected in experiments. However, the only combination of quarks which is stable is the one which produces a proton. All other combinations decay rapidly into protons, electrons, neutrinos, and gamma rays. (The quark combination making up a neutron is stable only when it is associated with protons in the nucleus of an atom.) In addition, quarks cannot exist individually. They must always be associated with at least one other quark.

Quarks have a fractional electric charge of plus or minus t. or f. However, it is not this electric charge which binds quark combinations together, but rather it is through the exchange of another set of particles called gluons. Gluons can exist in three different states or “colors.” Gluons moving between quarks give rise to the so-called strong force which binds quarks and nucleons together.

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