Unknown Periodic Table Part 3

The last episode of the article about the periodic table, except for the school one (corresponds to the title of the entire series). Today about the usually overlooked groups of the periodic table, artificial elements about which we know practically nothing, and about what awaits us in the eighth period.

, two rows, usually placed under the blackboard, are treated with disdain at school - the teacher will only mention their existence and move on to other, "more important" elements. Completely inadequate for the function they perform in the modern world. Uran i Pluto () form the basis of nuclear energy: a reliable source in any climatic conditions (unlike non-traditional energy), and when used correctly, it is safe and environmentally friendly. Lanthanides are currently a strategic raw material needed in many advanced technologies, for example, modern electronics would not exist without lanthanides. But even now, both groups are difficult to crack due to the law of periodicity.

Problem #1: Lanthanides

Mendeleev he knew only a few lanthanides and without much difficulty managed to include them in the table (1). In later years, however, discoveries of such metals challenged the very law of periodicity. It requires successive elements to lie in adjacent groups and have different valences. Meanwhile, lanthanides with slightly different atomic masses (which meant that they had to stand one after another in a continuous row) always had a valence equal to III (some also II or IV), that is, they were in the same group. Many theories have been put forward about lanthanides, for example, they were all considered the same element in different versions. The problem was solved by placing the lanthanides in one "box" of the system, but only quantum mechanics of the 20s and 30s explained their position in the periodic table.

1. Cerium, the most common lanthanide. All other lanthanides are similar and look like aluminum ()

The problem with lanthanides is still relevant today. In fact, it is not known with which element they begin and end. According to most textbooks, lanthanum, although it gave the name to the entire family, does not belong to it - it is scandalous of group 3lanthanides are metals from cerium to lutetium. However, opposing opinions indicate that the last member of the family scans much better. pray. Its physical properties and the chemical nature of the resulting compounds better match those of other group 3 elements (thus, lanthanides are metals from lanthanum to ytterbium). Discussions between chemists continue to this day, and the authors of books present the location of these elements in different ways (2).

2. The longest form of the periodic table. The lanthanides and actinides start right after group 2, and scandlers include lutetium and laurens.

Problem #2: Actinides

actinides, that is, elements from actinium to Laurence, constitute a group similar to the lanthanides, placed in the next period. The problem with the actinides was and remains the same as with the lanthanides. Mendeleev knew only two actinides and listed them without difficulty in the table (3). Tor with the strongest valence, group IV went to the group of titanium and zirconium, and group VI with the value Uranium found in the company of chromium, molybdenum and tungsten.

Discoveries at the turn of the XNUMXth and XNUMXth centuries shocked the system: there appeared a dozen or so radioactive elements. Some of them had the same masses, but different properties, while others had the same properties - different masses. This fact contradicted previous knowledge, but the discovery of isotopes clarified some problems. Most of the elements turned out to be mixtures of atoms with different masses (but, of course, with the same number of protons in the nucleus). Returning to actinides, two of them appeared at the beginning of the last century. Actinium immediately turned out to be similar to lanthanum (its radioactivity was found in sediments of lanthanides precipitated from solutions containing actinium), and protactinium fell into the group with vanadium, niobium and tantalum as a V-valuable element. Until the 40s, the first four actinides were quietly located in groups from 3 to 6.

3. The most common actinides: on the left is thorium, covered with a layer of dioxide, on the right is uranium ()

The problem arose during the implementation of the Manhattan Project. Physicists and chemists were sure that they had produced elements heavier than uraniumhowever, they were unable to detect their presence by chemical means. For example: element 93 next to uranium did not behave as a member of group 7. It was only the suggestion by Glen Seaborg, a later discoverer of several transuranium elements, that elements starting with actinium form rows similar to lanthanides solved the problem (4). The use of the developed methods for the separation of lanthanides, mainly chromatography, made it possible in subsequent years to identify artificial elements. The perfection of technology and the skill of experimenters is evidenced by the fact that the presence of some new elements was discovered with only a dozen of their atoms! (five).

But do not believe that thorium or uranium suddenly changed their properties and became lanthanide-like, trivalent metals. They still have more in common with titanides and chromium than with other actinides. Similarly with protactinates and even some transuranists. We can only talk about a certain similarity in the second half of the series. However, the lanthanides and actinides cannot be said to be related families (as if their placement on the periodic table suggests their location).

4. Glenn Seaborg (1912-99) gives the symbol for the element named after him.

5. The birthplace of many transuranists is the cyclotron at Lawrence Berkeley National Laboratory (August 1939)

No experimental data

Inaccurate knowledge of the properties of the heaviest transuranides is associated with an insufficient amount of material for research. The last element to be produced in a notable amount of milligrams is einstein, ranked 99th on the table. Of course, experiments are also carried out with other transuranides, but in their case, for example, the characteristic radiation arising in the precipitate is studied, and the formation of crystals of the compounds is not observed.

Physicochemical characteristics cannot be simply measured, but only estimated from indirect observations. A similar problem occurs with two lighter elements: astatu (No. 85) i French (No. 87). Because of their short lifetime, visible quantities have not yet been obtained, and the theory must fill in the gaps in observations and measurements. On the other hand, the annual production of plutonium, which is found in trace amounts in the earth's crust, is several tons and is better understood than many of the lighter persistent elements.

An even bigger problem is with the heaviest elements with a period of 7 after the actinides. In their case, one has to operate with literally separate atoms, the lifetime of which is calculated in fractions of a second. It is no longer possible to determine the radioactivity present in the precipitate of a compound bound to an element, but only to investigate certain physical properties (such as the rate of deposition on a given surface) and thus draw conclusions about chemical similarity. Of course, theory helps, or rather calculations based on quantum mechanics. However, the results are not 100% reliable, for example, according to a certain distribution of electrons on the shells, the description of chemical properties is still far away. The further we move away from direct observational data, the more unreliable becomes the conclusion of the "Method of Mendeleev" (that is, based on the properties of known elements).

Chemists and physicists are trying to predict the properties of even the heaviest, yet unobtained elements of the next period. Elements 119 and 120 will probably have features not much different from those located above francium and radium (in groups 1 and 2, the change in physicochemical properties is quite natural). In period 8, like the lanthanides and actinides, there will be as many as 18 very similar properties of the g-block elements that scientists have not yet dealt with.

Calculations suggest the existence of elements with a valency of up to 12 during this period. Probably, element 164 will be in the system, for heavier ones the nuclear charge will be so large that electrons will fall on them, and the orbital speed would exceed the speed of light. The calculations carried out do not give unambiguous results, some shift the boundaries of the periodic table by about a dozen elements.

The law of periodicity in the trash?

After reading the article, you probably came to the conclusion that a large number of exceptions from the rules given at the school, this gives you the right to answer the question in the affirmative. But don't give it too fast. Law of Periodicity it is formulated in a very general way (properties are periodically repeated), which is both its advantage and disadvantage. The advantage, indeed, by analyzing the position of an element in a table built on the basis of the law, one can draw conclusions about its physical and chemical properties. A disadvantage, as the conclusions are often inaccurate or even false.

Law of Periodicity works well in the main groups (1, 2, 13-18), better in their upper parts than in their lower ones. A perfect example is period 3: from sodium to argon, valency and chemical properties change regularly without any surprises. In the side groups (3-12) deviations are already clearly visible, but you read about the problems with lanthanides and actinides above. In general, the farther from the beginning of the system, the less exactly the law of periodicity is satisfied.

So in the trash? Universe of chemical elements and the relationships they create are a system so complex that even Mendeleev's brilliant idea cannot capture it entirely. The law certainly needs to be corrected and clarified. If we compare Mendeleev with Newton (since both the law of periodicity and the theory of gravity organized and explained large chunks of the world around us), we can conclude that chemists are still waiting for their Einstein.

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