Structure Of The Atomic Nucleus

Type of physical science: Nuclear physics

Field of study: Nuclei

The atomic nucleus consists of myriad particles that are often described in quantum terms. There are multiple levels of particles that may be described accurately as states of energy.

Overview

The first real science to address the subject of atomic structure occurred on April 29, 1897, when Joseph John Thomson announced that he had discovered exceedingly small subatomic particles, which he called corpuscles. Later, corpuscles were renamed electrons. In 1912, English physicist Ernest Rutherford, a colleague of Thomson, discovered that a very dense central core, known as the nucleus, was in the innermost part of the atom. Rutherfold also deduced that the neutron contains smaller positively charged particles, which he called protons.

The electron is much lighter in mass than the whole atom. Yet, the nucleus, which comprises such an overwhelming part of the mass (it is thousands of times heavier than the electron) is only a tiny part of the atom's total volume. The electron carries a net negative charge, which is counterbalanced by the nucleus' net positive charge.

Rutherford constructed a device that used a radioactive α particle emitter as a rudimentary particle accelerator and aimed the particles at a gold screen target with a fluorescent screen just beyond. If a few of the massive α particles would strike the nucleus of the atoms in the gold foil, the nucleus' nature would be revealed by the degree of deflection of the particles from the center of the fluorescent screen. It was discovered that the nucleus is extremely dense, very small, and surrounded by an orbiting cloud of electrons. This landmark discovery drew a clear line between classical physics and the newly emerging quantum physics.

Danish physicist Niels Bohr pondered Rutherford's model and found that the electrons could not possibly orbit the nucleus in the classical sense; the atom would be unstable and fly apart. A year after Rutherford made his announcement, Bohr radically changed physics when he announced that classical laws gave way to a peculiar set of quantum laws at atomic sizes.

Quantum physics makes a clear distinction between the macroscopic world larger than the atom and the microscopic world existing inside the electron shell in the interior of the atom. Atoms exhibit both wavelike and particle-like properties. In fact, all the atomic constituents exhibit this dual nature. Quantum physics defines these particles as fields that obey the laws of quantum physics and relativity. Thus, subatomic particles are actually defined by intense fields at a point.

In 1932, English physicist James Chadwick discovered that the nucleus contained another particle that he called the neutron, a neutrally charged particle. By the beginning of World War II, the atom was thought to consist of a nucleus of protons and neutrons embedded within a shell of electrons. In 1946, Japanese physicist Hideki Yukawa deduced that there must be an enormously powerful force holding together the quanta (protons and neutrons) inside the nucleus. He correctly noted that if the force binding the electrons had an associated quanta (photons), then the force binding the nuclear particles also should have an associated quantum particle, which he called the meson. With that conjecture, Yukawa initiated the discoveries of an endless profusion of particles.

In his elegant and revolutionary theories, Albert Einstein demonstrated unequivocally that matter and energy, under the right conditions, were interchangeable. In the quantum state, a binding force (such as the nucleus) can be transformed into a quantum particle and back again. In the stable form, atoms maintain an equilibrium where their quantum particle constituents are stable and well defined as its electrons and nucleus of protons and neutrons. As the atom is destabilized or blown apart in a particle accelerator, quantum forces that hold the atom together in a stable form may come apart and be identified as quantum particles. According to Yukawa, if the proton and neutron are separated from the nucleus, the quantum force binding them together would be seen as a particle he called the meson, later renamed the pion. In 1948, the existence of the pion, as the particle associated with the quantum force holding the proton and neutron together, was discovered in a particle accelerator experiment.

When the powerful particle accelerators were brought on line in the 1950's and 1960's, the short list of three nuclear particles grew dramatically. These machines probed deeper and deeper into the atomic nucleus, accelerating protons to very high velocities and slamming them into one another. The energy of the collision broke the protons into their constituent particles, which physicists photographed and identified. These scientists were trying to determine whether the protons and neutrons were composed of smaller particles called hadrons. The number and variety of hadrons appeared infinite, which stunned and disappointed physicists who were hoping for a simplification of matter. The American physicist Enrico Fermi commented that if he had known about hadrons, he would have gone into zoology instead of physics.

By 1960, physicists had determined that the nucleus was composed of protons and neutrons bound together by pions. As the protons and neutrons were broken apart, they appeared to be composed of an infinite variety of hadrons. In 1961, American physicist Murray Gell-Mann discovered that there was a pattern in the glut of hadrons. He indexed and cataloged them according to some of their most common characteristics, such as their spin, charge, and mass. He called his classification scheme the eightfold way. This eventually led to the postulation that hadrons were made up of simpler quantum particles, which he called quarks. According to Gell-Mann, an infinite variety of hadrons could be created in the breakup of a proton. Hadrons actually were very unstable proton parts composed of smaller quanta called quarks.

Physicists use models to describe nuclear interactions. Shell models describe the nature of the proton and neutron inside the nucleus, collectively called a nucleon. The nuclear shell model is analogous to the electronic shell models of atoms, except that the nuclear shell model describes stable nuclear configurations as they are bound together by the strong nuclear force.

The nuclear shell models predict stable configurations of nucleons inside the nucleus. The shell models describe how each nucleon moves about in the nucleus. They predict that each nucleon may be described in terms of their respective states and that these states of nucleons form shells or layers within the nucleus.

In a given nucleus, nucleons tend to assemble together in the most stable state possible.

In this process, the nuclear particles exist in a state of lowest possible internal energy while giving off external energy. Thus, the internal energy is negative, and most nuclei tend to attain the lowest or most negative possible energy. The reciprocal of this negative measure is called the nucleus' "binding energy." Thus, a very heavy nucleus with many nucleons bound together would have much more binding energy than a light nucleus with fewer nucleons. Observation of these properties has demonstrated that the internal or binding energy is almost directly proportional to the number of nucleons. Since the binding energy is proportional to the number of nucleons, the binding energy itself has been measured at about eight million electronvolts per nucleon.

As the nucleons in a given nucleus fill up these shells or layers within the nucleus, particularly stable orbital configurations are attained. These orbital configurations are defined in terms of how many nucleons occupy a given shell or layer. The number of neutrons and protons which correspond to particularly stable structures in closed shells are called "magic numbers."

The magic numbers for both protons and neutrons are 2, 8, 14, 20, 50, and 82. At this point, the magic numbers for protons and neutrons diverge. Beyond 82, the magic numbers for neutrons are 126 and 184, while the magic numbers corresponding to protons are 114 and 164.

Applications

High-energy particle physics requires increasingly powerful and larger instruments to look deeper into the atom. Particle accelerators are the ultimate microscopes; yet, they require huge energies and even large land areas to contain the enormous tracks down which the particles are accelerated. Current particle accelerators cover hundreds of meters. The newest accelerators will span tens of kilometers. Peering ever deeper into the atomic core requires not only huge machines costing billions of dollars, but also extraordinary energies. Breaking the bonds between the nuclear quanta requires that the colliding protons be traveling at enormous velocities.

Physicists have found that using the current from a common household outlet can enable them to look into the atom and distinguish the electron from the atomic nucleus. To look inside the nucleus requires far more energy, and to look inside the proton and neutron requires the energy of a small city.

Disassembling the atom ultimately will reveal whether all laws of the physical universe are characterized in a single set of laws that have been called the unified field theory. Physicists have been working to discover a concise law that describes all matter, forces, and gravity. Such a law is impossible until the exact nature of all matter and the forces that bind that matter are understood completely. The structure of the atomic nucleus also has applications in cosmology.

The big bang theory states that the universe came from a single, vast explosion 10 billion years ago from which matter condensed out of the energy of a primordial fireball. Investigation into the atomic interior relates to how that matter condensed and what universal constants fell out of such a condensation.

Other questions relating to the atomic nucleus are of vital importance to theoretical physics. Many theories relating to the nature of matter state that matter itself is ultimately unstable, that atoms should eventually disintegrate into energy. According to one theory, the proton is unstable and eventually decays. When the proton decays, the atom spontaneously disintegrates. Such theories are being tested now, although they are still inconclusive. Yet, even the radically new ideas such as those embodied in string theory also call for proton decay.

A new mathematics is needed to incorporate some of the discoveries being made in theoretical and experimental physics. Sir Isaac Newton needed to invent calculus to describe his concept of gravity, and Albert Einstein transformed Reimann geometry and tensor calculus to describe relativity. The new mathematics that will ultimately describe the new physics will have a ripple-down effect on not only physics but other science and technology as well.

Context

The social imperative to explore the universe has not always been linked to profit, a consideration which tends to drive many other technological efforts. There are few direct benefits from particle physics, yet some of the most profound discoveries in theoretical physics result from the use of particle accelerators. From the earliest philosophical questions about the nature of matter to the latest discoveries in string theory, scientists have found that matter is both tangible and ephemeral, with both particle-like and quantum characteristics. The structure of the atomic nucleus has depicted a world of quantum reality which challenges logic and requires a new philosophy of nature.

Principal terms

BINDING ENERGY: the amount of external energy emitted in the binding process of the nucleons

EIGHTFOLD WAY: the classification scheme applied to hadrons, which led to the discovery of quarks

HADRON: a quanta of protons and neutrons composed of quark combinations

MAGIC NUMBERS: the number associated with the quantity of nuclear particles required to form a stable shell or layer of protons or neutrons in an atomic nucleus

MESON: the quanta associated with the force binding protons and neutrons together in the nucleus; later called the pion

NUCLEON: the collective name for the proton and neutron inside the atomic nucleus bound together by the strong nuclear force

QUANTA: the designation of the constituents of atoms which are actually states of field that may be described either as wavelike energy or discrete particles

QUARK: the quanta of which protons and neutrons are composed

SHELL MODELS: nuclear models which describe the basic nuclear properties of a nucleus; ordering the protons and neutrons inside the nucleus in their most stable configurations

Bibliography

Crease, Robert P., and Charles C. Mann. THE SECOND CREATION. New York: Macmillan, 1986. Follows the making of twentieth century physics from its nineteenth century roots. Microscopically examines characters and personalities as well as the issues of physics. The atomic structure is defined as it is discovered alongside the other pioneering work of the mid- to late 1980's. Perhaps the most complete book ever written on the personalities and work of twentieth century particle physicists. Highly readable.

Hawking, Stephen W. A BRIEF HISTORY OF TIME. New York: Bantam, 1988. Examines the universe from the author's view of creation to the present. Examines the far-flung reaches of space and time from black holes to the interior of the atom and discusses the elementary particles of the atomic nucleus. Written for the lay public. Illustrated.

Pagels, Heinz R. THE COSMIC CODE. New York: Bantam, 1982. Describes quantum physics as "the language of nature." Pagels embarks on a literary quest to explain some of the most profoundly difficult topics in quantum physics to the lay reader. Opens up the atom to provide a clear view of its interior. Reads clearly. Illustrated.

Pagels, Heinz R. PERFECT SYMMETRY. New York: Bantam, 1985. In this follow-up book to The Cosmic Code, Pagels delves even deeper, providing a layperson's perspective. Covers the aspect of the atom's core and supports a lively discussion of the grand unified field theories and other frontier topics in physics. Very readable.

Sutton, Christine. THE PARTICLE CONNECTION. New York: Simon & Schuster, 1984. Explains the essentials behind the particle accelerator. Describes how the machine is used and the nature of the particle chase at CERN, the European particle accelerator laboratory. Illustrated in detail.

Atomic nuclei: hydrogen (top) and lithium (bottom)

Models of the Atomic Nucleus

Electrons and Atoms

The Periodic Table and the Atomic Shell Model

The Effect of Electric and Magnetic Fields on Quantum Systems