Each of the chemical elements represented in the Earth’s shells: atmosphere, lithosphere and hydrosphere can serve as a vivid example confirming the fundamental importance of atomic-molecular theory and the periodic law. They were formulated by the luminaries of natural science - Russian scientists M.V. Lomonosov and D. I. Mendeleev. Lanthanides and actinides are two families that contain 14 chemical elements, as well as the 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 electronic orbitals of their atoms.
Discovery story
At the end of the 18th century, Yu. Gadolin obtained the first compound from the group of rare-earth metals - yttrium oxide. Until the beginning of the 20th century, thanks to the research of G. Mosley in chemistry, it became known that a group of metals existed. 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 to the mid-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 energy levels, namely for lanthanides this is the fourth level of the f-sublevel, and for actinides - the fifth level of the f-sublevel. Next, we consider the electron shells of atoms of the above metals in more detail.
The structure of internal transition elements in the light of atomic-molecular theory
The ingenious discovery of the structure of chemicals of M.V. Lomonosov was the basis for further study of the electron shells of atoms. The Rutherford model of the structure of an elementary particle of a chemical element, the research of M. Planck, F. Gund allowed chemical scientists to find the right explanation for the existing laws of periodic changes in the physical and chemical properties that characterize lanthanides and actinides. One cannot ignore the most important role of the periodic law of D. I. Mendeleev in the study of 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 system of D. I. Mendeleev
In the third group of the sixth — longer period — behind lanthanum is a family of metals located from cerium to lutetium inclusive. For the lanthanum atom, the 4f sublevel is empty, and for lutetium it is completely filled with 14 electrons. The elements located between them are gradually filling the f-orbitals. In the actinoid family - from thorium to lawrence - the same principle of accumulation of negatively charged particles is observed with the only difference: filling with electrons occurs at the 5f sublevel. The structure of the external energy level and the number of negative particles on it (equal to two) are the same for all the above metals. This fact answers the question of why lanthanides and actinides, called internal transition elements, have many similarities.
In some sources of chemical literature, representatives of both families are combined into second secondary subgroups. They contain two metals from each family. In a short form of the periodic system of chemical elements of D.I. Mendeleev, representatives of these families are selected from the table itself and are arranged in separate rows. Therefore, the position of lanthanides and actinides in the periodic system corresponds to the general plan of the structure of atoms and the periodicity of filling of internal levels with electrons, and the presence of the same oxidation states caused the unification of internal transition metals into general groups. In them, chemical elements have signs and properties equivalent to lanthanum or sea anemone. That is why 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. So, ions of cerium, gadolinium, and other elements of the lanthanide family have high magnetic moments, which is associated with structural features of the f sublevel. This allowed the use of metals as alloying additives to obtain semiconductors with magnetic properties. Sulfides of elements of the actinium family (for example, protactinium sulfide, thorium) in their molecules have a mixed type of chemical bond: ion-covalent or covalent metal. This structural feature led to the appearance of a new physicochemical property and served as an answer to the question of why lanthanides and actinides possess luminescent properties. For example, a silver actinium sample in the dark glows with a bluish glow. This is explained by the action of electric current on metal ions, light photons, under the influence of which atoms are excited, and the electrons in them “jump” to higher energy levels and then return to their stationary orbits. It is for this reason that lanthanides and actinides belong to phosphors.
The consequences of reducing the ionic radii of atoms
In lanthanum and actinium, as well as in elements from their families, a monotonic decrease in the values of the radii of metal ions is observed. In chemistry, in such cases it is customary to talk about lanthanoid and actinoid compression. The following regularity has been established in chemistry: with an increase in the charge of the nucleus of atoms, if the elements belong to the same period, their radii decrease. This can be explained as follows: for metals such as cerium, praseodymium, neodymium, the number of energy levels in their atoms is invariably equal to six. However, the charges of the nuclei correspondingly increase by one and amount to +58, +59, +60. This means that the attractive force of the electrons of the inner shells to the positively charged nucleus increases. As a result, there is a decrease in the radii of atoms. In ionic metal compounds, with increasing sequence number, ionic radii also decrease. Similar changes are observed in elements of the actinium family. That is why lanthanides and actinides are called twins. A decrease in the radii of the ions primarily leads to a weakening of the basic properties of the hydroxides Ce (OH) 3 , Pr (OH) 3 , and the base of lutetium already exhibits amphoteric properties.
Surprising results are the filling of the 4f sublevel with unpaired electrons to half the orbitals of the europium atom. His atomic radius does not decrease, but, on the contrary, increases. The next gadolinium in the series of lanthanides of the gadolinium at the 5d sublevel has one electron of the 4f sublevel similar to Eu. Such a structure causes an abrupt decrease in the radius of the gadolinium atom. A similar phenomenon is observed in a pair of ytterbium - lutetium. In the first element, the atomic radius is large due to the complete filling of the 4f sublevel, while in lutetium it decreases stepwise, since the appearance of electrons is observed at the 5d sublevel. In sea anemone and other radioactive elements of this family, the radii of their atoms and ions do not change monotonously, but, like lanthanides, jumpwise. Thus, lanthanides and actinides are elements in which the properties of their compounds are correlatively dependent on the ionic radius and the structure of the electron shells of atoms.
Valence states
Lanthanides and actinides are elements whose characteristics are fairly similar. In particular, this concerns their oxidation states in ions and the valency of atoms. For example, thorium and protactinium, exhibiting a valence of three, in 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 metals from Lanthanum families: cerium, praseodymium, neodymium, etc. Lanthanides in these compounds will also be trivalent. These examples once again prove to us the correctness of the statement that lanthanides and actinides are twins. They have similar physical and chemical properties. This can be explained primarily by the structure of electronic orbitals in atoms of both families of internal transition elements.
Metallic properties
All representatives of both groups are metals in which 4f-, 5f-, and also d-sublevels are completed. Lanthanum and elements of its family are called rare earths. Their physical and chemical characteristics are so close that they are separated with great difficulty separately in the laboratory. Most often exhibiting an oxidation state of +3, the elements of the lanthanum series have many similarities with alkaline earth metals (barium, calcium, strontium). Actinides are also extremely active metals, and also radioactive.
The structural features of lanthanides and actinides also relate to such properties as, for example, pyrophoricity in a finely dispersed state. A decrease in the size of face-centered crystal lattices of metals is also observed. We add that all the chemical elements of both families are metals with a silvery sheen, which quickly darken in air due to their high reactivity. They are coated with a film of the corresponding oxide, which protects against 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.
Characteristic chemical reactions
As noted earlier, lanthanides and actinides are chemically active metals. So, lanthanum, cerium and other elements of the family are easily combined with simple substances - halogens, as well as phosphorus, carbon. Lanthanides can also interact with both carbon monoxide and carbon dioxide. They are also able to decompose water. In addition to simple salts, for example, such as SeCl 3 or PrF 3 , they form double salts. In analytical chemistry, an important place 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 highly soluble in water and 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 the corresponding ions in them. Solutions of salts of samarium or praseodymium are green, neodymium is red-violet, promethium and europium are pink. Since ions with an oxidation state of +3 are colored, this is used in analytical chemistry to recognize lanthanide metal ions (the so-called qualitative reactions). For the same purpose, chemical analysis methods such as fractional crystallization and ion exchange chromatography are also used.
In actinides, two groups of elements can be distinguished. These are Berkeley, Fermium, Mendelium, Nobel, Lawrence and Uranium, Neptunium, Plutonium, and Omeretia. 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 non-metals: sulfur, nitrogen, carbon. With oxygen-containing legends, they form complex compounds. As you can see, the metals of both families are close to each other in chemical behavior. This 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 oxidizing agent. That is why in the 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, as do the alkali metals located here. The place of hydrogen in the 7th group, along with halogen elements, indicates its reducing ability. In the sixth period is, as previously mentioned, a family of lanthanides, placed in a separate row for the convenience and compactness of the table. The seventh period contains a group of radioactive elements, in their characteristics similar to sea anemone. Actinides are located outside the table of chemical elements of D.I. Mendeleev 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 lanthanides and actinides belong to internal transition elements, and their physicochemical characteristics are very close to each other.
General methods for producing metals in industry
With the exception of thorium, protactinium, and uranium, which are extracted directly from ores, the remaining actinides can be obtained by irradiating uranium metal samples with fast-moving neutron fluxes. On an industrial scale, neptunium and plutonium are extracted from spent fuel from nuclear reactors. Note that the preparation 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 earths, are obtained by electrolysis of their chlorides or fluorides. To obtain ultrapure lanthanides, use the metallothermic method.
Where do internal transition elements apply?
The range of use of the metals studied by us is quite wide. For the actinium family, this is, first of all, nuclear weapons and energy. Actinides are also important in medicine, flaw detection, and activation analysis. The use of lanthanides and actinides as sources of neutron capture in nuclear reactors cannot be ignored. Lanthanides are also used as alloying additives for cast iron and steel, as well as in the production of phosphors.
Spread in nature
Oxides of actinides and lanthanides are often called zirconium, thorium, and yttrium earths. They are the main source for the production of the corresponding metals. Uranium, as the main representative of actinides, is in the outer layer of the lithosphere in the form of four types of ores or minerals. First of all, it is a uranium resin, which is uranium dioxide. It has the highest metal content. Often, uranium dioxide is accompanied by radium deposits (veins). They are found in Canada, France, Zaire. Thorium and uranium ore complexes often contain ores of other valuable metals, such as gold or silver.
The stocks of such raw materials are rich in Russia, the Republic of South Africa, Canada and Australia. Some sedimentary rocks contain the mineral carnotite. In addition to uranium, it also includes vanadium. The fourth type of uranium feedstock is phosphate ores and iron-uranium schists. Their reserves are in Morocco, Sweden and the USA. At present, lignite and coal deposits containing uranium impurities are also considered promising. They are mined in Spain, the Czech Republic, as well as in two American states - North and South Dakota.