Chemistry: basic concepts, definitions, terms and laws

Chemistry, the basic concepts of which we will consider, is a science that studies substances and their transformations, which occur with a change in structure and composition, and therefore, properties. First of all, it is necessary to determine what the term "substance" means. Speaking of it in a broad sense, it is a form of matter that has a lot of rest. A substance is any elementary particle, for example, a neutron. In chemistry, this concept is used in a narrower sense.

To begin with, we briefly describe the main terms and concepts of chemistry, atomic-molecular theory. After that, we will explain them, and also outline some important laws of this science.

The basic concepts of chemistry (matter, atom, molecule) are familiar to each of us from school. Below is a brief description of them, as well as other, less obvious terms and phenomena.

Atoms

First of all, all the substances that are studied in chemistry are composed of small particles called atoms. Neutrons are not the object of study of this science. It should also be said that atoms can be connected to each other, resulting in the formation of chemical bonds. In order to break this relationship, energy is needed. Consequently, atoms in ordinary conditions do not exist by themselves (except for "noble gases"). They connect with each other at least in pairs.

Continuous thermal motion

A continuous thermal motion characterizes all the particles that chemistry studies. The basic concepts of this science can not be stated without telling about it. With continuous motion, the average kinetic energy of the particles is proportional to temperature (but it should be noted that the energies of the individual particles are different). Ekkin = kT / 2, where k is the Boltzmann constant. This formula is valid for any kind of motion. Since Ekin = mV 2/2, the motion of massive particles is slower. For example, if the temperature is the same, oxygen molecules on average move 4 times slower than carbon molecules. This is because their mass is more than 16 times. Movement is vibrational, translational and rotational. Vibrational is observed in liquid, and in solid, and in gaseous substances. But the translational and rotational is most easily accomplished in gases. In liquids, it is more difficult, and in solids it is even more difficult.

Molecules

Let's continue to describe the basic concepts and definitions of chemistry. If atoms combine to form small groups (they are called molecules), such groups take part in the thermal movement, acting as a whole. Up to 100 atoms are present in typical molecules, and their number in so-called high-molecular compounds can reach 105.

Non-molecular substances

However, atoms are often combined into huge collectives from 107 to 1,027. In this form, they practically do not take part in the thermal movement. These associations are little like molecules. They are more like pieces of a solid body. These substances are usually called non-molecular. In this case, the thermal motion is carried out inside the piece, and it does not fly like a molecule. There is also a transitional region of dimensions, in which joins consisting of atoms in an amount from 105 to 107 are included. These particles are either very large molecules, or they represent small particles of powder.

Ions

It should be noted that atoms and their groups can have an electric charge. In this case, they are called ions in such a science as chemistry, the basic concepts of which we are studying. Since like charges always repel each other, the substance, where there is a significant excess of certain charges, can not be stable. Negative and positive charges in space always alternate. And the substance as a whole remains electrically neutral. We note that the charges considered to be large in electrostatics are insignificant from the point of view of chemistry (by 105-1015 atoms - Ie).

Objects of study in chemistry

It should be clarified that the objects of study in chemistry are those phenomena in which atoms do not arise and do not degrade, but only regroup, that is, they reunite in a new way. Some connections are broken, resulting in the formation of others. In other words, new substances appear from atoms that were in the composition of the initial substances. If the atoms and the bonds that exist between them are preserved (for example, during the evaporation of molecular substances), then these processes are part of the study of molecular physics rather than chemistry. In the case when atoms are formed or destroyed, we are talking about the subjects of studying nuclear or atomic physics. However, the boundary between chemical and physical phenomena is blurred. After all, the division into separate sciences is conditional, whereas nature is indivisible. Therefore, chemists are very useful knowledge of physics.

The basic concepts of chemistry were briefly described by us. Now we suggest you to consider them in more detail.

More about atoms

Atoms and molecules are what many people associate with chemistry. The basic concepts must be clearly defined. The fact that atoms exist, two thousand years ago was brilliantly guessed. Then, already in the 19th century, the scientists had experimental data (as yet indirect). We are talking about Avogadro's multiple relations, the laws of constancy of composition (below we will consider these basic concepts of chemistry). The atom was continued to be investigated in the 20th century, when a lot of direct experimental evidence arose. They were based on the data of spectroscopy, on the scattering of X-rays, alpha particles, neutrons, electrons, etc. The size of these particles is about 1 E = 1 ^-10 m. Their mass is about 10 ^-27 - 10 ^-25 kg. At the center of these particles is a positively charged core around which electrons with negative charge move. The size of the core is about 10 ^-15 m. It turns out that the electronic shell determines the size of the atom, but its mass is almost completely concentrated in the nucleus. Another definition should be introduced, considering the basic concepts of chemistry. The chemical element is the kind of atoms whose charge is the same.

Often there is a definition of an atom as the smallest particle of matter, chemically indivisible. How to understand "chemically"? As we have already noted, the division of phenomena into physical and chemical conditional. But the existence of atoms is unconditional. Therefore, chemistry is better defined through them, and not vice versa, atoms through chemistry.

Chemical bonding

This is why the atoms are held together. It does not allow them to fly apart under the influence of thermal motion. Let's note the main characteristics of the connections - this is the internuclear distance and energy. This is also the basic concepts of chemistry. The length of the bond is determined experimentally with a sufficiently high accuracy. Energy - too, but not always. For example, it is impossible to objectively determine what it is with respect to a single bond in a complex molecule. However, the energy of atomization of matter, which is necessary for breaking all available bonds, is always determined. Knowing the length of the connection, it is possible to determine which atoms are connected (they have a short distance), and which are not (a long distance).

Coordination number and coordination

The basic concepts of analytical chemistry include these two terms. What do they mean? Let's figure it out.

The coordination number is the number of nearest neighbors of a particular atom. In other words, this is the number of those with whom it is chemically bound. Coordination is the relative position, type and number of neighbors. In other words, this concept is more meaningful. For example, the coordination number of nitrogen inherent in ammonia and nitric acid molecules is the same - 3. However, their coordination is different - not planar and flat. It is determined independently of the concept of the nature of the bond, whereas the degree of oxidation and valence are conditional concepts that are created in order to predict coordination and composition in advance.

Determination of a molecule

We have already touched on this concept, considering the basic concepts and laws of chemistry briefly. Now let us dwell on it in more detail. In textbooks, there is often a definition of a molecule as the smallest neutral particle of a substance that has its chemical properties, and is also able to exist independently. It should be noted that this definition is now obsolete. First, what all physicists and chemists call a molecule, the properties of matter do not preserve. Water dissociates, but this requires a minimum of 2 molecules. The degree of water dissociation is 10 ^-7 . In other words, only one molecule of 10 million can be subjected to this process. If you have one molecule, or there is even one hundred, you can not get an idea of its dissociation. The fact is that the thermal effects of reactions in chemistry usually involve the energy of interaction between molecules. Therefore, they can not be found by one of them. Both the chemical and physical properties of a molecular substance can be determined only by a large group of molecules. In addition, there are substances in which the "smallest" particle capable of independently exists is vaguely large and very different from the usual molecules. A molecule is actually a group of atoms that is not electrically charged. In the particular case it can be one atom, for example, Ne. This group should be able to participate in diffusion, as well as in other types of thermal motion, acting as a whole.

As you can see, the basic concepts of chemistry are not so simple. A molecule is something that must be carefully studied. It has its own properties, as well as its molecular mass. We will now talk about the latter.

Molecular mass

How to determine the molecular weight by experience? One way - based on Avogadro's law, on the relative density of steam. The most accurate method is mass spectrometric. The electron is knocked out of the molecule. The resulting ion is first accelerated in an electric field, then rejected by a magnetic path. The ratio of charge to mass is determined precisely by the magnitude of the deviation. There are also methods based on the properties that solutions possess. However, the molecules in all these cases must necessarily be in motion - in solution, in a vacuum, in a gas. If they do not move, it is impossible to objectively calculate their mass. And their very existence in this case is difficult to detect.

Features of non-molecular substances

Speaking of them, they note that they consist of atoms, not molecules. However, this is also true of noble gases. These atoms move freely, therefore, it is better to consider them as monatomic molecules. However, this is not the main thing. More importantly, in non-molecular substances there are many atoms that are bound together. It should be noted that the division of all substances into non-molecular and molecular ones is insufficient. Division by connectivity is more meaningful. Consider, for example, the difference in the properties of graphite and diamond. Both are carbon, but the first is soft, and the second is hard. What are they different from each other? The difference lies precisely in their coherence. If we consider the structure of graphite, we will see that there are strong bonds only in two dimensions. But in the third, interatomic distances are very significant, therefore, there is no strong connection. Graphite easily slips and breaks through these layers.

Structure Connectivity

Otherwise, it is called spatial dimension. It represents the number of dimensions of space characterized by the continuous (almost infinite) system of skeleton (strong bonds) in them. The values that it can take are 0, 1, 2, and 3. Therefore, it is necessary to distinguish three-dimensionally connected, layered, chain and island (molecular) structures.

Law of constancy of composition

We have already studied the basic concepts of chemistry. The substance was briefly considered by us. Now let's talk about the law that applies to him. Usually it is formulated as follows: any individual substance (that is, pure), regardless of how it was received, has the same quantitative and qualitative composition. But what does the concept of "pure substance" mean? Let's figure it out.

Two thousand years ago, when the structure of substances could not yet be studied by direct methods, when even the basic chemical concepts and laws of chemistry, which we were accustomed to, did not even exist, it was defined descriptively. For example, water is a fluid that forms the basis of seas and rivers. It has no smell, color, taste. It has such temperatures of freezing and melting, copper sulfate turns blue from it . Salty sea water is because it is not clean. However, the salts can be separated by distillation. Approximately, the basic chemical concepts and laws of chemistry were defined by a descriptive method.

For scientists of the time it was not obvious that the liquid, which is isolated in different ways (burning hydrogen, dehydrating vitriol, distilling sea water), has the same composition. A great discovery in science was the proof of this fact. It became clear that the ratio of oxygen and hydrogen can not smoothly change. This means that the elements consist of atoms - indivisible portions. So the formulas of substances were obtained, and also the representation of scientists about molecules is justified.

In our time, any substance, either explicitly or implicitly, is determined primarily by a formula, not by a melting point, taste or color. Water - H ₂ O. If other molecules are present, it will no longer be pure. Consequently, a pure molecular substance is one that is composed of molecules of only one species.

However, how in this case be with electrolytes? After all, they contain ions, not just molecules. A more rigorous definition is needed. A pure molecular substance is one that is composed of molecules of one species, and possibly also products of their reversible rapid transformation (isomerization, association, dissociation). The word "fast" in this context means that we can not get rid of these products, they immediately appear again. The word "reversible" indicates that the transformation is not complete. If it is, then it is better to say that it is unstable. In this case, it is not a pure substance.

The law of conservation of mass of matter

From the earliest times this law was known in metaphorical form. He said that the substance is uncreatable and indestructible. Then came his quantitative formulation. According to her, the weight (and from the end of the 17th century - the mass) is a measure of the amount of matter.

This law, in the form customary for us, was opened in 1748 by Lomonosov. In 1789 he was supplemented by A. Lavoisier, a French scientist. Its modern formulation sounds like this: the mass of substances that enter into a chemical reaction equals the mass of the substances that result from it.

Avogadro's law, the law of gas volume relationships

The last of these was formulated in 1808 by JL Gay-Lussac, a French scientist. At present, this law is called the law of Gay-Lussac. According to him, the volumes of reacting gases refer to each other, and also to the volumes of the gaseous products obtained as small integers.

The law discovered by Gay-Lussac, explains the law, which was discovered a little later, in 1811, Amedeo Avogadro, an Italian scientist. It states that under equal conditions (pressure and temperature) in gases having the same volumes, the same number of molecules are present.

Two important consequences follow from Avogadro's law. The first is that under the same conditions, one mole of any gas occupies an equal volume. The volume of any of them under normal conditions (which are the temperature of 0 ° C, as well as the pressure of 101.325 kPa) is 22.4 liters. The second consequence of this law is the following: under equal conditions, the ratio of the masses of gases having the same volume is equal to the ratio of their molar masses.

There is one more law that must be mentioned. Let's talk about it in brief.

Periodic law and table

DI Mendeleyev, based on the chemical properties of elements and atomic-molecular theory, discovered this law. This event occurred on March 1, 1869. The periodic law is one of the most important in nature. It can be formulated as follows: the properties of the elements and the complex and simple substances they form have a periodic dependence on the charges of the nuclei of their atoms.

The periodic table, which Mendeleev created, consists of seven periods and eight groups. Groups are called its vertical columns. The elements inside each of them have similar physical and chemical properties. The group, in turn, is divided into subgroups (main and side).

The horizontal rows of this table are called periods. The elements that are in them differ, but they have a common thing - that their last electrons are located on the same energy level. In the first period there are only two elements. It is hydrogen H and helium He. Eight elements are available in the second period. In the fourth there are already 18. Mendeleev marked this period as the first big one. In the fifth there are also 18 elements, its structure is similar to the fourth one. In the sixth - 32 elements. The seventh is not finished. This period begins with France (Fr). We can assume that it will contain 32 elements, as well as the sixth. However, so far only 24 have been found.

The rule of the report

According to the rule of the test, all elements tend to acquire an electron or lose it in order to have the 8-electron configuration of the noble gas closest to them. The energy of ionization is the amount of energy that is needed to separate an electron from an atom. The rule of the decree states that when moving from left to right on a periodic table, more energy is required to detach the electron. Therefore, the elements on the left side tend to lose the electron. On the contrary, those on the right are eager to acquire it.

The laws and basic concepts of chemistry were briefly described. Of course, this is only general information. Within the framework of one article it is impossible to tell in detail about such a serious science. The basic concepts and laws of chemistry, summarized in this article, are only a starting point for further study. After all, there are many sections in this science. There is, for example, organic and inorganic chemistry. The basic concepts of each of the sections of this science can be studied for a very long time. But those that are presented above are related to general issues. Therefore, we can say that these are the basic concepts of organic chemistry, as well as inorganic chemistry.