Transmutation and Radioactivity

Submitted by ChemPRIME Staff on Wed, 12/08/2010 - 23:58

Transmutation of one elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means. into another requires a change in the structures of the nuclei of the atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. involved. For example, the first step in the spontaneousCapable of proceeding without an outside source of energy; refers to a reaction in which the products are thermodynamically favored (product-favored reaction). radioactiveDescribes a substance that gives off radiation‐alpha particles, beta particles, or gamma rays‐by the disintegration of its nucleus. decay of uranium is emission of an α particle, 42He2+, from the nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass. 23892U. Since the α particle consists of two protons and two neutrons, the atomic numberThe number of protons in the nucleus of an atom; used to define the position of an element in the periodic table; represented by the letter Z. must be reduced by 2 and the mass numberThe sum of the numbers of protons and neutrons in an atom; these two kinds of particles contain almost all of the mass of an atom. by 4. The productA substance produced by a chemical reaction. of this nuclear reaction is therefore 23490Th. In other words, loss of an α particle changes (transmutes) uranium into thorium. In the equation for the decay, the sum of the atomic numbers on the left and right are equal, as is the sum of the massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. numbers on the left and right:

{}_{\text{92}}^{\text{238}}\text{U }\to \text{ }{}_{\text{90}}^{\text{234}}\text{Th* + }{}_{\text{2}}^{\text{4}}\alpha      (1)

Alpha decay is typical for large nuclei, because it reduces their size rapidly. Every element above Z = 83 (Bi) is radioactive, apparently because no number of neutrons can stabilize the nucleus against the repulsions between large numbers of protons.

Loss of a β particle (electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass.) from an atomic nucleus leaves the nucleus with an extra unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. of positive charge, that is, an extra proton. This increases the atomic number by 1 and also changes one element to another. For example, the 23490Th mentioned above emits β particles. Its atomic number increases by 1, but its mass number remains the same. (The β particle is an electron and has a very small mass.) In effect one neutron is converted to a proton and an electron. Thus the thorium transmutes to protactinium 23491Pa. (Note carefully that the β particle is an electron emitted from the nucleus of the thorium atom, not one of the electrons from outside the nucleus.) Using the standard symbol

{}_{\text{90}}^{\text{234}}\text{Th  }\to \text{ }{}_{\text{91}}^{\text{234}}\text{Pa + }{}_{-\text{1}}^{\text{0}}\beta      (2)

Beta decay increases the number of protons, so it occurs when a nucleus has a high n/p ratio, compared to the stable nuclei of that element. If the nucleus has a low n/p ratio, it can reduce the number of protons by "positronA positively charged particle having the same mass and magnitude of charge as an electron." emission:

{}_{\text{6}}^{\text{11}}\text{C }\to \text{ }{}_{\text{5}}^{\text{11}}\text{B + }{}_{\text{+1}}^{\text{0}}\beta      (3)

Positrons ( {}_{\text{+1}}^{\text{0}}\beta ) are the basis of medical "PET (Positron Emission Tomography) Scans", in which they annihilate their antiparticle, the beta:

{}_{\text{+1}}^{\text{0}}\beta +{}_{\text{-1}}^{\text{0}}\beta \to 2 ~{}_{\text{0}}^{\text{0}}\gamma       (4)

The two gammas leave in opposite directions from the point of the annihilation, so the PET machine can "trace" their origin to create an image.

A gamma rayHigh energy electromagnetic radiation emitted during radioactive decay. is not a particle, and so its emission from a nucleus does not involve a change in atomic number or mass number. Rather it involves a change in the way the same protons and neutrons are packed together in the nucleus. In equation (1) above, the product Th is shown with an asterisk, indicating that the decay leaves it in an excited state. It releases its extra energyA system's capacity to do work. in the form of a gamma:

{}_{\text{90}}^{\text{234}}\text{Th* }\to \text{ }{}_{\text{90}}^{\text{234}}\text{Th + }{}_{\text{0}}^{\text{0}}\gamma       (5)

It is important to note, however, that radioactivityThe release of particles and/or energy from an unstable nucleus. and transmutation both involve changes within the atomic nucleus. Such nuclear reactions will be discussed in more detail in the section devoted to Nuclear Chemistry. Because protons and neutrons are held tightly in the nucleus, nuclear reactions are much less common in everyday life than chemical reactions. The latter involve electrons surrounding the nucleus, and these are much less rigidly held.