Radioactivity is the emission of radiation by unstable nuclei. That radiation may exist in the form of subatomic particles (primarily alpha and beta particles) or in the form of energy (primarily gamma rays).
Radioactivity was discovered accidentally in 1896 by French physicist Henri Becquerel (1852–1908). In the decades that followed Becquerel's discovery, research on radioactivity produced revolutionary breakthroughs in our understanding of the nature of matter and led to a number of important practical applications. These applications include a host of new devices and techniques ranging from nuclear weapons and nuclear power plants to medical techniques that can be used for diagnosing and treating serious diseases.
The nucleus of all atoms (with the exception of hydrogen) contains one or more protons and one or more neutrons. The nucleus of most carbon atoms, for instance, contains six protons and six neutrons. In most cases, the nuclei of atoms are stable; that is, they do not undergo changes on their own. A carbon nucleus will look exactly the same a hundred years from now (or a million years from now) as it does today.
But some nuclei are unstable. An unstable nucleus is one that undergoes some internal change spontaneously. In this change, the nucleus gives off a subatomic particle, or a burst of energy, or both. As an example, an isotope of carbon, carbon-14, has a nucleus consisting of six protons and eight (rather than six) neutrons. A nucleus that gives off a particle or energy is said to undergo radioactive decay, or just decay.
Alpha particle: The nucleus of a helium atom, consisting of two protons and two neutrons.
Beta particle: An electron emitted by an atomic nucleus.
Gamma ray: A high-energy form of electromagnetic radiation.
Isotopes: Two or more forms of an element with the same number of protons but different numbers of neutrons in their atomic nuclei.
Nucleus (atomic): The core of an atom, usually consisting of one or more protons and neutrons.
Radioactive decay: The process by which an atomic nucleus gives off radiation and changes into a new nucleus.
Radioactive family: A group of radioactive isotopes in which the decay of one isotope leads to the formation of another radioactive isotope.
Stable nucleus: An atomic nucleus that does not undergo any changes spontaneously.
Subatomic particle: Basic unit of matter and energy (proton, neutron, electron, neutrino, and positron) smaller than an atom.
Unstable nucleus: An atomic nucleus that undergoes some internal change spontaneously.
Scientists are not entirely clear as to what makes a nucleus unstable. It seems that some nuclei contain an excess number of protons or neutrons or an excess amount of energy. These nuclei restore what must for them be a proper balance of protons, neutrons, and energy by giving off a subatomic particle or a burst of energy.
In this process, the nucleus changes its composition and may actually become a different nucleus entirely. For example, in its attempt to achieve stability, a carbon-14 nucleus gives off a beta particle. After the carbon-14 nucleus has lost the beta particle, it consists of seven protons and seven neutrons. But a nucleus consisting of seven protons and seven neutrons is no longer a carbon nucleus. It is now the nucleus of a nitrogen atom. By giving off a beta particle, the carbon-14 atom has changed into a nitrogen atom.
The forms of radiation most commonly emitted by a radioactive nucleus are called alpha particles, beta particles, and gamma rays. An alpha particle is the nucleus of a helium atom. It consists of two protons and two neutrons. Consider the case of a radium-226 atom. The nucleus of a radium-226 atom consists of 88 protons and 138 neutrons. If that nucleus gives off an alpha particle, it must lose the two protons and two neutrons of which the alpha particle is made. After emission of the alpha particle, the remaining nucleus contains only 86 protons (88 − 2) and 136 neutrons (138 − 2). This nucleus is the nucleus of a radon atom, not a radium atom. By emitting an alpha particle, the radium-226 atom has changed into an atom of radon.
The emission of beta particles from nuclei was a source of confusion for scientists for many years. A beta particle is an electron. The problem is that electrons do not exist in the nuclei of atoms. They can be found outside the nucleus but not within it. How is it possible, then, for an unstable nucleus to give off a beta particle (electron)?
The answer is that the beta particle is produced when a neutron inside the atomic nucleus breaks apart to form a proton and an electron:
neutron → proton + electron
Recall that a proton carries a single positive charge and the electron a single negative charge. That means that a neutron, which carries no electrical charge at all, can break apart to form two new particles (a proton and an electron) whose electrical charges add up to make zero.
Think back to the example of carbon-14, mentioned earlier. A carbon-14 nucleus decays by giving off a beta particle. That means that one neutron in the carbon-14 nucleus breaks apart to form a proton and an electron. The electron is given off as a beta ray, and the proton remains behind in the nucleus. The new nucleus contains seven protons (its original six plus one new proton) and seven neutrons (its original eight reduced by the breakdown of one).
The loss of an alpha particle or a beta particle from an unstable nucleus is often accompanied by the loss of a gamma ray. A gamma ray is a form of high-energy radiation. It is similar to an X ray but of somewhat greater energy. Some unstable nuclei can decay by the emission of gamma rays only. When they have lost the energy carried away by the gamma rays, they become stable.
Many radioactive elements occur in nature. In fact, all of the elements heavier than bismuth (atomic number 83) are radioactive. They have no stable isotopes.
The heaviest of the radioactive elements are involved in sequences known as radioactive families. A radioactive family is a group of elements in which the decay of one radioactive element produces another element that is also radioactive. As an example, the parent isotope of one radioactive family is uranium-238. When uranium-238 decays, it forms thorium-234. But thorium-234 is also radioactive. When it decays, it forms protactinium-234. Protactinium-234, in turn, is also radioactive and decays to form uranium-234. The process continues for another eleven steps. Finally, the isotope polonium-210 decays to form lead-206, which is stable.
Many lighter elements also have radioactive isotopes. Some examples include hydrogen-3, carbon-14, potassium-40, and tellurium-123.
Radioactive isotopes can also be made artificially. The usual process is to bombard a stable nucleus with protons, neutrons, alpha particles, or other subatomic particles. The bombardment process can be accomplished with particle accelerators (atom-smashers) or in nuclear reactors. When one of the bombarding particles (bullets) hits a stable nucleus, it may cause that nucleus to become unstable and, therefore, turn radioactive.