Lanthanides and actinides: position in the periodic table

Each of the chemical elements, represented in the Earth's shells: the atmosphere, the lithosphere and the hydrosphere - can serve as a vivid example, confirming the fundamental importance of atomic-molecular teaching and periodic law. They were formulated by the coryphaeans of natural science - Russian scientists MV Lomonosov and DI Mendeleev. Lanthanides and actinides are two families that contain 14 chemical elements, as well as metals themselves - lanthanum and actinium. Their properties - both physical and chemical - will be considered by us in this paper. In addition, we will establish how the position in the periodic system of hydrogen, lanthanides, actinides depends on the structure of the electron orbitals of their atoms.

History of the discovery

At the end of the 18th century, Yu. Gadolin obtained the first compound from the group of rare-earth metals, yttrium oxide. Before the beginning of the 20th century, thanks to the research of G. Mosely in chemistry, it became known about the existence of a group of metals. They were located in the periodic system between lanthanum and hafnium. Another chemical element, actinium, like lanthanum, forms a family of 14 radioactive chemical elements called actinides. Their discovery in science occurred, from 1879 until the middle of the 20th century. Lanthanides and actinides have many similarities in both physical and chemical properties. This can be explained by the arrangement of electrons in the atoms of these metals, which are at the energy levels, namely for the lanthanides this is the fourth level of the f-sublevel, and for the actinides, the fifth level, the f-sublevel. Next, we consider the electronic shells of the atoms of the above metals in more detail.

The structure of internal transition elements in the light of atomic-molecular doctrine

The brilliant discovery of the chemical structure of MV Lomonosov was the basis for further study of the electron shells of atoms. The Rutherford model of the structure of the elementary particle of the chemical element, the investigations of M. Planck, F. Gund allowed the chemists to find the correct explanation for the existing patterns of periodic changes in the physical and chemical properties that characterize the lanthanides and actinides. One can not ignore the crucial role of D. Mendeleyev's periodic law in studying the structure of atoms of transition elements. Let us dwell on this issue in more detail.

The place of internal transition elements in the periodic table of DI Mendeleyev

In the third group of the sixth - a longer period - behind the lanthanum is a family of metals located from cerium to lutetium inclusive. At the lanthanum atom, the 4f sublevel is empty, and in lutetium it is completely filled with 14 electrons. The elements located between them are gradually filled with f-orbitals. In the family of actinides - from thorium to laurentium - the same principle of accumulation of negatively charged particles is observed with the only difference: electron filling occurs on the 5f sublevel. The structure of the external energy level and the number of negative particles on it (equal to two) for all of the above metals are the same. This fact answers the question of why the lanthanides and actinides, called internal transition elements, have many similarities.

In some sources of chemical literature, representatives of both families are grouped into second secondary subgroups. They contain two metals from each family. In the short form of the periodic system of chemical elements DI Mendeleev representatives of these families are distinguished from the table itself and are located in separate rows. Therefore, the position of the lanthanides and actinides in the periodic system corresponds to the general plan of the structure of atoms and the periodicity of the filling of internal levels by electrons, and the presence of identical degrees of oxidation caused the combination of internal transition metals in common groups. In them, chemical elements possess characteristics and properties equivalent to lanthanum or actinium. That is why the lanthanides and actinides are removed from the table of chemical elements.

How the electronic configuration of the f-sublevel affects the properties of metals

As we said earlier, the position of lanthanides and actinides in the periodic system directly determines their physical and chemical characteristics. Thus, ions of cerium, gadolinium and other elements of the family of lanthanides have high magnetic moments, which is due to the peculiarities of the structure of the f-sublevel. This allowed the use of metals as alloying additives for the production of semiconductors with magnetic properties. Sulfides of the elements of the family of actinium (for example, protactinium sulfide, thorium) in their molecules have a mixed type of chemical bond: ion-covalent or covalent-metal. This feature of the structure led to the appearance of a new physico-chemical property and served as a response to the question of why lanthanides and actinides possess luminescent properties. For example, the actinium of silvery color in the dark glows a bluish glow. This is explained by the action on the ions of metals of electric current, photons of light, under the influence of which there is excitation of atoms, and electrons in them "jump" to higher energy levels and then return to their stationary orbits. It is for this reason that the lanthanides and actinides belong to the phosphors.

The consequences of decreasing the ionic radii of atoms

In lanthanum and actinium, as in the case of elements from their families, there is a monotonous decrease in the value of the parameters of the radii of metal ions. In chemistry in such cases it is customary to speak of lanthanoid and actinide compression. In chemistry, the following regularity is established: with an increase in the charge of the atomic nucleus, in the case when the elements belong to the same period, their radii decrease. This can be explained in the following way: for metals such as cerium, praseodymium, neodymium, the number of energy levels in their atoms is invariably equal to six. However, the nuclear charges are correspondingly increased by one and are +58, +59, +60. This means that the force of attraction of the electrons of the inner shells to the positively charged core increases. As a consequence, the radii of the atoms decrease. In ionic metal compounds, as the ordinal number increases, the ionic radii also decrease. Similar changes are observed in the elements of the family of actinium. That is why lanthanoids and actinides are called twins. Reduction of the ion radii primarily leads to a weakening of the basic properties of the hydroxides Ce (OH) ₃ , Pr (OH) ₃ , and the base of the lutetium already exhibits amphoteric properties.

To unexpected results, the filling of the 4f sublevel with unpaired electrons leads to half the orbitals of the europium atom. His radius of the atom does not decrease, but, on the contrary, it increases. Next to it, in the gadolinium lanthanide series, one electron of the 4f sublevel appears on the 5d sublevel, similarly to Eu. Such a structure causes a spasmodic decrease in the radius of the gadolinium atom. A similar phenomenon is observed in a pair of ytterbium - lutetium. In the first element, the radius of the atom is large because of the full filling of the 4f sublevel, while in lutetium it decreases in an abrupt way, since electrons appear on the 5d sublevel. In actinium and other radioactive elements of this family, the radii of their atoms and ions do not change monotonically, but, like the lanthanoids, they change discontinuously. Thus, lanthanides and actinides are elements in which the properties of their compounds depend correlatively on the ionic radius and the structure of the electron shells of atoms.

Valency states

Lanthanides and actinides are elements whose characteristics are quite similar. In particular, this concerns their degrees of oxidation in ions and the valence of atoms. For example, thorium and protactinium, exhibiting a valence of three, in the compounds Th (OH) ₃ , PaCl ₃ , ThF ₃ , Pa ₂ (CO ₃ ) _3. All these substances are insoluble and have the same chemical properties as the metals from Lanthanum families: cerium, praseodymium, neodymium, etc. The lanthanides in these compounds will also be trivalent. These examples once again prove to us the correctness of the statement that the lanthanides and actinides are twins. They have similar physical and chemical properties. This can be explained, first of all, by the structure of the electron orbitals of the atoms of both families of internal transition elements.

Metal properties

All representatives of both groups are metals, in which 4f-, 5f-, and also d-sublevels are added. Lanthanum and elements of its family are called rare earth. Their physical and chemical characteristics are so close that separately in the laboratory they are separated with great difficulty. Using the +3 oxidation state most often, the elements of the lanthanum series have many similar features with alkaline earth metals (barium, calcium, strontium). Actinides are also extremely active metals, also radioactive.

Features of the structure of lanthanides and actinides also affect properties such as, for example, pyrophoricity in the finely dispersed state. There is also a decrease in the size of face-centered crystal lattices of metals. We add that all the chemical elements of both families are metals with silvery sheen, because of their high reactivity rapidly darkening in the air. They are covered with a film of the corresponding oxide, which protects from further oxidation. All elements are sufficiently refractory, with the exception of neptunium and plutonium, the melting point of which is much lower than 1000 ° C.

Typical chemical reactions

As noted earlier, lanthanides and actinides are chemically active metals. Thus, lanthanum, cerium and other elements of the family are easily combined with simple substances - halogens, as well as with phosphorus, carbon. Lanthanides can also interact with both carbon monoxide and carbon dioxide. They are also capable of decomposing water. In addition to simple salts, for example such as SeCl ₃ or PrF ₃ , they form double salts. An important place in analytical chemistry is occupied by the reactions of lanthanide metals with aminoacetic and citric acids. Complex compounds formed as a result of such processes are used to separate a mixture of lanthanides, for example, in ores.

When interacting with nitrate, chloride and sulfate acids, metals form the corresponding salts. They are readily soluble in water and are readily capable of forming crystalline hydrates. It should be noted that aqueous solutions of lanthanide salts are colored, which is explained by the presence of corresponding ions in them. Solutions of salts of samarium or praseodymium are green, neodymium - red-violet, promethium and europium - pink. Since ions with an oxidation state of +3 are colored, it is used in analytical chemistry to recognize ions of lanthanide metals (so-called qualitative reactions). For the same purpose, chemical analysis methods such as fractional crystallization and ion-exchange chromatography are also used.

Actinides can be divided into two groups of elements. These are berkelium, fermium, mendelevium, nobelium, laurentium and uranium, neptunium, plutonium, and omeresis. The chemical properties of the first of them are similar to lanthanum and metals from its family. Elements of the second group have very similar chemical characteristics (almost identical to each other). All actinides quickly interact with nonmetals: sulfur, nitrogen, carbon. With oxygen-containing ligands they form complex compounds. As we see, the metals of both families are close to each other in chemical behavior. That is why lanthanides and actinides are often called twin metals.

Position in the periodic system of hydrogen, lanthanides, actinides

It is necessary to take into account the fact that hydrogen is a sufficiently reactive substance. It manifests itself depending on the conditions of the chemical reaction: both a reducing agent and an oxidizer. That is why in a periodic system hydrogen is located simultaneously in the main subgroups of two groups at once.

In the first, hydrogen plays the role of a reducing agent, like the alkali metals located here. The place of hydrogen in the 7th group along with the elements halogens indicates its reducing ability. In the sixth period, as already mentioned, a family of lanthanides is placed in a separate row for the convenience and compactness of the table. The seventh period contains a group of radioactive elements, similar in their characteristics to actinium. Actinides are located outside the table of chemical elements of DI Mendeleyev under a number of the lanthanum family. These elements are the least studied, since the nuclei of their atoms are very unstable due to radioactivity. Recall that the lanthanides and actinides belong to the elements of the internal transition, and their physico-chemical characteristics are very close to each other.

General methods of production of metals in industry

With the exception of thorium, protactinium and uranium, which are extracted directly from the ores, the remaining actinides can be obtained by irradiating uranium metal samples with rapidly moving neutron fluxes. On an industrial scale, neptunium and plutonium are extracted from spent fuel from nuclear reactors. We note that the production of actinides is a rather complex and expensive process, the main methods of which are ion exchange and multi-stage extraction. Lanthanides, which are called rare earth elements, are obtained by electrolysis of their chlorides or fluorides. To produce ultra-pure lanthanides, a metallothermic method is used.

Where internal transition elements are used

The range of use of the metals we are studying is quite wide. For the family of actinium - it is, first of all, nuclear weapons and energy. Of great importance are actinides in medicine, defectoscopy, activation analysis. We can not ignore the use of lanthanides and actinides as sources of neutron capture in nuclear reactors. Lanthanides are also used as alloying additives to iron and steel, as well as in the production of phosphors.

Distribution in nature

Oxides of actinides and lanthanides are often called zirconium, thorium, yttrium. They are the main source for obtaining the corresponding metals. Uranium, as the main representative of actinides, is located in the outer layer of the lithosphere in the form of four kinds of ores or minerals. First of all, it is uranium tar, which is uranium dioxide. In it, the metal content is the highest. Often, uranium dioxide is accompanied by radium deposits (veins). They are found in Canada, France, Zaire. Complexes of thorium and uranium ore often contain ores of other valuable metals, for example gold or silver.

The stocks of such raw materials are rich in Russia, South Africa, Canada and Australia. Some sedimentary rocks contain mineral carnotite. In its composition, in addition to uranium, is also vanadium. The fourth type of uranium raw materials is phosphate ores and iron-iron shale. Their reserves are in Morocco, Sweden and the United States. At present, deposits of lignite and coal containing uranium impurities are also promising. They are mined in Spain, the Czech Republic, and also in two American states - North and South Dakota.