Period (chemistry). Period of the periodic system What is a group and period in chemistry
A period is a row of the periodic system of chemical elements, a sequence of atoms in order of increasing nuclear charge and filling the outer electron shell with electrons.
The periodic table has seven periods. The first period, containing 2 elements, as well as the second and third, containing 8 elements each, are called small. The remaining periods with 18 or more elements are large. The seventh period is not completed. The number of the period to which a chemical element belongs is determined by the number of its electron shells.
Each period begins with a typical metal and ends with a noble gas, preceded by a typical non-metal.
In the first period, besides helium, there is only one element - hydrogen, which combines properties typical of both metals and non-metals. The 1s subshell of these elements is filled with electrons.
For elements of the second and third periods, the s- and p-subshells are sequentially filled. Elements of short periods are characterized by a fairly rapid increase in electronegativity with increasing nuclear charges, a weakening of metallic properties and an increase in non-metallic properties.
The fourth and fifth periods contain decades of transition d-elements, in which, after filling the outer s-subshell with electrons, the d-subshell of the previous energy level is filled, according to the Klechkovsky rule.
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p 6f 7d 7f ...
In the sixth and seventh periods, the 4f- and 5f-subshells become saturated, as a result of which they contain 14 more elements compared to the 4th and 5th periods.
Due to the differences in periods in length and other characteristics, there are different ways of their relative arrangement in the periodic system. In the short-period version, short periods contain one row of elements, large periods have two rows. In the long-period version, all periods consist of one series. The lanthanide and actinide series are usually written separately at the bottom of the table.
Elements of the same period have similar atomic masses, but different physical and chemical properties, unlike elements of the same group. With an increase in the nuclear charge of elements of the same period, the atomic radius decreases and the number of valence electrons increases, as a result of which the metallic and non-metallic properties of the elements are weakened, the reducing properties are weakened and the oxidative properties of the substances they form are strengthened.
The sequence of atoms in order of increasing nuclear charge and filling the outer electron shell with electrons.
The periodic table has seven periods. The first period, containing 2 elements, as well as the second and third, containing 8 elements each, are called small. Other periods with 18 or more elements - big. The seventh period is completed. The eighth period is not completed. The number of the period to which a chemical element belongs is determined by the number of its electron shells (energy levels).
Each period (except for the first) begins with a typical metal (, Na, , , ,) and ends with a noble gas (, , , Xe, ,), which is preceded by a typical non-metal.
In the first period, besides helium, there is only one element - hydrogen, which combines properties typical of both metals and (to a greater extent) non-metals. These elements are filled with electrons 1 s- subshell.
The elements of the second and third periods undergo sequential filling s- And r- subshells. Elements of short periods are characterized by a fairly rapid increase in electronegativity with increasing nuclear charges, a weakening of metallic properties and an increase in non-metallic properties.
The fourth and fifth periods contain decades of transition d-elements (from scandium to zinc and from yttrium to cadmium), in which, after filling the outer s-subshells are filled according to Klechkovsky’s rule, d-subshell of the previous energy level.
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p 6f 7d 7f ...
In the sixth and seventh periods, saturation occurs 4 f- and 5 f-subshells, as a result of which they contain 14 more elements compared to the 4th and 5th periods (lanthanides in the sixth and actinides in the seventh period).
Due to the differences in periods in length and other characteristics, there are different ways of their relative arrangement in the periodic system. In the short-period version, small periods contain one a number of elements, large ones have two rows. In the long-period version, all periods consist of one series. The lanthanide and actinide series are usually written separately at the bottom of the table.
Elements of the same period have similar atomic masses, but different physical and chemical properties, unlike elements of the same period
What does the period number indicate? Chemistry and got the best answer
Answer from TheLastDreamer[guru]
A period is a row of the periodic system of chemical elements, a sequence of atoms in order of increasing nuclear charge and filling the outer electron shell with electrons.
The periodic table has seven periods. The first period, containing 2 elements, as well as the second and third, containing 8 elements each, are called small. The remaining periods with 18 or more elements are large. The seventh period is not completed. The number of the period to which a chemical element belongs is determined by the number of its electron shells (energy levels).
Each period (except for the first) begins with a typical metal (Li, Na, K, Rb, Cs, Fr) and ends with a noble gas (He, Ne, Ar, Kr, Xe, Rn), which is preceded by a typical non-metal.
The charge number of the atomic nucleus (synonyms: atomic number, atomic number, ordinal number of a chemical element) is the number of protons in the atomic nucleus. The charge number is equal to the charge of the nucleus in units of elementary charge and at the same time equal to the serial number of the chemical element corresponding to the nucleus in the periodic table.
A group of the periodic system of chemical elements is a sequence of atoms in increasing nuclear charge that have the same electronic structure.
The group number is determined by the number of electrons in the outer shell of the atom (valence electrons) and, as a rule, corresponds to the highest valence of the atom.
In the short-period version of the periodic system, the groups are divided into subgroups - main (or subgroups A), starting with elements of the first and second periods, and secondary (subgroups B), containing d-elements. Subgroups are also named after the element with the lowest nuclear charge (usually the element of the second period for the main subgroups and the element of the fourth period for secondary subgroups). Elements of the same subgroup have similar chemical properties.
With an increase in the nuclear charge of elements of the same group, due to an increase in the number of electron shells, the atomic radii increase, resulting in a decrease in electronegativity, an increase in the metallic and weakening of the non-metallic properties of the elements, an increase in the reducing and weakening of the oxidative properties of the substances they form.
TheLastDreamer
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Read above.
Reply from Yoldar Baizhanov[guru]
It shows the number of electrons and protons.
Reply from 2 answers[guru]
Hello! Here is a selection of topics with answers to your question: What does the period number show? Chemistry
The question of the subject of chemistry, like any philosophical question, has a historical retrospective.
Pre-alchemical period
As a field of practical activity, chemistry has roots in ancient times. Long before our era, man became acquainted with the transformations of various substances and learned to use them for his needs. The origins of chemistry include the atomistic doctrine, which was alternative at that time, and the doctrine of the elemental elements of ancient natural philosophy.
Alchemical period
In the 3rd-4th centuries AD. e. Alchemy arose in Alexandria, which recognized the possibility of transforming base metals into noble ones - gold and silver - with the help of the philosopher's stone. The main thing in the chemical teaching of this period was the observation of individual properties of substances and their explanation with the help of substances (principles) supposedly included in the composition of these substances.
Period of unification of chemistry
In the 15th and 16th centuries, Europe began a period of rapid growth in trade and material production. By the 16th century, technology in Europe had reached a level significantly higher than during the heyday of the Ancient World. At the same time, changes in technical techniques were ahead of their theoretical understanding. Further improvement of technology rested on the main contradiction of the era - the contradiction between the relatively high level of technological knowledge achieved by that time and the sharp lag in theoretical natural science.
At the beginning of the 17th century, major philosophical works appeared that had a significant impact on the development of natural science. The English philosopher Francis Bacon put forward the thesis that the decisive argument in a scientific discussion should be experiment. The seventeenth century in philosophy was also marked by the revival of atomistic ideas. Mathematician (founder of analytical geometry) and philosopher René Descartes argued that all bodies consist of corpuscles of various shapes and sizes; the shape of the corpuscles is related to the properties of the substance. At the same time, Descartes believed that corpuscles are divisible and consist of a single matter. Descartes denied Democritus' ideas about indivisible atoms moving in emptiness, not daring to admit the existence of emptiness. Corpuscular ideas, very close to the ancient ideas of Epicurus, were also expressed by the French philosopher Pierre Gassendi. Gassendi called groups of atoms that form compounds molecules (from lat. moles- a bunch). Gassendi's corpuscular concepts have gained fairly wide recognition among natural scientists.
In the 17th century, new experimental natural science became a tool for resolving the contradiction between a high level of technology and an extremely low level of knowledge about nature.
One of the consequences of the scientific revolution that occurred in the second half of the 17th century was the creation of a new scientific chemistry. Robert Boyle is traditionally considered the creator of scientific chemistry, who proved the inconsistency of alchemical ideas, gave the first scientific definition of the concept of a chemical element, and thereby for the first time raised chemistry to the level of science.
British scientist Robert Boyle was one of the greatest chemists, physicists and philosophers of his time. Boyle's main scientific achievements in chemistry include his foundation of analytical chemistry (qualitative analysis), studies of the properties of acids, the introduction of indicators into chemical practice, and the study of the densities of liquids using the hydrometer he invented. It is impossible not to mention the law discovered by Boyle, which bears his name (also called the Boyle-Mariotte law).
However, Boyle's main merit was his proposed new system of chemical philosophy, outlined in the book “The Skeptical Chemist” (1661). The book was devoted to searching for an answer to the question of what exactly should be considered elements, based on the current level of development of chemistry. Boyle wrote:
“Chemists have hitherto been guided by overly narrow principles that did not require a particularly broad mental outlook; They saw their task in the preparation of medicines, in the production and transformation of metals. I look at chemistry from a completely different point of view: not as a doctor, not as an alchemist, but as a philosopher should look at it. I have outlined here a plan for chemical philosophy, which I hope to carry out and improve with my experiments and observations.”.
The book is structured in the form of a conversation between four philosophers: Themist, a peripatetic (follower of Aristotle), Philoponus, a spagyricist (supporter of Paracelsus), Carneades, who expounds the views of “Mr. Boyle,” and Eleutherius, who impartially evaluates the arguments of the disputants. The discussion of philosophers led the reader to the conclusion that neither the four elements of Aristotle, nor the three principles of the alchemists can be recognized as elements. Boyle emphasized:
“There is no reason to assign to a given body the name of this or that element simply because it resembles it in one easily noticeable property; after all, with the same right I could refuse it this name, since other properties are different.”.
Based on experimental data, Boyle showed that the concepts of modern chemistry must be revised and brought into line with experiment.
Elements, according to Boyle, are practically indecomposable bodies (substances), consisting of similar homogeneous (consisting of primary matter) corpuscles, from which all complex bodies are composed and into which they can be decomposed. Corpuscles can vary in shape, size, and mass. The corpuscles from which bodies are formed remain unchanged during the transformations of the latter.
Boyle saw the main task of chemistry in the study of the composition of substances and the dependence of the properties of a substance on its composition. At the same time, Boyle considered it possible to use the concept of composition only when it is possible to restore the original body from the elements isolated from a given complex body (i.e., he actually took synthesis as a criterion for the correctness of the analysis). Boyle in his writings did not name a single element in the new understanding of this concept; He did not indicate the number of elements, noting only that:
"It would not be absurd to assume that the number is much more than three or four".
Thus, the book “The Skeptical Chemist” is not an answer to pressing questions of chemical philosophy, but the setting of a new goal for chemistry. The main significance of Boyle's work is this:
1. Formulation of a new goal of chemistry - studying the composition of substances and the dependence of the properties of a substance on its composition.
2. Proposal of a program for searching and studying real chemical elements;
3. Introduction to chemistry of the inductive method;
Boyle's ideas about the element as a practically indecomposable substance quickly gained wide acceptance among natural scientists. However, the creation of theoretical ideas about the composition of bodies that can replace the teachings of Aristotle and the mercury-sulfur theory turned out to be a very difficult task. In the last quarter of the 17th century, eclectic views appeared, the creators of which tried to link alchemical traditions and new ideas about chemical elements. The views of the French chemist Nicolas Lemery, author of the widely known textbook “Course of Chemistry,” had a great influence on his contemporaries.
Lemery's textbook began with a definition of the subject of chemistry:
“Chemistry is an art that teaches how to separate various substances contained in mixed bodies. I understand by mixed bodies those that are formed in nature, namely: minerals, plant and animal bodies.”.
Next, Lemery listed the “chemical principles,” that is, the main components of bodies. After a certain “universal spirit” (which the author himself admits to be “somewhat metaphysical”), Lemery, based on analysis through fire, identified five basic material principles of substances: alcohol (otherwise “mercury”), oil (otherwise “sulfur”), salt, water ( "phlegm") and earth. The first three principles are active, water and earth are passive.
Lemery, however, noted that these substances are “beginnings” for us only insofar as chemists were unable to further decompose these bodies; Obviously, these “beginnings” can in turn be divided into simpler ones. Thus, what is accepted as the principles are the substances obtained by the separation of mixed bodies and separated only so far as the means at the disposal of chemists allow this to be done.
At the turn of the 17th and 18th centuries, scientific chemistry was only at the very beginning of its journey; The most important obstacles that had to be overcome were still strong alchemical traditions (neither Boyle nor Lemery denied the fundamental possibility of transmutation), false ideas about the firing of metals as decomposition, and the speculative (speculative) nature of atomism.
The philosophy of the 18th century is a philosophy of mind, reason, scientific thought. The human mind tries to understand the world around us with the help of scientific knowledge, considerations, observations and logical conclusions, as opposed to medieval scholasticism and blind adherence to church dogmas. This also affected the chemistry. The first theories of scientific chemistry began to appear.
The first theory of scientific chemistry - the theory of phlogiston - was largely based on traditional ideas about the composition of substances and about elements as carriers of certain properties. Nevertheless, it was precisely this that became in the 18th century the main condition and the main driving force for the development of the doctrine of the elements and contributed to the complete liberation of chemistry from alchemy. It was during the almost century-long existence of the phlogiston theory that the transformation of alchemy into chemistry, begun by Boyle, was completed.
The phlogiston theory of combustion was created to describe the processes of firing metals, the study of which was one of the most important problems in chemistry at the end of the 18th century. Metallurgy at this time was faced with two problems, the resolution of which was impossible without serious scientific research - large losses in the smelting of metals and the fuel crisis caused by the almost complete destruction of forests in Europe.
The basis for the theory of phlogiston was the traditional idea of combustion as the decomposition of a body. The phenomenological picture of firing metals was well known: the metal turns into scale, the mass of which is greater than the mass of the original metal; In addition, during combustion, gaseous products of unknown nature are released. The goal of chemical theory was a rational explanation of this phenomenon, which could be used to solve specific technical problems. Neither Aristotle’s ideas nor alchemical views on combustion met the last condition.
The creators of the phlogiston theory are considered to be German chemists Johann Joachim Becher and Georg Ernst Stahl. Becher, in his book "Underground Physics", outlined his very eclectic views on the constituent parts of bodies. These, in his opinion, are three types of earth: the first is fusible and rocky (terra lapidea), the second is greasy and combustible (terra pinguis) and the third is volatile (terra fluida s. mercurialis). The flammability of bodies, according to Becher, is due to the presence of a second, fatty, earth in their composition. Becher's system is very similar to the alchemical doctrine of the three principles, in which flammability is due to the presence of sulfur; however, Becher believes that sulfur is a complex body formed by acid and terra pinguis. In fact, Becher's theory represented one of the first attempts to offer something new to replace the alchemical doctrine of the three principles. Becher traditionally explained the increase in the mass of metal during firing by the addition of “fiery matter.” These views of Becher served as a prerequisite for the creation of the phlogiston theory proposed by Stahl in 1703, although they have very little in common with it. However, Stahl himself always claimed that the author of the theory belongs to Becher.
The essence of the phlogiston theory can be summarized in the following basic principles:
1. There is a material substance contained in all combustible bodies - phlogiston (from the Greek φλογιστοζ - combustible).
2. Combustion is the decomposition of a body with the release of phlogiston, which is irreversibly dispersed into the air. The vortex-like movements of phlogiston released from a burning body represent visible fire. Only plants can extract phlogiston from the air.
3. Phlogiston is always combined with other substances and cannot be isolated in its pure form; The substances that are richest in phlogiston are those that burn without leaving a residue.
4. Phlogiston has negative mass.
Stahl's theory, like all its predecessors, was also based on the idea that the properties of a substance are determined by the presence in them of a special carrier of these properties. The position of the phlogiston theory about the negative mass of phlogiston was intended to explain the fact that the mass of scale (or all combustion products, including gaseous ones) is greater than the mass of the burned metal.
The process of firing a metal within the framework of the phlogiston theory can be represented by the following similar chemical equation:
Metal = Scale + Phlogiston
To obtain metal from scale (or ore), according to theory, you can use any body rich in phlogiston (i.e., burns without residue) - charcoal or coal, fat, vegetable oil, etc.:
Scale + Phlogiston-rich body = Metal
It must be emphasized that experiment can only confirm the validity of this assumption; this was a good argument in favor of Stahl's theory. The phlogiston theory was eventually extended to any combustion processes. The identity of phlogiston in all combustible bodies was substantiated by Stahl experimentally: coal equally reduces sulfuric acid into sulfur and earth into metals. Respiration and rusting of iron, according to Stahl's followers, represent the same process of decomposition of bodies containing phlogiston, but proceeding more slowly than combustion.
The phlogiston theory made it possible, in particular, to give an acceptable explanation for the processes of smelting metals from ore, which consists of the following: ore, in which the phlogiston content is low, is heated with charcoal, which is very rich in phlogiston; In this case, phlogiston passes from coal into ore, and phlogiston-rich metal and phlogiston-poor ash are formed.
It should be noted that in the historical literature there are serious disagreements in assessing the role of the phlogiston theory - from sharply negative to positive. However, it cannot be denied that the phlogiston theory had a number of undoubted advantages:
– it simply and adequately describes experimental facts concerning combustion processes;
– the theory is internally consistent, i.e. none of the consequences contradicts the main provisions;
– the theory of phlogiston is entirely based on experimental facts;
– the phlogiston theory had predictive ability.
The phlogiston theory, the first truly scientific theory of chemistry, served as a powerful stimulus for the development of quantitative analysis of complex bodies, without which experimental confirmation of ideas about chemical elements would have been absolutely impossible. It should be noted that the statement about the negative mass of phlogiston was actually made on the basis of the law of conservation of mass, which was discovered much later. This assumption in itself contributed to the further intensification of quantitative research. Another result of the creation of the phlogiston theory was the active study by chemists of gases in general and gaseous combustion products in particular. By the middle of the 18th century, pneumatic chemistry became one of the most important branches of chemistry, whose founders Joseph Black, Daniel Rutherford, Henry Cavendish, Joseph Priestley and Karl Wilhelm Scheele were the creators of a whole system of quantitative methods in chemistry.
In the second half of the 18th century, the phlogiston theory gained almost universal recognition among chemists. Based on phlogiston concepts, a nomenclature of substances was formed; Attempts have been made to connect such properties of a substance as color, transparency, alkalinity, etc., with the content of phlogiston in it. The French chemist Pierre Joseph Maceur, author of the very popular textbook "Elements of Chemistry" and "Chemical Dictionary", wrote in 1778 that the phlogiston theory
"... is the clearest and most consistent with chemical phenomena. Differing from systems generated by the imagination without agreement with nature and destroyed by experience, Stahl's theory is the most reliable guide in chemical research. Numerous experiments... are not only far from disproving it, but, on the contrary , become evidence in her favor".
Ironically, Maceur's textbook and dictionary appeared at a time when the age of phlogiston theory was coming to an end.
Non-phlogiston ideas about combustion and respiration even arose somewhat earlier than the phlogiston theory. Jean Rey, to whom science owes the postulate “all bodies are heavy,” suggested as early as 1630 that the increase in the mass of metal during firing was due to the addition of air. In 1665, Robert Hooke, in his work “Micrography,” also suggested the presence in the air of a special substance similar to the substance contained in a bound state in saltpeter.
These views were further developed in the book “On saltpeter and airborne alcohol of saltpeter,” which was written in 1669 by the English chemist John Mayow. Mayow tried to prove that the air contains a special gas (spiritus nitroaëreus) that supports combustion and is necessary for breathing; He substantiated this assumption with famous experiments with a burning candle under a bell. However, it was possible to isolate this spiritus nitroaëreus in a free state only after more than a hundred years. The discovery of oxygen was made independently of each other almost simultaneously by several scientists.
Karl Wilhelm Scheele obtained oxygen in 1771, calling it “fiery air”; however, the results of Scheele’s experiments were published only in 1777. According to Scheele, “fiery air” was “acidic thin matter combined with phlogiston.”
Joseph Priestley isolated oxygen in 1774 by heating mercuric oxide. Priestley believed that the gas he obtained was air completely devoid of phlogiston, as a result of which combustion proceeded better in this “dephlogisticated air” than in ordinary air.
In addition, the discovery of hydrogen by Cavendish in 1766 and nitrogen by Rutherford in 1772 (it should be noted that Cavendish mistook hydrogen for pure phlogiston) was of great importance for the creation of the oxygen theory of combustion.
The significance of the discovery made by Scheele and Priestley was able to be correctly assessed by the French chemist Antoine Laurent Lavoisier. In 1774, Lavoisier published the treatise “Small Works on Physics and Chemistry,” where he suggested that during combustion, part of the atmospheric air is added to bodies. After Priestley visited Paris in 1774 and told Lavoisier about the discovery of “dephlogisticated air,” Lavoisier repeated his experiments and in 1775 published the work “On the nature of a substance that combines with metals when calcined and increases their weight” (however , Lavoisier attributed the priority of the discovery of oxygen to himself). Finally, in 1777, Lavoisier formulated the main principles of the oxygen theory of combustion:
1. Bodies burn only in “clean air”.
2. “Clean air” is absorbed during combustion, and the increase in the mass of the burned body is equal to the decrease in the mass of air.
3. When heated, metals turn into “earths”. Sulfur or phosphorus, combining with “clean air”, turns into acids.
The new oxygen theory of combustion (the term oxygen – oxygenium – appeared in 1877 in Lavoisier’s work “General consideration of the nature of acids and the principles of their combination”) had a number of significant advantages over the phlogistic theory. It is simpler than the phlogiston one, did not contain “unnatural” assumptions about the presence of negative mass in bodies, and, most importantly, was not based on the existence of substances not isolated experimentally. As a result, the oxygen theory of combustion quickly gained wide acceptance among natural scientists (although the controversy between Lavoisier and phlogistics continued for many years).
At the end of the 18th century and the beginning of the 19th, a movement called Scientism (from science) prevailed in philosophy, which manifests itself in admiration for science, the cult of science and human knowledge. A person is proud of his knowledge and intelligence, freedom, and is confident in his ability to solve all problems that arise. Academies became the main centers of scientific activity. At this time, a revolution was taking place in chemical science.
The significance of the oxygen theory turned out to be much greater than just an explanation of the phenomena of combustion and respiration. The rejection of the phlogiston theory required a revision of all the basic principles and concepts of chemistry, a change in terminology and nomenclature of substances. Therefore, with the creation of the oxygen theory, a turning point in the development of chemistry began, called the “chemical revolution”.
In 1785-1787 Four outstanding French chemists - Antoine Laurent Lavoisier, Claude Louis Berthollet, Louis Bernard Guiton de Morveau and Antoine Francois de Fourcroy - on behalf of the Paris Academy of Sciences, developed a new system of chemical nomenclature. The logic of the new nomenclature involved constructing the name of a substance based on the names of the elements of which the substance consists. The basic principles of this nomenclature are still used today.