Subatomic particles are particles that are smaller than an atom. In 1940, the number of subatomic particles known to science could be counted on the fingers of one hand: protons, neutrons, electrons, neutrinos, and positrons. The first three particles were known to be the building blocks from which atoms are made: protons and neutrons in atomic nuclei and electrons in orbit around those nuclei. Neutrinos and positrons were somewhat peculiar particles discovered outside Earth's atmosphere and of uncertain origin or significance.
That view of matter changed dramatically over the next two decades. With the invention of particle accelerators (atom-smashers) and the discovery of nuclear fission and fusion, the number of known subatomic particles increased. Scientists discovered a number of particles that exist at energies higher than those normally observed in our everyday lives: sigma particles, lambda particles, delta particles, epsilon particles, and other particles in positive, negative, and neutral forms. By the end of the 1950s, so many subatomic particles had been discovered that some physicists referred to their list as a "particle zoo."
The quark model
In 1964, American physicist Murray Gell-Mann (1929– ) and Swiss physicist George Zweig (1937– ) independently suggested a way out of the particle zoo. They suggested that the nearly 100 subatomic particles that had been discovered so far were not really elementary (fundamental) particles. Instead, they suggested that only a relatively few elementary particles existed, and the other subatomic particles that had been discovered were composed of various combinations of these truly elementary particles.
Words to Know
Antiparticles: Subatomic particles similar to the proton, neutron, electron, and other subatomic particles, but having one property (such as electric charge) opposite them.
Atomic mass unit (amu): A unit of mass measurement for small particles.
Atomic number: The number of protons in the nucleus of an atom.
Elementary particle: A subatomic particle that cannot be broken down into any simpler particle.
Energy levels: The regions in an atom in which electrons are most likely to be found.
Gluon: The elementary particle thought to be responsible for carrying the strong force (which binds together neutrons and protons in the atomic nucleus).
Graviton: The elementary particle thought to be responsible for carrying the gravitational force.
Isotopes: Forms of an element in which atoms have the same number of protons but different numbers of neutrons.
Lepton: A type of elementary particle.
Photon: An elementary particle that carries electromagnetic force.
Quark: A type of elementary particle.
Spin: A fundamental property of all subatomic particles corresponding to their rotation on their axes.
The truly elementary particles were given the names quarks and leptons. Each group of particles, in turn, consists of six different types of particles. The six quarks, for example, were given the rather fanciful names of up, down, charm, strange, top (or truth), and bottom (or beauty). These six quarks could be combined, according to Gell-Mann and Zweig, to produce particles such as the proton (two up quarks and one down quark) and the neutron (one up quark and two down quarks).
In addition to quarks and leptons, scientists hypothesized the existence of certain particles that "carry" various kinds of forces. One of those particles was already well known, the photon. The photon is a strange type of particle with no mass that apparently is responsible for the transmission of electromagnetic energy from one place to another.
In the 1980s, three other force-carrying particles were also discovered: the W + , W − , and Z 0 bosons. These particles carry certain forces that can be observed during the radioactive decay of matter. (Radioactive elements spontaneously emit energy in the form of particles or waves by disintegration of their atomic nuclei.) Scientists have hypothesized the existence of two other force-carrying particles, one that carries the strong force, the gluon (which binds together protons and neutrons in the nucleus), and one that carries gravitational force, the graviton.
Five important subatomic particles
For most beginning science students, the five most important sub-atomic particles are the proton, neutron, electron, neutrino, and positron. Each of these particles can be described completely by its mass, electric charge, and spin. Because the mass of subatomic particles is so small, it is usually not measured in ounces or grams but in atomic mass units (label: amu) or electron volts (label: eV). An atomic mass unit is approximately equal to the mass of a proton or neutron. An electron volt is actually a unit of energy but can be used to measure mass because of the relationship between mass and energy (E = mc 2 ).
All subatomic particles (indeed, all particles) can have one of three electric charges: positive, negative, or none (neutral). All subatomic particles also have a property known as spin, meaning that they rotate on their axes in much the same way that planets such as Earth do. In general, the spin of a subatomic particle can be clockwise or counterclockwise, although the details of particle spin can become quite complex.
Proton. The proton is a positively charged subatomic particle with an atomic mass of about 1 amu. Protons are one of the fundamental constituents of all atoms. Along with neutrons, they are found in a very concentrated region of space within atoms referred to as the nucleus.
The number of protons determines the chemical identity of an atom. This property is so important that it is given a special name: the atomic number. Each element in the periodic table has a unique number of protons in its nucleus and, hence, a unique atomic number.
Neutron. A neutron has a mass of about 1 amu and no electric charge. It is found in the nuclei of atoms along with protons. The neutron is normally a stable particle in that it can remain unchanged within the nucleus for an infinite period of time. Under some circumstances, however, a neutron can undergo spontaneous decay, breaking apart into a proton and an electron. When not contained with an atomic nucleus, the half-life for this change—the time required for half of any sample of neutrons to undergo decay—is about 11 minutes.
The nuclei of all atoms with the exception of the hydrogen-1 isotope contain neutrons. The nuclei of atoms of any one element may contain different numbers of neutrons. For example, the element carbon is made of at least three different kinds of atoms. The nuclei of all three kinds of atoms contain six protons. But some nuclei contain six neutrons, others contain seven neutrons, and still others contain eight neutrons. These forms of an element that contain the same number of protons but different numbers of neutrons are known as isotopes of the element.
Electron. Electrons are particles carrying a single unit of negative electricity with a mass of about 1/1800 amu, or 0.0055 amu. All atoms contain one or more electrons located in the space outside the atomic nucleus. Electrons are arranged in specific regions of the atom known as energy levels. Each energy level in an atom may contain some maximum number of electrons, ranging from a minimum of two to a maximum of eight.
Electrons are leptons. Unlike protons and neutrons, they are not thought to consist of any smaller particles but are regarded themselves as elementary particles that cannot be broken down into anything simpler.
All electrical phenomena are caused by the existence or absence of electrons or by their movement through a material.
Neutrino. Neutrinos are elusive subatomic particles that are created by some of the most basic physical processes of the universe, like decay of radioactive elements and fusion reactions that power the Sun. They were originally hypothesized in 1930 by Swiss physicist Wolfgang Pauli (1900–1958). Pauli was trying to find a way to explain the apparent loss of energy that occurs during certain nuclear reactions.
Neutrinos ("little neutrons") proved very difficult to actually find in nature, however. They have no electrical charge and possibly no mass. They rarely interact with other matter. They can penetrate nearly any form of matter by sliding through the spaces between atoms. Because of these properties, neutrinos escaped detection for 25 years after Pauli's prediction.
Then, in 1956, American physicists Frederick Reines and Clyde Cowan succeeded in detecting neutrinos produced by the nuclear reactors at the Savannah River Reactor. By 1962, the particle accelerator at Brookhaven National Laboratory was generating enough neutrinos to conduct an experiment on their properties. Later, physicists discovered a second type of neutrino, the muon neutrino.
Traditionally, scientists have thought that neutrinos have zero mass because no experiment has ever detected mass. If neutrinos do have a mass, it must be less than about one hundred-millionth the mass of the proton, the sensitivity limit of the experiments. Experiments conducted during late 1994 at Los Alamos National Laboratory hinted at the possibility that neutrinos do have a very small, but nonzero, mass. Then in 1998, Japanese researchers found evidence that neutrinos have at least a small mass, but their experiments did not allow them to determine the exact value for the mass.
In 2000, at the Fermi National Accelerator Laboratory near Chicago, a team of 54 physicists from the United States, Japan, South Korea, and
Greece detected a third type of neutrino, the tau neutrino, considered to be the most elusive member of the neutrino family.
Positron. A positron is a subatomic particle identical in every way to an electron except for its electric charge. It carries a single unit of positive electricity rather than a single unit of negative electricity.
The positron was hypothesized in the late 1900s by English physicist Paul Dirac (1902–1984) and was first observed by American physicist Carl Anderson (1905–1991) in a cosmic ray shower. The positron was the first antiparticle discovered—the first particle that has properties similar to protons, neutrons, and electrons, but with one property exactly the opposite of them.