What is the positive charge of the nucleus of an atom. Atomic nucleus: nuclear charge

From planetary model structure of atoms, we know that an atom is a nucleus, and a cloud of electrons revolving around it. Moreover, the distance between the electrons and the nucleus is tens and hundreds of thousands of times greater than the size of the nucleus itself.

What is the core itself? Is it a small hard indivisible ball or is it made up of smaller particles? Not a single microscope that exists in the world is able to clearly show us what is happening at this level. Everything is too small. Then how to be? Is it even possible to study the physics of the atomic nucleus? How to find out the composition and characteristics of the atomic nucleus, if it is not possible to study it?

The charge of the nucleus of an atom

With a wide variety of indirect experiments, expressing hypotheses and testing them in practice, through trial and error, scientists managed to investigate the structure of the atomic nucleus. It turned out that the nucleus consists of even smaller particles. The number of these particles determines the size of the nucleus, its charge and Chemical properties substances. Moreover, these particles have a positive charge, which compensates for the negative charge of the electrons of the atom. These particles are called protons. Their number in the normal state is always equal to the number of electrons. The question of how to determine the charge of the nucleus no longer stood. The charge of the nucleus of an atom in a neutral state is always equal to the number of electrons revolving around it and is opposite in sign to the charge of the electrons. And physicists have already learned how to determine the number and charge of electrons.

The structure of the atomic nucleus: protons and neutrons

However, in the course of further research, new problem. It turned out that protons, having the same charge, in some cases differ twice in mass. This caused a lot of questions and inconsistencies. In the end, it was possible to establish that the composition of the atomic nucleus, in addition to protons, also includes some particles that are almost equal in mass to protons, but do not have any charge. These particles are called neutrons. The detection of neutrons resolved all inconsistencies in the calculations. As a result, protons and neutrons, as the constituent elements of the nucleus, were called nucleons. The calculation of any values relating to the characteristics of the core has become much easier to understand. Neutrons do not take part in the formation of the nuclear charge, therefore, their influence on the chemical properties of matter is practically not manifested, however, neutrons participate in the formation of the mass of nuclei, respectively, affect the gravitational properties of the atomic nucleus. Thus, there is some indirect influence of neutrons on the properties of matter, but it is extremely insignificant.

Atoms of any substance are electrically neutral particles. An atom consists of a nucleus and a collection of electrons. The nucleus carries a positive charge, the total charge of which is equal to the sum of the charges of all the electrons of the atom.

General information about the charge of the nucleus of an atom

The charge of the nucleus of an atom determines the location of the element in periodic system DI. Mendeleev and, accordingly, the chemical properties of a substance consisting of these atoms and compounds of these substances. The value of the nuclear charge is:

where Z is the number of the element in the periodic table, e is the value of the electron charge or.

Elements with the same numbers Z, but different atomic masses are called isotopes. If the elements have the same Z, then their nucleus has an equal number of protons, and if the atomic masses are different, then the number of neutrons in the nuclei of these atoms is different. For example, hydrogen has two isotopes: deuterium and tritium.

The nuclei of atoms have a positive charge because they are made up of protons and neutrons. A proton is a stable particle belonging to the class of hadrons, which is the nucleus of a hydrogen atom. A proton is a positively charged particle. Its charge is equal in modulus to the elementary charge, that is, the magnitude of the charge of the electron. The charge of a proton is often denoted as , then we can write that:

The rest mass of a proton () is approximately equal to:

You can learn more about the proton by reading the section "Charge of the proton".

Nuclear charge experiments

Moseley was the first to measure the nuclear charges in 1913. The measurements were indirect. The scientist determined the relationship between the frequency x-ray radiation() and the nuclear charge Z.

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

Chadwick measured the nuclear charge directly in 1920. He carried out the scattering of particles on metal films, in fact, repeating Rutherford's experiments, which led Rutherford to build nuclear model atom.

In these experiments, the particles were passed through a thin metal foil. Rutherford found that in most cases the particles passed through the foil, deviating by small angles from the original direction of motion. This is explained by the fact that - particles are deflected under the influence of electric forces of electrons, which have a much smaller mass than - particles. Sometimes, quite rarely, the particles were deflected at angles exceeding 90 o . Rutherford explained this fact by the presence of a charge in the atom, which is localized in a small volume, and this charge is associated with a mass that is much larger than that of the particle.

For a mathematical description of the results of his experiments, Rutherford derived a formula that determines angular distribution- particles after their scattering by atoms. When deriving this formula, the scientist used Coulomb's law for point charges and at the same time believed that the mass of the nucleus of an atom is much greater than the mass of particles. The Rutherford formula can be written as:

where n is the number of scattering nuclei per unit area of the foil; N is the number of - particles that pass in 1 second through a single area, perpendicular to the direction of the flow - particles; - the number of particles that are scattered inside the solid angle - the charge of the scattering center; - mass - particles; - deflection angle - particles; v - speed - particles.

The Rutherford formula (3) can be used to find the charge of the nucleus of an atom (Z), if we compare the number of incident particles (N) with the number (dN) of particles scattered at an angle, then the function will depend only on the charge of the scattering nucleus. By conducting experiments and applying Rutherford's formula, Chadwick found the charges of the nuclei of platinum, silver and copper.

Examples of problem solving

EXAMPLE 1

The task

A metal plate is irradiated - with particles having a high speed. Some part of these particles during elastic interaction with the nuclei of metal atoms changes the direction of their movement to the opposite. What is the charge of the nucleus of metal atoms (q), if the minimum distance between the particle and the nucleus is r. The mass of a particle is equal to its velocity v. When solving the problem, relativistic effects can be neglected. Particles are considered point, the nucleus is immobile and point.

Solution

Let's make a drawing.

Moving towards the nucleus of an atom, the particle overcomes the Coulomb force, which repels it from the nucleus, since the particle and the nucleus have positive charges. The kinetic energy of a moving particle is converted into the potential energy of interaction between the nucleus of the metal atom and the particle. The law of conservation of energy should be taken as the basis for solving the problem.:

We find the potential energy of point charged particles as:

where the charge of the particles is: , since and - the particles are the nucleus of the helium atom, which consists of two protons and two neutrons, since we assume that the experiment is carried out in air.

Kinetic energy - particles before collision with the nucleus of an atom is equal to:

In accordance with (1.1), we equate the right parts of expressions (1.2) and (1.3), we have:

From formula (1.4) we express the charge of the nucleus:

Answer

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. Long time scientists thought that it was indivisible, one, but for long hours, days, months and years, studies, studies and experiments were carried out, which 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, 3 years later, he received Nobel Prize. in textbooks and scientific papers they are designated 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 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 core, compared to general sizes of the whole atom, 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 chemical element.

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.
Electrons, which are an integral part of every atom, also carry certain kind electric charge. They are negatively charged elementary particles, and in writing they are defined as −1.
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.
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 meaning charges: from 1 to 118.

Another important concept, which you need to know young chemists- 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.

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 sizes atom as a whole (~10 -10 m). Only near such a small positive charge can there be 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 is the charge all the electrons in an 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 chemical element - its relative atomic mass that it is what 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 its serial number. And, moreover, 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 main characteristic 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 can also be seen 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 a particle with a charge of two elementary charges, the charge of the nucleus, and hence its serial number (now they usually say - the 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.

Investigating the passage of an α-particle through a thin gold foil (see Section 6.2), E. Rutherford came to the conclusion that an atom consists of a heavy positively charged nucleus and electrons surrounding it.

core called the center of the atom,in which almost all the mass of an atom and its positive charge is concentrated.

IN composition of the atomic nucleus are included elementary particles : protons And neutrons (nucleons from the Latin word nucleus- core). Such a proton-neutron model of the nucleus was proposed by the Soviet physicist in 1932 D.D. Ivanenko. The proton has a positive charge e + = 1.06 10 -19 C and a rest mass m p\u003d 1.673 10 -27 kg \u003d 1836 me. Neutron ( n) is a neutral particle with rest mass m n= 1.675 10 -27 kg = 1839 me(where the mass of the electron me, is equal to 0.91 10 -31 kg). On fig. 9.1 shows the structure of the helium atom according to the ideas of the end of XX - early XXI in.

Core charge equals Ze, where e is the charge of the proton, Z- charge number equal to serial number chemical element in Mendeleev's periodic system of elements, i.e. the number of protons in the nucleus. The number of neutrons in a nucleus is denoted N. Usually Z > N.

Nuclei with Z= 1 to Z = 107 – 118.

Number of nucleons in the nucleus A = Z + N called mass number . nuclei with the same Z, but different BUT called isotopes. Kernels, which, at the same A have different Z, are called isobars.

The nucleus is denoted by the same symbol as the neutral atom, where X is the symbol for a chemical element. For example: hydrogen Z= 1 has three isotopes: – protium ( Z = 1, N= 0), is deuterium ( Z = 1, N= 1), – tritium ( Z = 1, N= 2), tin has 10 isotopes, and so on. In the vast majority of isotopes of the same chemical element, they have the same chemical and close physical properties. In total, about 300 stable isotopes and more than 2000 natural and artificially obtained are known. radioactive isotopes.

The size of the nucleus is characterized by the radius of the nucleus, which has a conditional meaning due to the blurring of the nucleus boundary. Even E. Rutherford, analyzing his experiments, showed that the size of the nucleus is approximately 10–15 m (the size of an atom is 10–10 m). There is an empirical formula for calculating the core radius:

(9.1.1)

where R 0 = (1.3 - 1.7) 10 -15 m. From this it can be seen that the volume of the nucleus is proportional to the number of nucleons.

The density of the nuclear substance is on the order of 10 17 kg/m 3 and is constant for all nuclei. It greatly exceeds the density of the densest ordinary substances.

Protons and neutrons are fermions, because have spin ħ /2.

The nucleus of an atom has own angular momentum – nuclear spin :

(9.1.2)

where I – internal(complete)spin quantum number.

Number I accepts integer or half-integer values 0, 1/2, 1, 3/2, 2, etc. Kernels with even BUT have integer spin(in units ħ ) and obey the statistics Bose–Einstein(bosons). Kernels with odd BUT have half-integer spin(in units ħ ) and obey the statistics Fermi–Dirac(those. nuclei are fermions).

Nuclear particles have their own magnetic moments, which determine the magnetic moment of the nucleus as a whole. The unit for measuring the magnetic moments of nuclei is nuclear magneton μ poison:

(9.1.3)

Here e – absolute value electron charge, m p is the mass of the proton.

Nuclear magneton in m p/me= 1836.5 times smaller than the Bohr magneton, hence it follows that the magnetic properties of atoms are determined magnetic properties its electrons .

There is a relationship between the spin of the nucleus and its magnetic moment:

(9.1.4)

where γ poison - nuclear gyromagnetic ratio.

The neutron has a negative magnetic moment μ n≈ – 1.913μ poison because the direction of the neutron spin and its magnetic moment are opposite. The magnetic moment of the proton is positive and equal to μ R≈ 2.793μ poison. Its direction coincides with the direction of the proton spin.

The distribution of the electric charge of protons over the nucleus is generally asymmetric. The measure of deviation of this distribution from spherically symmetric is quadrupole electric moment of the nucleus Q. If the charge density is assumed to be the same everywhere, then Q determined only by the shape of the nucleus. So, for an ellipsoid of revolution

(9.1.5)

where b is the semiaxis of the ellipsoid along the spin direction, but- axis in the perpendicular direction. For a nucleus stretched along the direction of the spin, b > but And Q> 0. For a nucleus oblate in this direction, b < a And Q < 0. Для сферического распределения заряда в ядре b = a And Q= 0. This is true for nuclei with spin equal to 0 or ħ /2.

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