Each of the chemical elements presented in the shells of the Earth: the atmosphere, lithosphere and hydrosphere - can serve as a vivid example, confirming the fundamental importance of the atomic and 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 each, 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 history
At the end of the 18th century, Y. 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. Moseley 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 radioactivechemical elements called actinides. Their discovery in science occurred from 1879 to the middle of the 20th century. Lanthanides and actinides have a lot of 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 f-sublevel, and for actinides - the fifth level f-sublevel. Next, we will consider the electron shells of the atoms of the above metals in more detail.
The structure of internal transitional elements in the light of atomic and molecular teachings
The ingenious discovery of the structure of chemicals by MV 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 studies of M. Planck, F. Gund allowed chemists to find the correct explanation for the existing patterns of periodic changes in physical and chemical properties that characterize lanthanides and actinides. It is impossible to 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's dwell on this issue in more detail.
Place of internal transition elements in D. I. Mendeleev's Periodic Table
In the third group of the sixth - larger period - behind lanthanum is a family of metals ranging from cerium to lutetium inclusive. The 4f sublevel of the lanthanum atom is empty, while the lutetium atom is completely filled with the 14thelectrons. The elements located between them are gradually filling f-orbitals. In the family of actinides - from thorium to lawrencium - 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 of the above metals. 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 combined into second side subgroups. They contain two metals from each family. In the short form of the periodic system of chemical elements of D. I. Mendeleev, representatives of these families are separated from the table itself and 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 internal levels with electrons, and the presence of the same oxidation states caused the association of internal transition metals into common groups. In them, chemical elements have features 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 periodicsystem directly determines their physical and chemical characteristics. Thus, 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 made it possible to use metals as dopants to obtain semiconductors with magnetic properties. Sulfides of elements of the actinium family (for example, sulfide of protactinium, thorium) in the composition of their molecules have a mixed type of chemical bond: ionic-covalent or covalent-metal. This feature of the structure led to the emergence of a new physicochemical property and served as an answer to the question of why lanthanides and actinides have luminescent properties. For example, a sample of actinium silver in the dark glows with a bluish glow. This is due to the action of electric current, photons of light on metal ions, 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 are classified as phosphors.
Consequences of decreasing the ionic radii of atoms
In lanthanum and actinium, as in the elements from their families, there is a monotonous decrease in the value of the radii of metal ions. In chemistry, in such cases it is customary to speak of lanthanide and actinide compression. In chemistry, the following pattern has been established: 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 followsway: for such metals as cerium, praseodymium, neodymium, the number of energy levels in their atoms is unchanged and equal to six. However, the charges of the nuclei respectively increase 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 nucleus increases. As a result, the atomic radii decrease. In ionic compounds of metals, with an increase in the atomic number, the ionic radii also decrease. Similar changes are observed in the elements of the anemone family. That is why the lanthanides and actinides are called twins. A decrease in the radii of ions leads, first of all, to a weakening of the basic properties of the hydroxides Ce(OH)3, Pr(OH)3 properties.
The filling of the 4f-sublevel with unpaired electrons up to half of the orbitals of the europium atom leads to unexpected results. Its atomic radius does not decrease, but, on the contrary, increases. Gadolinium, which follows it in the lanthanide series, has one electron in the 4f sublevel at the 5d sublevel, similarly to Eu. This structure causes an abrupt decrease in the radius of the gadolinium atom. A similar phenomenon is observed in a pair of ytterbium - lutetium. For the first element, the atomic radius is large due to the complete filling of the 4f sublevel, while for lutetium it abruptly decreases, since the appearance of electrons is observed at the 5d sublevel. In actinium and other radioactive elements of this family, the radii of their atoms and ions do not change monotonously, but, like in the case of lanthanides, stepwise. Thus, the lanthanides andactinides are elements whose properties of their compounds correlatively depend on the ionic radius and the structure of the electron shells of atoms.
Valence states
Lanthanides and actinides are elements whose characteristics are quite similar. In particular, this concerns their oxidation states in ions and the valency of atoms. For example, thorium and protactinium, which exhibit a valence of three, in the compounds Th(OH)3, PaCl3, ThF3 , Pa2(CO3)3. All these substances are insoluble and have the same chemical properties as the metals from the lanthanum family: 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 lanthanides and actinides are twins. They have similar physical and chemical properties. This can be explained primarily 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 completed. Lanthanum and the elements of its family are called rare earths. Their physical and chemical characteristics are so close that they are separated separately under laboratory conditions with great difficulty. 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 are 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 the 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, due to their high reactivity, they quickly darken in air. They are covered with a film of the corresponding oxide, which protects against further oxidation. All elements are sufficiently refractory, with the exception of neptunium and plutonium, whose melting point is well below 1000 °C.
Characteristic chemical reactions
As noted earlier, lanthanides and actinides are reactive metals. So, lanthanum, cerium and other elements of the family easily combine with simple substances - halogens, as well as with phosphorus, carbon. The lanthanides can also interact with both carbon monoxide and carbon dioxide. They are also capable of decomposing water. In addition to simple s alts, such as SeCl3 or PrF3, for example, they form double s alts. In analytical chemistry, reactions of lanthanide metals with aminoacetic and citric acids occupy an important place. The 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, metalsform the corresponding s alts. They are highly soluble in water and easily capable of forming crystalline hydrates. It should be noted that aqueous solutions of lanthanide s alts are colored, which is explained by the presence of the corresponding ions in them. Solutions of samarium or praseodymium s alts are green, neodymium - red-violet, promethium and europium - pink. Since ions with an oxidation state of +3 are colored, this is used in analytical chemistry to recognize lanthanide metal ions (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, lawrencium and uranium, neptunium, plutonium, omercium. The chemical properties of the first of these are similar to lanthanum and metals from its family. The 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. They form complex compounds with oxygen-containing legends. As we can see, the metals of both families are close to each other in chemical behavior. This is why the lanthanides and actinides are often referred to as twin metals.
Position in the periodic system of hydrogen, lanthanides, actinides
It is necessary to take into account the fact that hydrogen is a fairly reactive substance. It manifests itself depending on the conditions of the chemical reaction: both as a reducing agent and as an oxidizing agent. That is why in the periodic systemhydrogen 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 the elements halogens, indicates its reducing ability. In the sixth period, as already mentioned, the lanthanide family is located, placed in a separate row for the convenience and compactness of the table. The seventh period contains a group of radioactive elements similar in characteristics to actinium. Actinides are located outside the table of chemical elements of D. I. Mendeleev under the row 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 are 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 mined directly from ores, the rest of the actinides can be obtained by irradiating samples of metallic uranium with fast-moving neutron streams. On an industrial scale, neptunium and plutonium are mined from spent fuel from nuclear reactors. Note that the production of actinides is a rather complicated and expensive process, the main methods of which are ion exchange and multistage extraction. Lanthanides, which are called rare earth elements, are obtained by electrolysis of their chlorides or fluorides. The metallothermic method is used to extract ultrapure lanthanides.
Where internal transition elements are used
The range of use of the metals we study is quite wide. For the anemone family, this is, first of all, nuclear weapons and energy. Actinides are also important in medicine, flaw detection, and activation analysis. It is impossible to ignore the use of lanthanides and actinides as sources of neutron capture in nuclear reactors. Lanthanides are also used as alloying additions to 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, yttrium earths. They are the main source for obtaining the corresponding metals. Uranium, as the main representative of actinides, is found in the outer layer of the lithosphere in the form of four types of ores or minerals. First of all, it is uranium pitch, 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. Complexes of thorium and uranium ores often contain ores of other valuable metals, such as gold or silver.
The reserves of such raw materials are rich in Russia, South Africa, Canada and Australia. Some sedimentary rocks contain the mineral carnotite. In addition to uranium, it also contains vanadium. Fourththe type of uranium raw materials is phosphate ores and iron-uranium shales. Their reserves are located in Morocco, Sweden and the USA. At present, deposits of lignite and coal containing uranium impurities are also considered promising. They are mined in Spain, the Czech Republic, as well as in two US states - North and South Dakota.
