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Characteristics and Behavior

beta emission, Periodic Law, nuclear charge, neutral atom, beta particles

At the time of the discovery of radioactivity physicists believed that the atom was the ultimate, indivisible building block of matter. The recognition of alpha and beta particles as discrete units of matter and of radioactivity as a process by means of which atoms are transformed into new kinds of atoms possessing new chemical properties because of the emission of one or the other of these particles brought with it the realization that atoms themselves must have structure and that they are not the ultimate, fundamental particles of nature. In 1911 Rutherford proved the existence of a nucleus within the atom by experiments in which alpha particles were scattered by thin metal foils (see Atom). The nuclear hypothesis has since grown into a refined and fully accepted theory of atomic structure, in terms of which the entire phenomenon of radioactivity can be explained. Briefly, the atom is thought to consist of a dense central nucleus surrounded by a cloud of electrons. The nucleus, in turn, is composed of protons equal in number to the electrons (in an electrically neutral atom), and neutrons. An alpha particle, or doubly charged helium ion, consists of two neutrons and two protons, and hence can be emitted only from the nucleus of an atom. Loss of an alpha particle by a nucleus results in the formation of a new nucleus, lighter than the original by four mass units (the masses of the neutron and of the proton are about one unit each). An atom of the uranium isotope of mass 238, upon emitting an alpha particle, becomes an atom of another element of mass 234. Each of the two protons that form part of the alpha particle emitted from an atom of uranium-238 possesses a unit of positive electric charge. The number of positive charges in the nucleus, balanced by the same number of negative electrons in the orbits outside the nucleus, determines the chemical nature of the atom. Because the charge on the uranium-238 nucleus decreases by two units as a result of alpha emission, the atomic number of the resultant atom is 2 less than that of the original, which was 92. The new atom has an atomic number of 90 and hence is an isotope of the element thorium. See Elements, Chemical; Nuclear Chemistry; Periodic Law.

Thorium-234 emits beta particles, which are electrons. According to current theory, beta emission is accomplished by the transformation of a neutron into a proton, thus resulting in an increase in nuclear charge (or atomic number) of one unit. The mass of the electron is negligible, thus the isotope that results from thorium-234 decay has mass number 234 but atomic number 91 and is, therefore, a protactinium isotope.

Article key phrases:

beta emission, Periodic Law, nuclear charge, neutral atom, beta particles, mass number, Nuclear Chemistry, alpha particles, mass units, Thorium, atomic number, neutrons, protons, neutron, orbits, electrons, Rutherford, proton, decay, realization, experiments, masses, existence, formation, transformation, Elements, increase, Loss, means, unit, turn, process, time, results, terms

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