CC BY
2
UDC 37.66.30
Nabiyeva G. O.
assistant of the department of physics and chemistry Andijan Institute of Agriculture and Agro technologies
Uzbekistan, Andijan
THEMATIC CONTROL OF STUDENTS' KNOWLEDGE AS AN EFFECTIVE PROCESS OF TEACHING CHEMISTRY
Abstract: The relevance of this work lies in increasing the effectiveness of the educational process and obtaining objective information by the teacher about the course of educational and cognitive activities of students. The teacher receives this information in the process of monitoring the educational and cognitive activity of students.
Key words: valence, chemical element, electrons, formulas, affects, hydrogen
bond.
The properties of chemical elements and the simple and complex substances formed by them are periodically dependent on the charge of the nucleus of the atoms of these elements.
So the theory of the structure of atoms made it possible to clarify the formulation of the periodic law and reveal its essence more deeply.
Determination of the patterns of distribution of electrons by energy levels made it possible to clarify the essence of the phenomenon of periodicity: with an increase in the charge of the nucleus of atoms, similar properties of elements are periodically repeated, the atoms of which have the same number of valence electrons.
Valence refers to an important concept in chemistry. The w ord "Valencia" (from the Latin "Valentia") originated in the middle of the 19th century. During the completion of the chemical-analytical stage of the development of chemistry. At present, the number of covalent bonds by which a given atom is connected to other atoms usually determines the valence of chemical elements.
Valence is the ability of an atom of a chemical element to form a certain number of chemical bonds.
Valence is the ability of atoms of one element to attach a certain number of atoms of another element.
Valence and valence capabilities are important characteristics of a chemical element. They are determined by the structure of atoms and change periodically with an increase in the charges of the nuclei.
What do you think the concept of valence means? (students' opinion). Valence means opportunity - a means, a condition necessary for the implementation of something; possible - one that can happen, acceptable.
The valence capabilities of atoms are the permissible valences of an element, the entire range of their values in various compounds.
Since the valence of an atom depends on the number of unpaired electrons, we will consider the structures of atoms in excited states, taking into account the valence capabilities. Let us write down the electron diffraction formulas for the distribution of electrons over orbitals in a carbon atom.
The periodic change in the valence of elements and, consequently, their properties is due to the fact that with an increase in the charges of atomic nuclei, elements with a similar electronic structure, for example, lithium Li, sodium Na and potassium K, are periodically repeated; beryllium Be, magnesium Mg, etc.
In a periodic sequence, the atomic radii of these elements increase. So, for example, in the 2nd period from the element of lithium Li to the element of fluorine F there is a gradual decrease in atomic radii, and from the element fluorine F to the element sodium Na - a sharp increase in atomic radii. This phenomenon is explained as follows.
When passing from lithium Li to fluorine F, the charges of the atomic nuclei of these elements gradually increase. In this regard, in the series, the force of attraction of the outer electrons to the nucleus gradually increases and the sizes of atoms decrease. And with the transition from the fluorine element F to the sodium element Na, the subsequent electron is placed on the third energy level farther from the nucleus. Therefore, the size of the atoms of the sodium element Na increases greatly.
The size of atoms, in turn, affects their properties. So, for example, the atoms of the elements lithium Li, sodium Na, potassium K have the largest dimensions in comparison with the atoms of other elements in the same periods. In this regard, the outer electrons in the atoms of alkali metals are farther from the nucleus, are less attracted to it and can be easily removed. This explains why alkali metals are electron donors, i.e. strong reducing agents. With the transition in periods from typical metallic elements to halogens, the sizes of atoms decrease, the force of attraction of outer electrons to the nucleus increases, which leads to a decrease in reducing and an increase in oxidizing properties.
In the periods of the periodic system, from left to right, the numerical values of electro negativities increase, and in groups from top to bottom, they decrease. Since the type of bond depends on the difference in the values of the electro negativities of the connecting atoms of the elements, it is impossible to draw a sharp boundary between the individual types of chemical bonds. Depending on which of the limiting cases, the chemical bond is closer in nature, it is referred to as ionic or covalent polar.
Ionic bond. An ionic bond is formed by the interaction of atoms, which differ greatly in electronegativity. For example, typical metals - lithium Li, sodium Na, potassium K, calcium Ca, barium Ba - form an ionic bond with typical non-metals, mainly halogens.
It should be borne in mind that when even alkali metals interact with such electronegative elements as oxygen and sulfur, an ionic bond in the full sense of this concept arises. So, for example, in compounds Li? O, Na? S, there is not an ionic, but a covalent strongly polar bond.
In addition to the halides of the most active metals, the ionic bond is characteristic of alkalis and salts, which include oxygen atoms and active metals. For example, sodium hydroxide Na OH and sodium sulfate Na? SO? only bonds between sodium and oxygen atoms are ionic. The rest of the bonds are covalent polar. In a dilute aqueous solution, alkali and salts dissociate as follows:
Strong electrostatic attraction forces exist between the ions. Therefore, ionic compounds have relatively high melting points.
Covalent non-polar bond. When atoms with the same electro negativities combine, molecules with a covalent non-polar bond are formed. Recall that such a bond, for example, exists in molecules of gaseous substances consisting of the same atoms: H ?, F ?, Cl ?, O ?, N ?. In these cases, chemical bonds are formed due to common electron pairs, i.e. when the corresponding electron clouds overlap, due to the electron-nuclear interaction, which occurs when atoms approach each other.
Hydrogen bond. The bond between the hydrogen atoms of one molecule and the strongly negative elements (O, N, F) of another molecule is called a hydrogen bond.
The question may arise: why exactly hydrogen is able to form such specific chemical bonds? This is due to the fact that the hydrogen atom has a very small radius and when a single electron is displaced or given up, hydrogen acquires a relatively strong positive charge, which acts on electronegative elements in the molecules of substances. Let's look at some examples. We are used to depicting the composition of water by the formula H? O, but it would be more correct to denote the composition of water by the formula (H? O) n, where n is 2, 3, 4, etc., since individual water molecules are connected by hydrogen bonds, which are schematically depicted by dots:
The hydrogen bond is much weaker than the ionic or covalent bond, but stronger than the intermolecular interaction.
The formation of hydrogen bonds explains why the volume of water, in contrast to the volume of other substances, increases with decreasing temperature.
When studying organic chemistry, the following question arose: why are the boiling points of alcohols much higher than those of the corresponding hydrocarbons? It turns out that a chemical interaction also occurs between the molecules of alcohols with the formation of hydrogen bonds:
You already know that D.I. Mendeleev studied the process of dissolving alcohol in water. He came to the conclusion that during dissolution, chemical processes also occur simultaneously, for example, the interaction of alcohol molecules with water molecules:
The hydrogen bond is characteristic of many organic compounds (phenols, aldehydes, carboxylic acids, etc.). Due to the hydrogen bond, the secondary structure of proteins, the double helix of DNA, is formed.
While studying inorganic chemistry, you acquired the first understanding of solutions and the process of dissolving substances in ode. It was also mentioned there that when substances are mixed with water, both homogeneous systems (a characteristic property of solutions) and heterogeneous ones are formed, i.e.
suspensions and emulsions. Many disperse systems are known. They differ depending on what kind of particles (solid, liquid, gaseous) and in what medium (liquid, gaseous) they are distributed. The teacher must be able to make the learning process not only effective, but also interesting for children. Systematic control of students' knowledge and skills is one of the main conditions for improving the quality of education. Skillful mastery of various forms of control of knowledge and skills by the teacher helps to increase the interest of students in learning, prevents lagging, ensures the active work of each student. Control for students should be educational, and therefore, it will be both developing and educating.
References:
1. Baranov S.P. Learning principles. - M., 1981. p. 354
2. Smirnov S.A. Pedagogy: pedagogical theories, systems, technologies. Tutorial. -M., 1998, p. 468
3. N.E. Kuznetsova. and other Methods of teaching chemistry. M. Enlightenment. 1984.
4. Volkov A.I. and others. Programmed control of current knowledge in general chemistry. Toolkit. Minsk. 1988.
