actinoid element, also called actinide element, any of a series of 15 consecutive chemical elements in the periodic table from actinium to lawrencium (atomic numbers 89–103). As a group, they are significant largely because of their radioactivity. Although several members of the group, including uranium (the most familiar), occur naturally, most are man-made. Both uranium and plutonium have been used in atomic weapons for their explosive power and currently are being employed in nuclear plants for the production of electrical power. These elements are also called the actinide elements. However, the International Union of Pure and Applied Chemistry, the international body in charge of chemical nomenclature, prefers the term actinoid, since the -ide ending is usually reserved for negatively charged ions.
General similarities of the actinoid elements
The actinoid elements follow one another in the seventh series of the periodic table. Each has 86 electrons arranged as in the atoms of the noble gas radon (which precedes actinium by three columns in the table), with three more electrons that may be positioned in the 6d and 7s orbitals (the seventh shell is outermost), and with additional electrons packing into inside orbitals. Specifically, the series is formed by the insertion of one more electron for each successive new element into an underlying 5f orbital. The valence electrons, however, are found mainly in the 6d and 7s orbitals. Thus, the chief difference between the atoms of the elements of the series is the presence of additional 5f electrons deep within the electron cloud. Because of its position in the 5th shell, this distinguishing electron subshell actually affects the chemical properties of the actinoids only in a relatively minor way; 5f electrons do not usually contribute to the formation of chemical bonds with other atoms.
As with the elements of any group, there are a number of exceptions to these generalities, particularly in the lower members of the series, but, for most of these elements, the concept of a series of chemically similar actinoid elements is a useful guide for predicting their chemical and physical properties.
Like all elements, each actinoid has its own unique atomic number, equal to the number of protons in the nucleus and, consequently, to the number of electrons. At the same time, the atoms of an element are capable of existing in a number of forms (isotopes), each of which has a different number of neutrons in its nucleus and hence a different atomic mass. Although isotopes of a given element behave alike chemically, they have differing stabilities in relation to radioactive decay, which is a property of the nucleus. No element beyond bismuth in the periodic table—i.e., no element that has an atomic number greater than 83—has any stable isotopes; radioactive isotopes of every element in the table can be produced in the laboratory. The actinoids are unusual in forming a series of 15 elements having no stable isotopes; every actinoid isotope undergoes radioactive decay, and, as a result, only a few of the lighter, stabler members of the series (such as thorium and uranium) are found in nature. The half-life, or the precise time required for one-half of any amount of a particular isotope to disappear as a result of radioactive decay, is a measure of the stability of that isotope. Three naturally occurring isotopes in the actinoid series (232Th, 235U, and 238U) have long half-lives, of the order of billions of years. These isotopes are described as primordial, because they are believed to have been present when Earth accreted. Some of the isotopes to which the primordial actinoid isotopes decay are also found in nature, but the half-lives of the isotopes in the 232Th, 235U, or 238U decay chains are much shorter. See actinium and protactinium.
Actinium, thorium, protactinium, and uranium are the only actinoid elements found in nature to any significant extent. The remaining actinoid elements, commonly called the transuranium elements, are all man-made by bombarding naturally occurring actinoids with neutrons in reactors or with heavy ions (charged particles) in particle accelerators (such as cyclotrons). The actinoids beyond uranium do not occur in nature (except, in some cases, in trace amounts), because the stability of their isotopes decreases with increase in atomic number and whatever quantities may be produced decay too fast to accumulate. The half-life of uranium-238, the stablest uranium isotope, is 4.5 ×109 years. Plutonium-239 has a half-life of 24,400 years and is produced in reactors in ton amounts, but nobelium and lawrencium, elements 102 and 103, with half-lives of seconds, are produced a few atoms at a time. The first of these synthetic actinoid elements to be discovered (1940) was neptunium, atomic number 93, which was prepared by bombardment of uranium metal with neutrons.