Neutrons

FIELDS OF STUDY: Nuclear Chemistry

ABSTRACT

The discovery and basic properties of neutrons are described. Neutrons are found in all but the simplest hydrogen atom. They stabilize the protons against the force of electrostatic repulsion. They are electrically neutral but seem to have an internal structure that includes electrical charge.

Models of the Atom

Various models of atomic structure were proposed throughout the nineteenth and twentieth centuries, the most useful being the "planetary" model and the "plum pudding" model. In the first, an atom was envisioned to be composed of a nucleus with electrical charges whirling about it like the planets around the sun. In the latter model, the atom was envisioned to be a round, positively charged blob with dots of negative electrical charge embedded throughout. In 1897, while conducting research on the nature of cathode rays in cathode ray tubes, J. J. Thomson (1856–1940) discovered that the cathode rays behaved more like streams of charged particles than like electromagnetic radiation. He called the particles "corpuscles" at first, though they soon became known as electrons. Based on this discovery, Thomson proposed the plum-pudding model of the atom in 1904.

Thomson’s discoveries verified that atoms had an internal structure consisting of fundamental particles, but the existence of subatomic particles other than electrons had yet to be proved. In 1909, Ernest Rutherford (1871–1937), Hans Geiger (1882–1945), and Ernest Marsden (1889–1970) performed what is known as the gold-foil experiment, which produced more conclusive results. By directing a beam of α (alpha) particles at a very thin target of gold foil, they found that while most of the particles passed directly through the foil as though it were not there, some of the particles were deflected at various angles, as though they had bounced off something very small and very dense inside the metal. These observations led Rutherford to propose the planetary model of atomic structure, with the majority of the atom’s mass concentrated in a small, dense nucleus. He continued experimenting with alpha particles, and in 1917 he discovered that the collision of an alpha particle with the nucleus of a nitrogen atom released a positively charged particle identical to a hydrogen nucleus—in other words, a proton, as the most common isotope of hydrogen contains no neutrons. Rutherford also posited the existence of an as-yet-undiscovered neutral particle, though he believed it to be composed of one electron and one proton, which was later disproved. The existence of the neutron was finally experimentally verified by James Chadwick (1891–1974) in 1932.

Properties and Function of Neutrons

The atomic number of an element is equal to the number of protons in its nucleus, as it is the number of protons that defines an atom’s identity as an element. In contrast, the number of neutrons in an atom’s nucleus can vary without changing its elemental identity. Rather, atoms containing the same number of protons but different numbers of neutrons are said to be different isotopes of the same element. The simplest example of this is deuterium, an isotope of hydrogen that contains one proton and one neutron in its nucleus (the most common isotope of hydrogen has one proton and no neutron). Deuterium can also be called hydrogen-2, where the "2" is the total number of protons and neutrons in the nucleus and is approximately equal to the atomic mass of the isotope.

The positively charged protons would readily fly apart from each other due to electrostatic repulsion between like charges if not for the presence of neutrons, which counteract this repulsion. Studies in high-energy particle physics suggest that the neutron has an internal structure that effectively places a negative charge within the neutron next to the positive charge of the proton, as shown below:

In this way, the neutron acts as a sort of glue for protons. Neutrons interact strongly with nuclei of other atoms but produce only weak ionization from collisions with electrons. Free neutrons have a half-life of about twelve minutes and decompose into a proton, an electron, and a neutrino, which is an electrically neutral physical particle having no detectable mass.

The Discovery of the Neutron

James Chadwick had studied under Rutherford and later worked alongside him. He, too, was convinced of the existence of the neutron, and throughout the 1920s he conducted several experiments designed to prove it. However, it was not until the end of the decade that new developments paved the way for Chadwick to make his discovery.

In 1930, Walther Bothe (1891–1957) and his student Herbert Becker reported on an experiment in which they had bombarded beryllium and other light elements, including boron and lithium, with alpha particles. The bombardment produced an unidentified radiation that easily penetrated other materials, which they believed to consist of γ (gamma) rays. Irène Joliot-Curie (1897–1956) and her husband, Frédéric Joliot-Curie (1900–1958), investigated this new radiation further by directing it at a sheet of paraffin, a type of wax composed of hydrogen and carbon atoms. They reported in early 1932 that when the paraffin was struck by the unidentified radiation, it emitted protons with high kinetic energy.

In an elastic collision between two particles, the kinetic energy of one particle is transferred completely to the other, and the velocity of the rebounding particles is directly related to their mass. Chadwick realized that the Joliot-Curies’ discovery was consistent with the idea that the unidentified radiation was composed not of gamma rays but of neutrally charged particles and that, if this were true, the mass of such a particle had to be similar to that of the proton to account for the transfer of energy. In January 1932, the same month that the Joliot-Curies published their observations, Chadwick immediately began to replicate their experiment with some modifications. He theorized, and his research confirmed, that when the nucleus of an atom of beryllium-9 was struck by an alpha particle, the product would not be an atom of carbon-13 accompanied by the emission of a gamma ray, but rather an atom of carbon-12 accompanied by the ejection of a neutron—the source of the unidentified radiation.

PRINCIPAL TERMS

• atomic mass: the total mass of the protons, neutrons, and electrons in an individual atom.
• deuterium: an isotope of hydrogen that contains one neutron and one proton; occurs naturally in about 1 in 6,500 hydrogen atoms.
• fundamental particle: one of the smaller, indivisible particles that make up a larger, composite particle; commonly used to refer to electrons, protons, and neutrons, although these are themselves composed of various actual fundamental particles, such as quarks, leptons, and certain types of bosons.
• isotope: an atom of a specific element that contains the usual number of protons in its nucleus but a different number of neutrons.
• nucleus: the central core of an atom, consisting of specific numbers of protons and neutrons and accounting for at least 99.98 percent of the atomic mass.

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