Belkin I.K. The charge of the atomic nucleus and Mendeleev's periodic system of elements // Kvant. - 1984. - No. 3. - S. 31-32.

By special agreement with the editorial board and the editors of the journal "Kvant"

Modern ideas about the structure of the atom arose in 1911-1913, after the famous experiments of Rutherford on the scattering of alpha particles. In these experiments, it was shown that α -particles (their charge is positive), falling on a thin metal foil, are sometimes deflected at large angles and even thrown back. This could only be explained by the fact that the positive charge in the atom is concentrated in a negligible volume. If we imagine it in the form of a ball, then, as Rutherford established, the radius of this ball should be approximately 10 -14 -10 -15 m, which is tens and hundreds of thousands of times smaller than the size of the atom as a whole (~10 -10 m) . Only near such a small positive charge can there be an electric field capable of discarding α - a particle moving at a speed of about 20,000 km/s. Rutherford called this part of the atom the nucleus.

This is how the idea arose that an atom of any substance consists of a positively charged nucleus and negatively charged electrons, the existence of which in atoms was established earlier. Obviously, since the atom as a whole is electrically neutral, the charge of the nucleus must be numerically equal to the charge of all the electrons present in the atom. If we denote the electron charge modulus by the letter e(elementary charge), then the charge q i cores should be equal q i = Ze, where Z is an integer equal to the number of electrons in the atom. But what is the number Z? What is the charge q i core?

From the experiments of Rutherford, which made it possible to determine the size of the nucleus, in principle, it is possible to determine the value of the charge of the nucleus. After all, the electric field that rejects α -particle, depends not only on the size, but also on the charge of the nucleus. And Rutherford really estimated the charge of the nucleus. According to Rutherford, the nuclear charge of an atom of a chemical element is approximately equal to half of its relative atomic mass BUT, multiplied by the elementary charge e, i.e

\(~Z = \frac(1)(2)A\).

But, oddly enough, the true charge of the nucleus was established not by Rutherford, but by one of the readers of his articles and reports, the Dutch scientist Van den Broek (1870-1926). It is strange because Van den Broek was not a physicist by education and profession, but a lawyer.

Why did Rutherford, when evaluating the charges of atomic nuclei, correlate them with atomic masses? The fact is that when in 1869 D. I. Mendeleev created a periodic system of chemical elements, he arranged the elements in the order of increasing their relative atomic masses. And over the past forty years, everyone has become accustomed to the fact that the most important characteristic of a chemical element is its relative atomic mass, that it is this that distinguishes one element from another.

Meanwhile, it was at this time, at the beginning of the 20th century, that difficulties arose with the system of elements. In the study of the phenomenon of radioactivity, a number of new radioactive elements were discovered. And there seemed to be no place for them in Mendeleev's system. It seemed that Mendeleev's system needed to be changed. This was what Van den Broek was especially concerned about. Over the course of several years, he proposed several options for an expanded system of elements, in which there would be enough space not only for the still undiscovered stable elements (D. I. Mendeleev himself “took care” of the places for them), but also for radioactive elements too. Van den Broek's last version was published in early 1913, it had 120 places, and uranium occupied cell number 118.

In the same year, 1913, the results of the latest research on scattering were published. α -particles at large angles, carried out by Rutherford's collaborators Geiger and Marsden. Analyzing these results, Van den Broek made an important discovery. He found that the number Z in formula q i = Ze is not equal to half the relative mass of an atom of a chemical element, but to its serial number. And besides, the ordinal number of the element in the Mendeleev system, and not in his, Van den Broek, 120-local system. Mendeleev's system, it turns out, did not need to be changed!

It follows from the idea of ​​Van den Broek that every atom consists of an atomic nucleus, the charge of which is equal to the serial number of the corresponding element in the Mendeleev system, multiplied by the elementary charge, and electrons, the number of which in the atom is also equal to the serial number of the element. (A copper atom, for example, consists of a nucleus with a charge of 29 e, and 29 electrons.) It became clear that D. I. Mendeleev intuitively arranged the chemical elements in ascending order not of the atomic mass of the element, but of the charge of its nucleus (although he did not know about this). Consequently, one chemical element differs from another not by its atomic mass, but by the charge of the atomic nucleus. The charge of the nucleus of an atom is the main characteristic of a chemical element. There are atoms of completely different elements, but with the same atomic masses (they have a special name - isobars).

The fact that it is not atomic masses that determine the position of an element in the system is also evident from the periodic table: in three places, the rule of increasing atomic mass is violated. So, the relative atomic mass of nickel (No. 28) is less than that of cobalt (No. 27), for potassium (No. 19) it is less than that of argon (No. 18), for iodine (No. 53) it is less than that of tellurium ( No. 52).

The assumption about the relationship between the charge of the atomic nucleus and the ordinal number of the element easily explained the displacement rules for radioactive transformations, discovered in the same 1913 (Physics 10, § 103). Indeed, when emitted by the nucleus α -particle, the charge of which is equal to two elementary charges, the charge of the nucleus, and hence its serial number (now they usually say - atomic number) should decrease by two units. When emitting β -particle, that is, a negatively charged electron, it must increase by one unit. This is what the displacement rules are about.

The idea of ​​Van den Broek very soon (literally in the same year) received the first, albeit indirect, experimental confirmation. Somewhat later, its correctness was proved by direct measurements of the charge of the nuclei of many elements. It is clear that it played an important role in the further development of the physics of the atom and the atomic nucleus.

The nuclear charge () determines the location of the chemical element in the D.I. table. Mendeleev. The Z number is the number of protons in the nucleus. Cl is the charge of the proton, which is equal in magnitude to the charge of the electron.

We emphasize once again that the charge of the nucleus determines the number of positive elementary charges carried by protons. And since the atom is generally a neutral system, the charge of the nucleus also determines the number of electrons in the atom. And we remember that the electron has a negative elementary charge. Electrons in an atom are distributed over energy shells and subshells depending on their number, therefore, the charge of the nucleus has a significant effect on the distribution of electrons over their states. The chemical properties of an atom depend on the number of electrons at the last energy level. It turns out that the charge of the nucleus determines the chemical properties of the substance.

It is now customary to denote various chemical elements as follows: , where X is the symbol of a chemical element in the periodic table, which corresponds to the charge.

Elements that have the same Z but different atomic masses (A) (which means that the nucleus has the same number of protons but a different number of neutrons) are called isotopes. So, hydrogen has two isotopes: 1 1 H-hydrogen; 2 1 H-deuterium; 3 1 H-tritium

There are stable and unstable isotopes.

Nuclei with the same masses but different charges are called isobars. Isobars are mainly found among heavy nuclei, and in pairs or triads. For example, and .

The first indirect measurement of the nuclear charge was made by Moseley in 1913. He established a relationship between the frequency of the characteristic X-ray radiation () and the nuclear charge (Z):

where C and B are constants independent of the element for the series of radiation under consideration.

The charge of the nucleus was directly determined by Chadwick in 1920 while studying the scattering of nuclei of the helium atom on metal films.

Core Composition

The nucleus of a hydrogen atom is called a proton. The mass of a proton is:

The nucleus is made up of protons and neutrons (collectively called nucleons). The neutron was discovered in 1932. The mass of the neutron is very close to the mass of the proton. The neutron has no electric charge.

The sum of the number of protons (Z) and the number of neutrons (N) in the nucleus is called the mass number A:

Since the masses of the neutron and proton are very close, each of them is equal to almost an atomic mass unit. The mass of electrons in an atom is much less than the mass of the nucleus, so it is believed that the mass number of the nucleus is approximately equal to the relative atomic mass of the element, if rounded to the nearest integer.

Examples of problem solving

EXAMPLE 1

Exercise

Nuclei are very stable systems, therefore, protons and neutrons must be kept inside the nucleus by some kind of force. What can you say about these forces?

Decision

It can be immediately noted that the forces that bind nucleons do not belong to gravitational ones, which are too weak. The stability of the nucleus cannot be explained by the presence of electromagnetic forces, since between protons, as particles carrying charges of the same sign, there can only be electrical repulsion. Neutrons are electrically neutral particles. A special kind of force acts between nucleons, which are called nuclear forces. These forces are almost 100 times stronger than electrical forces. Nuclear forces are the most powerful of all known forces in nature. The interaction of particles in the nucleus is called strong. The next feature of nuclear forces is that they are short-range. Nuclear forces become noticeable only at a distance of the order of cm, that is, at a distance of the size of the nucleus.

EXAMPLE 2

Exercise

At what minimum distance can the nucleus of a helium atom, having a kinetic energy equal to that of a head-on collision, approach the motionless nucleus of a lead atom?

Decision

Let's make a drawing. Consider the motion of the nucleus of a helium atom ( - particles) in an electrostatic field, which creates a motionless nucleus of a lead atom. - the particle moves towards the nucleus of the lead atom with a speed decreasing to zero, since repulsive forces act between like-charged particles. The kinetic energy that the particle possessed will turn into the potential energy of interaction - the particle and the field (), which creates the nucleus of the lead atom: We express the potential energy of a particle in an electrostatic field as: where is the charge of the nucleus of a helium atom; - the intensity of the electrostatic field, which creates the nucleus of the lead atom. From (2.1) - (2.3) we get:

An atom is the smallest particle of a chemical element that retains all of its chemical properties. An atom consists of a positively charged nucleus and negatively charged electrons. The charge of the nucleus of any chemical element is equal to the product of Z and e, where Z is the serial number of this element in the periodic system of chemical elements, e is the value of the elementary electric charge.

Electron- this is the smallest particle of a substance with a negative electric charge e=1.6·10 -19 coulombs, taken as an elementary electric charge. Electrons, rotating around the nucleus, are located on the electron shells K, L, M, etc. K is the shell closest to the nucleus. The size of an atom is determined by the size of its electron shell. An atom can lose electrons and become a positive ion, or gain electrons and become a negative ion. The charge of an ion determines the number of electrons lost or gained. The process of turning a neutral atom into a charged ion is called ionization.

atomic nucleus(the central part of the atom) consists of elementary nuclear particles - protons and neutrons. The radius of the nucleus is about a hundred thousand times smaller than the radius of the atom. The density of the atomic nucleus is extremely high. Protons- These are stable elementary particles having a unit positive electric charge and a mass 1836 times greater than the mass of an electron. The proton is the nucleus of the lightest element, hydrogen. The number of protons in the nucleus is Z. Neutron is a neutral (not having an electric charge) elementary particle with a mass very close to the mass of a proton. Since the mass of the nucleus is the sum of the mass of protons and neutrons, the number of neutrons in the nucleus of an atom is A - Z, where A is the mass number of a given isotope (see). The proton and neutron that make up the nucleus are called nucleons. In the nucleus, nucleons are bound by special nuclear forces.

The atomic nucleus has a huge store of energy, which is released during nuclear reactions. Nuclear reactions occur when atomic nuclei interact with elementary particles or with the nuclei of other elements. As a result of nuclear reactions, new nuclei are formed. For example, a neutron can transform into a proton. In this case, a beta particle, i.e., an electron, is ejected from the nucleus.

The transition in the nucleus of a proton into a neutron can be carried out in two ways: either a particle with a mass equal to the mass of an electron, but with a positive charge, called a positron (positron decay), is emitted from the nucleus, or the nucleus captures one of the electrons from the nearest K-shell (K -capture).

Sometimes the formed nucleus has an excess of energy (it is in an excited state) and, passing into the normal state, releases excess energy in the form of electromagnetic radiation with a very short wavelength -. The energy released during nuclear reactions is practically used in various industries.

An atom (Greek atomos - indivisible) is the smallest particle of a chemical element that has its chemical properties. Each element is made up of certain types of atoms. The structure of an atom includes the kernel bearing a positive electric charge, and negatively charged electrons (see), forming its electronic shells. The value of the electric charge of the nucleus is equal to Z-e, where e is the elementary electric charge, equal in magnitude to the charge of the electron (4.8 10 -10 e.-st. units), and Z is the atomic number of this element in the periodic system of chemical elements (see .). Since a non-ionized atom is neutral, the number of electrons included in it is also equal to Z. The composition of the nucleus (see. Atomic nucleus) includes nucleons, elementary particles with a mass approximately 1840 times greater than the mass of an electron (equal to 9.1 10 - 28 g), protons (see), positively charged, and chargeless neutrons (see). The number of nucleons in the nucleus is called the mass number and is denoted by the letter A. The number of protons in the nucleus, equal to Z, determines the number of electrons entering the atom, the structure of the electron shells and the chemical properties of the atom. The number of neutrons in the nucleus is A-Z. Isotopes are called varieties of the same element, the atoms of which differ from each other in mass number A, but have the same Z. Thus, in the nuclei of atoms of different isotopes of one element there are a different number of neutrons with the same number of protons. When designating isotopes, the mass number A is written at the top of the element symbol, and the atomic number at the bottom; for example, isotopes of oxygen are denoted:

The dimensions of the atom are determined by the dimensions of the electron shells and for all Z are about 10 -8 cm. Since the mass of all the electrons of the atom is several thousand times less than the mass of the nucleus, the mass of the atom is proportional to the mass number. The relative mass of an atom of a given isotope is determined in relation to the mass of an atom of the carbon isotope C 12, taken as 12 units, and is called the isotopic mass. It turns out to be close to the mass number of the corresponding isotope. The relative weight of an atom of a chemical element is the average (taking into account the relative abundance of isotopes of a given element) value of the isotopic weight and is called the atomic weight (mass).

An atom is a microscopic system, and its structure and properties can only be explained with the help of quantum theory, created mainly in the 20s of the 20th century and intended to describe phenomena on an atomic scale. Experiments have shown that microparticles - electrons, protons, atoms, etc. - in addition to corpuscular, have wave properties that manifest themselves in diffraction and interference. In quantum theory, a certain wave field characterized by a wave function (Ψ-function) is used to describe the state of micro-objects. This function determines the probabilities of possible states of a micro-object, i.e., it characterizes the potential possibilities for the manifestation of one or another of its properties. The law of variation of the function Ψ in space and time (the Schrödinger equation), which makes it possible to find this function, plays the same role in quantum theory as Newton's laws of motion in classical mechanics. The solution of the Schrödinger equation in many cases leads to discrete possible states of the system. So, for example, in the case of an atom, a series of wave functions for electrons is obtained corresponding to different (quantized) energy values. The system of energy levels of the atom, calculated by the methods of quantum theory, has received brilliant confirmation in spectroscopy. The transition of an atom from the ground state corresponding to the lowest energy level E 0 to any of the excited states E i occurs when a certain portion of energy E i - E 0 is absorbed. An excited atom goes into a less excited or ground state, usually with the emission of a photon. In this case, the photon energy hv is equal to the difference between the energies of an atom in two states: hv= E i - E k where h is Planck's constant (6.62·10 -27 erg·sec), v is the frequency of light.

In addition to atomic spectra, quantum theory has made it possible to explain other properties of atoms. In particular, valency, the nature of the chemical bond and the structure of molecules were explained, and the theory of the periodic system of elements was created.

At the heart of any science lies something small and important. In biology it is a cell, in linguistics it is a letter and sound, in engineering it is a cog, in construction it is a grain of sand, and for chemistry and physics the most important thing is the atom, its structure.

This article is intended for persons over 18 years of age.

Are you over 18 already?

An atom is that smallest particle of everything that surrounds us, which carries all the necessary information, a particle that determines characteristics and charges. For a long time, scientists thought that it was indivisible, one, but for long hours, days, months and years, studies, studies and experiments were carried out that proved that the atom also has its own structure. In other words, this microscopic ball consists of even smaller components that affect the size of its nucleus, properties and charge. The structure of these particles is as follows:

• electrons;
• the nucleus of an atom.

The latter can also be divided into very elementary parts, which in science are called protons and neurons, of which there are a clear number in each case.

The number of protons that are in the nucleus indicates the structure of the shell, which consists of electrons. This shell, in turn, contains all the necessary properties of a particular material, substance or object. Calculating the sum of protons is very simple - it is enough to know the serial number of the smallest part of the substance (atom) in the well-known periodic table. This value is also called the atomic number and is denoted by the Latin letter "Z". It is important to remember that protons have a positive charge, and in writing this value is defined as +1.

Neurons are the second component of the nucleus of an atom. This is an elementary subatomic particle that does not carry any charge, unlike electrons or protons. Neurons were discovered in 1932 by J. Chadwick, for which he received the Nobel Prize 3 years later. In textbooks and scientific papers, they are referred to as the Latin character "n".

The third component of the atom is the electron, which is in monotonous motion around the nucleus, thus creating a cloud. It is this particle that is the lightest of all known to modern science, which means that its charge is also the smallest. The electron is denoted in the letter from -1.

It is the combination of positive and negative particles in the structure that makes the atom an uncharged or neutrally charged particle. The nucleus, in comparison with the total size of the entire atom, is very small, but it is in it that all the weight is concentrated, which indicates its high density.

How to determine the charge of the nucleus of an atom?

To determine the charge of the nucleus of an atom, you need to be well versed in the structure, structure of the atom itself and its nucleus, understand the basic laws of physics and chemistry, and also be armed with the periodic table of Mendeleev to determine the atomic number of a chemical element.

1. The knowledge that a microscopic particle of any substance has a nucleus and electrons in its structure, which create a shell around it in the form of a cloud. The nucleus, in turn, includes two types of elementary indivisible particles: protons and neurons, each of which has its own properties and characteristics. Neurons do not have an electronic charge in their arsenal. This means that their charge is neither equal nor greater than or less than zero. Protons, unlike their counterparts, carry a positive charge. In other words, their electric charge can be denoted as +1.
2. Electrons, which are an integral part of every atom, also carry a certain kind of electrical charge. They are negatively charged elementary particles, and in writing they are defined as −1.
3. To calculate the charge of an atom, you need knowledge about its structure (we just remembered the necessary information), the number of elementary particles in the composition. And in order to find out the sum of the charge of an atom, you need to mathematically add the number of some particles (protons) to others (electrons). Usually, the characteristic of an atom says that it is electron neutral. In other words, the value of electrons is equal to the number of protons. The result is that the value of the charge of such an atom is equal to zero.
4. An important nuance: there are situations when the number of positively and negatively charged elementary particles in the nucleus may not be equal. This suggests that the atom becomes an ion with a positive or negative charge.

The designation of the nucleus of an atom in the scientific field looks like Ze. Deciphering this is quite simple: Z is the number assigned to the element in the well-known periodic table, it is also called the ordinal or charging number. And it indicates the number of protons in the nucleus of an atom, and e is just the charge of a proton.

In modern science, there are nuclei with different charge values: from 1 to 118.

Another important concept that young chemists need to know is the mass number. This concept indicates the total amount of the charge of nucleons (these are the very smallest components of the nucleus of an atom of a chemical element). And you can find this number if you use the formula: A = Z + N where A is the desired mass number, Z is the number of protons, and N is the number of neutrons in the nucleus.

What is the nuclear charge of a bromine atom?

In order to demonstrate in practice how to find the charge of an atom of a necessary element (in our case, bromine), it is worth referring to the periodic table of chemical elements and finding bromine there. Its atomic number is 35. This means that the charge of its nucleus is also 35, since it depends on the number of protons in the nucleus. And the number of protons is indicated by the number under which the chemical element stands in the great work of Mendeleev.

Here are a few more examples to make it easier for young chemists to calculate the necessary data in the future:

• the charge of the nucleus of the sodium atom (na) is 11, since it is under this number that it can be found in the table of chemical elements.
• the charge of the phosphorus nucleus (whose symbolic designation is P) has a value of 15, because that is how many protons are in its nucleus;
• sulfur (with graphic designation S) is a neighbor in the table of the previous element, therefore, its nuclear charge is 16;
• iron (and we can find it in the designation Fe) is at number 26, which indicates the same number of protons in its nucleus, and hence the charge of the atom;
• carbon (aka C) is under the 6th number of the periodic table, which indicates the information we need;
• magnesium has atomic number 12, and in international symbolism it is known as Mg;
• chlorine in the periodic table, where it is written as Cl, is number 17, so its atomic number (namely, we need it) is the same - 17;
• calcium (Ca), which is so useful for young organisms, is found at number 20;
• the charge of the nucleus of the nitrogen atom (with the written designation N) is 7, it is in this order that it is presented in the periodic table;
• barium stands at number 56, which is equal to its atomic mass;
• the chemical element selenium (Se) has 34 protons in its nucleus, and this shows that this will be the charge of the nucleus of its atom;
• silver (or written Ag) has a serial number and an atomic mass of 47;
• if you need to find out the charge of the nucleus of the lithium atom (Li), then you need to turn to the beginning of the great work of Mendeleev, where he is at number 3;
• Aurum or our favorite gold (Au) has an atomic mass of 79;
• for argon, this value is 18;
• rubidium has an atomic mass of 37, while strontium has an atomic mass of 38.

It is possible to list all the components of Mendeleev's periodic table for a very long time, because there are a lot of them (these components). The main thing is that the essence of this phenomenon is clear, and if you need to calculate the atomic number of potassium, oxygen, silicon, zinc, aluminum, hydrogen, beryllium, boron, fluorine, copper, fluorine, arsenic, mercury, neon, manganese, titanium, then you only need refer to the table of chemical elements and find out the serial number of a particular substance.

CORE CHARGE

Moseley's law. The electric charge of the nucleus is formed by the protons that make up its composition. Number of protons Z called its charge, meaning that the absolute value of the charge of the nucleus is equal to Ze. The charge of the nucleus is the same as the serial number Z element in Mendeleev's periodic system of elements. For the first time, the charges of atomic nuclei were determined by the English physicist Moseley in 1913. By measuring the wavelength with a crystal λ characteristic X-ray radiation for the atoms of certain elements, Moseley discovered a regular change in wavelength λ for elements following one after another in the periodic system (Fig. 2.1). Moseley interpreted this observation as the dependence λ from some atomic constant Z, changing by one from element to element and equal to one for hydrogen:

where and are constants. From experiments on the scattering of X-ray quanta by atomic electrons and α -particles by atomic nuclei, it was already known that the charge of the nucleus is approximately equal to half the atomic mass and, therefore, is close to the ordinal number of the element. Since the emission of characteristic X-ray radiation is a consequence of electrical processes in the atom, Moseley concluded that the atomic constant found in his experiments, which determines the wavelength of the characteristic X-ray radiation and coincides with the serial number of the element, can only be the charge of the atomic nucleus (Moseley's law).

Rice. 2.1. X-ray spectra of atoms of neighboring elements obtained by Moseley

The measurement of X-ray wavelengths is carried out with great precision, so that on the basis of Moseley's law, the belonging of an atom to a chemical element is established absolutely reliably. However, the fact that the constant Z in the last equation is the charge of the nucleus, although it is justified by indirect experiments, it ultimately rests on the postulate - Moseley's law. Therefore, after Moseley's discovery, the charges of nuclei were repeatedly measured in scattering experiments. α -particles based on Coulomb's law. In 1920, Chadwig improved the method for measuring the proportion of scattered α -particles and received the charges of the nuclei of atoms of copper, silver and platinum (see table 2.1). Chadwig's data leave no doubt about the validity of Moseley's law. In addition to the indicated elements, the charges of the nuclei of magnesium, aluminum, argon, and gold were also determined in the experiments.

Table 2.1. The results of Chadwick's experiments

Definitions. After Moseley's discovery, it became clear that the main characteristic of an atom is the charge of the nucleus, and not its atomic mass, as chemists of the 19th century assumed, because the charge of the nucleus determines the number of atomic electrons, and hence the chemical properties of atoms. The reason for the difference between the atoms of chemical elements is precisely that their nuclei have a different number of protons in their composition. On the contrary, a different number of neutrons in the nuclei of atoms with the same number of protons does not change the chemical properties of atoms in any way. Atoms that differ only in the number of neutrons in their nuclei are called isotopes chemical element.