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
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:
, and Z
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 AND 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.
Properties of the Leptons