James Richard Fromm
Nuclear reactions can be of many types, but chemists generally need consider only a few of them. In this section we consider the three basic forms of nuclear decay known as alpha emission, beta emission, and gamma emission. Two forms of energetic nuclear reactions, nuclear fission and nuclear fusion, and some of their consequences are discussed in a later section.
The emission of alpha radiation, which we now know is the emission of an alpha particle or helium nucleus 4He, is one of the common pathways by which a radioactive isotope can decay to a stable isotope. The atomic number of the nucleus decreases by two and the atomic mass decreases by four. The kinetic energy of the helium nucleus emitted as alpha radiation is large by chemical standards and will cause secondary reactions in molecules impacted by the helium nucleus. Alpha radiation is therefore damaging, but it is not penetrating; alpha radiation can be stopped by a sheet of paper, a short distance in air, or human skin. Alpha sources are very dangerous if their contents are breathed or consumed, because then the decaying nuclei are within the body and, short-range or not, can be highly destructive of human tissue. Plutonium is dangerous in this way, because the artificially produced isotope plutonium-239 is an alpha emitter; its decay reaction is
239Pu 4He + 235U.
The emission of a beta particle, or beta radiation, is the emission of a very energetic electron from the nucleus of an atom. The emission of a beta particle has the effect of leaving the nuclear mass unchanged but increasing the nuclear charge by one, and so the element produced by beta decay is one column to the right in the periodic chart from the original element. For this reason beta decay is sometimes considered to be the transformation of one neutron into a proton and an electron, even though that is an oversimplified picture of what actually occurs in the nucleus. Carbon-14 is one of the radioactive isotopes which undergo beta decay; its decay reaction is
14C 14N + 0e-.
Beta radiation is somewhat more penetrating than is alpha radiation.
Emission of gamma radiation is the emission of very high-energy photons, not the emission of particles. Many alpha and beta emissions are accompanied by gamma emissions because the nucleus loses excess energy through gamma radiation. The alpha emission reaction
238U 4He + 234Th + 2hv
involves emission of two photons of gamma radiation, of different energies, in addition to the emission of the energetic alpha particle.
Gamma radiation is both highly destructive and highly penetrating. Gamma radiation can only be stopped by meter-thick shields of a dense material such as lead or even thicker shields of concrete. Very high-energy gamma radiation, called cosmic radiation, comes to earth continually from distant regions of outer space. Cosmic radiation has sufficient energy to penetrate many kilometers of rock.