a family of 14 chemical elements with atomic numbers 90 to 103, which follow actinium (Ac) in Period 7 of Mendeleev’s periodic table and, like Ac, are in Group III. The actinides include thorium (symbol Th; atomic number Z = 90), protactinium (Pa; 91), uranium (U; 92), neptunium (Np; 93), plutonium (Pu; 94), americium (Am; 95), curium (Cm; 96), berkelium (Bk; 97), californium (Cf; 98), einsteinium (Es; 99), fermium (Fm; 100), mendelevium (Md; 101), element No. 102, which does not yet have a generally accepted name, and lawrencium (Lr; 103). All actinides are radioactive; that is, they have no stable isotopes. Th, Pa, and U are naturally radioactive elements; they occur in nature and were discovered before the other actinides. The remaining actinides, which are often called transuranium elements, were artificially produced between 1940 and 1963 by means of nuclear reactions. Of these, only Np and Pu have been detected in negligibly small amounts in some radioactive ores, and the “heavier” actinides (those with higher atomic numbers) have not been found in nature. Great credit for the study of the actinides is due the American chemist G. T. Seaborg, who advanced the hypothesis of the existence of the actinides in 1942; nine of the actinides were synthesized for the first time under his direction or with his participation. The actinides were classified as a special family because of the similarity of their chemical properties to each other and to those of actinium, a similarity which is due to the similar structure of the outer electron shells of their atoms.

The name “actinides” denotes similarity to actinium. It was given to the actinides by analogy with the lanthanides, a family of 14 elements which also belong to Group III of the periodic system and follow lanthanum in Period 6. The properties of the elements of these two families are similar in many respects.

The close resemblance of the chemical properties of the actinides and their similarity to the lanthanides are related to the distinctive structure of the electron shells in these families. Atoms consist of nuclei and electron shells, with the number of electron shells equaling the number of the period of an element in Mendeleev’s table; the actinides have seven electron shells. The actinides (and similarly the lanthanides) differ from other elements in that each new electron which appears in the atom parallel with the increase in the atomic number (Z)—in the progression from the first actinide (Th; Z = 90) to the second (Pa; 91), up to the last (Lr; 103)—occupies not the outer (sixth or seventh) electron shells, but the fifth shell, which lies closer to the nucleus. In the lanthanides, where the number of shells is six, the electrons similarly fill the fourth shell rather than the outer, fifth and sixth shells. Thus, in the elements of both of these families the electron shell third from the outside becomes filled; but the structure of their two outer shells is similar. As a rule, the number of electrons in these outer shells in the actinides and lanthanides does not vary

by more than one, and in nearly all cases, those representatives of the two families that are equidistant from lanthanum and actinum, respectively, contain a strictly identical number of electrons in their two outer shells. (Only the principle of the filling of the electron shells in atoms of the two families is presented here; in many cases, especially in the actinides, the order of filling is more complex.) The electron configurations of the actinides are shown in Table 1.

The fact that during the transition from Th to Lr the number of electrons in the two outer shells as a rule does not change, while the positive nuclear charge gradually increases, causes a stronger attraction of the outer electrons toward the nucleus and leads to the so-called actinide contraction: as the atomic number increases, the radii of the neutral actinide atoms and of their ions with the same valence do not increase as is usually the case, but even decrease somewhat. (For example, the radius of U³⁺ is 1.03 Å, that of Np³⁺ is 1.01 Å, that of Pu³⁺ is 1.00 Å, and that of Am³⁺ is 0.99 Å.)

Since the chemical properties of an element depend primarily on the number of electrons in its outer shells and on its atomic and ionic radii, it is not surprising that the actinides resemble each other in their properties and that the chemical behavior of the actinides and the lanthanides is very similar. This similarity is especially noticeable when the elements are in the same valence state. Thus, the trivalent actinides form the same insoluble compounds (hydroxides, fluorides, carbonates, oxalates, and so on) as do the trivalent lanthanides. The trifluorides, trichlorides, and other analogous compounds of the trivalent actinides form isostructural series; in other words, the compounds belonging to such series—for example, to the series MeCl₃ (where Me is an actinide atom)—have similar crystalline lattices, the parameters of which gradually decrease with increasing atomic number (Z) of the actinide. The dioxides, tetrafluorides, hexafluorides, and other compounds of the actinides form similar isostructural series. In their susceptibility to hydrolysis, the compounds of the pentavalent actinides—for example, their pentachlorides—resemble each other very closely. In solution, hexavalent actinides exist as MeO₂²⁺ ions, and so on. The examples cited here do not nearly exhaust the instances of similarities of actinides, but they suffice to show the presence of such similarities.

However, in addition to their common features, there is also an essential difference between the actinides and lanthanides. The actinides often form compounds in oxidation states significantly higher than +3; this is not characteristic of the lanthanides. In their compounds, the actinides exhibit the following valences (the most typical valence is shown in boldface): Th (3, 4), Pa (3, 4, 5), U (3, 4, 5, 6), Np (3, 4, 5, 6, 7), Pu (3, 4, 5, 6, 7), Am (3, 4, 5, 6), Cm (3, 4), Bk (3, 4), Cf (2, 3), Es (3), Fm (3), Md (2, 3), and No. 102 (2, 3). Thus, a valence of 3 is characteristic of the actinides only after Am. The first members of the actinide family (Th, Pa, and U) are more often tetra-, penta-, and hexavalent, respectively, in their compounds. The actinides form complexes more readily than do the lanthanides. These distinctive features of the actinides are explained by the fact that the electrons “newly arrived” at the fifth shell from the nucleus (the so-called 5f electrons, or electrons of the 5f subshell) are very close in their nuclear binding energy to electrons in the sixth shell (the so-called 6d electrons, or electrons of the 6d subshell); these 6d electrons can act as additional valence electrons (see Table 1). In the lanthanides, on the other hand, the “newly arrived” 4f electrons are always bound considerably more firmly by the nucleus than are the 5d electrons. As a result of the closeness of the nuclear binding energy of the 5f and 6d electrons in Th, Pa, and U, their 5f electrons can also act as valence electrons. A valence of 3 is not characteristic of these elements, and in many respects they resemble not the Group III elements, but those of Groups IV, V, and VI, respectively, with which they actually had long been classified. In the 1930’s and early 1940’s, it was believed that the elements Np and Pu, which follow U, should similarly belong to Groups VII and VIII, respectively, or that because of their observed similarities to U, they should be placed in Group VI together with U. However, analysis of the principles governing the changes in the chemical properties of the elements in Period 7 suggested to Seaborg that the transuranium elements should be regarded as analogues of actinium—especially after the discovery of the actinides that follow Pu, as well as the experimentally observed similarity of the crystallographic, spectroscopic, and magnetic characteristics of compounds of the elements that follow actinium to the corresponding characteristics of compounds of the lanthanides. As a result of these considerations, the hypothesis of the existence of an actinide family was advanced. This hypothesis contributed significantly to the discovery of Am, Cm, and the subsequent actinides, since it predicted that the most characteristic valence of these elements would be 3, rather than 4 and 6 as originally proposed, and that one should therefore look for them in this valence state.

For the reasons examined above, the properties of the elements classified by Seaborg as actinides differ considerably more among themselves than do the properties of the lanthanides, and therefore the question of whether an actinide family in Period 7 analogous to the lanthanide family in Period 6 actually exists, or whether the structure of this part of Period 7 is more complex, was not completely settled for a long time. Of decisive importance for arriving at the final conclusion regarding the structure of Period 7 was the study of the chemical properties of the 104th element, kurchatovium (discovered by G. N. Flerov and his collaborators in 1964), conducted in 1966 under the direction of the Czech chemist I. Zvara at Dubna, USSR. It was discovered that kurchatovium differs sharply in its properties from the preceding elements and that it is an analogue of hafnium, which is in Group IV. Theoretical calculations show that the number of elements in which the f subshell of the third electron shell from the outside is filled must be 14. It must therefore be assumed that the family of 14 actinides begins with Th and ends with Lr. Present practice is to place all actinides in a separate row at the bottom of the periodic table, as is also done with the lanthanides.

Because of the close similarity of the chemical properties of the actinides (especially the transuranium actinides), they are very difficult to separate. The method of ion-exchange chromatography, which is also widely used for the separation of the lanthanides, is very useful. Since this method has played an important part in the discovery and study of the actinides and, moreover, gives a clear idea of the nature of work with these elements, it warrants examination in greater detail. A vertical glass tube is filled with a special organic polymer or resin. Then a solution is added containing, for example, trivalent actinide ions. The ions react with the polymer and are chemically bound to it. In order to extract the actinide from the tube, columns running through it introduce a solution which contains substances that form more stable bonds with the actinide ions than does the organic polymer. The order of elution of the actinides from the column depends primarily on the ionic radius of the elements, and conditions can be arranged so that the ions with the smallest radii will leave the column first. Since the radii of the actinide ions gradually decrease from Th to Lr, they will be eluted in the order of decreasing atomic number Z. The elution sequence of the actinides is so strictly maintained that it is possible, from the presence of radioactive atoms in a particular portion of solution that has passed through the column, to deduce what elements are present in the mixture and to determine their atomic number accurately. This method is highly selective, takes little time, and is applicable even when only a few atoms of an element are present. It was used in particular in the discovery of Bk, Cf, Es, Fm, and Md.

At present, practical use of the actinides is restricted primarily to Th, U, and Pu. The isotopes ²³³U, ²³⁵U, and ²³⁹Pu serve as nuclear fuel in atomic reactors and as the explosive in atom bombs. Certain isotopes of actinides (²³⁸Pu, ²⁴²Cm, and others), which emit high-energy α-particles, can be used in the development of power sources with a lifetime of ten years or more, which would be necessary, for example, for supplying the navigational radio equipment of space satellites. In such power sources, the heat evolved during radioactive decay is converted by means of special equipment into electric current. The study of the properties of the actinides is of great theoretical importance, since it broadens our understanding of nuclear properties, the chemical behavior of the elements, and so on.