“Atom and atomic nucleus” - Isotopes. Biography of the atom. Problem solving. Discovery of the neutron. The contributions are unequal. Model of an atom in the form of a positively charged ball. Quiz. Designation of the nucleus of an atom. An atom has a nucleus. Nuclear forces. In the depths of space. Ideas about the structure of the atom. Core theory. The nucleus of an atom consists of nucleons. Proton-neutron model of the atomic nucleus.
“Nuclear binding energy” - The decrease in specific binding energy of light elements is explained by surface effects. - Mass defect. Coulomb forces tend to tear the nucleus apart. Specific binding energy. Elements with mass numbers from 50 to 60 have the maximum binding energy (8.6 MeV/nucleon). Binding energy of atomic nuclei. The binding energy of nucleons on the surface is less than that of nucleons inside the nucleus.
"The Nucleus of the Atom" - Ernest Rutherford. Test (continued). Isotopes of one element differ in the number... in the nucleus. The nucleus of an atom consists of protons and neutrons. Information about the composition of the atomic nucleus is indicated as follows: Molecules Ions Protons. And the total charge of electrons is Ze. Test: For example, into the nucleus of an oxygen atom. Protons are carriers of an elementary positive charge, neutrons are electrically neutral.
“Atomic nucleus” - 1932 Ivanenko and Heisenberg proposed a proton-neutron model of the atomic nucleus. Quanta of nuclear interactions. Discovery of the structure of the nucleus. Kernel model. However, inside a stable nucleus, neutrons are bound to protons and do not decay spontaneously. J. Chadwick repeated the experiment. The discovery of the neutron was an important step forward.
"Fundamental interactions" - Newton's theory of universal gravitation. Superunion. Association models. Types of interaction of elementary particles. Creation of a unified theory of fundamental interactions. Theoretical achievements. Elementary particle. Symbol for weak interaction. Interactions. Lever scales. Electromagnetic interaction.
“Physics of the atomic nucleus” - The activity of the drug is the number of nuclei that decay in a unit period of time: Types of nuclear reactions. Virtual particles. I. Nucleon. 3. Strangeness. Conservation laws. Elementary particles are particles that behave as structureless. Synthesis of transuranic chemical elements. The spectrum of radiation is discrete.
There are 9 presentations in total
When it became clear that the nuclei of atoms have a complex structure, the question arose of what particles they consist of.
In 1913, Rutherford put forward the hypothesis that one of the particles that make up the atomic nuclei of all chemical elements is the nucleus of the hydrogen atom.
The basis for this assumption was a number of facts that had emerged by that time and obtained experimentally. In particular, it was known that the masses of atoms of chemical elements exceed the mass of a hydrogen atom by an integer number of times (i.e., they are multiples of it). In 1919, Rutherford set up an experiment to study the interaction of α-particles with the nuclei of nitrogen atoms.
In this experiment, an alpha particle flying at enormous speed, when it hit the nucleus of a nitrogen atom, knocked out some particle from it. According to Rutherford's assumption, this particle was the nucleus of the hydrogen atom, which Rutherford called a proton (from the Greek protos - first). But since the observation of these particles was carried out by the scintillation method, it was impossible to determine exactly which particle was emitted from the nucleus of the nitrogen atom.
It was possible to verify that a proton actually emitted from the nucleus of an atom only a few years later, when the reaction between an α particle and the nucleus of a nitrogen atom was carried out in a cloud chamber.
Through the transparent round window of a cloud chamber, even with the naked eye you can see the tracks (i.e., trajectories) of particles moving quickly in it (Fig. 161).
Rice. 161. Photographs of tracks of charged particles obtained in a cloud chamber
The figure shows straight lines diverging like a fan. These are traces of α-particles that flew through the chamber space without colliding with the nuclei of nitrogen atoms. But the trace of one alpha particle bifurcates, forming a so-called “fork”. This means that at the bifurcation point of the track, an α-particle interacted with the nucleus of a nitrogen atom, resulting in the formation of nuclei of oxygen and hydrogen atoms. The fact that these particular nuclei are formed was determined by the nature of the curvature of the tracks when a cloud chamber was placed in a magnetic field.
The reaction of the interaction of a nitrogen nucleus with α-particles with the formation of oxygen and hydrogen nuclei is written as follows:
where the symbol H denotes a proton, i.e., the nucleus of a hydrogen atom, with a mass approximately equal to 1 a. amu (more precisely, 1.0072765 amu), and a positive charge equal to the elementary charge (i.e., the modulus of the electron charge). The symbol is also used to denote a proton.
Subsequently, the interaction of alpha particles with the nuclei of atoms of other elements was studied: boron (B), sodium (Na), aluminum (Al), magnesium (Mg) and many others. As a result, it turned out that alpha particles knocked out protons from all these nuclei. This gave reason to believe that protons are part of the atomic nuclei of all chemical elements.
The discovery of the proton did not provide a complete answer to the question of what particles the nuclei of atoms consist of. If we assume that atomic nuclei consist only of protons, then a contradiction arises.
Let us show, using the example of the nucleus of a beryllium atom (), what this contradiction is.
Let us assume that the nucleus consists only of protons. Since the charge of each proton is equal to one elementary charge, the number of protons in the nucleus must be equal to the charge number, in this case four.
But if the beryllium nucleus really consisted of only four protons, then its mass would be approximately 4 a. amu (since the mass of each proton is approximately 1 amu).
However, this contradicts experimental data, according to which the mass of the nucleus of a beryllium atom is approximately 9 AU. eat.
Thus, it becomes clear that in addition to protons, the nuclei of atoms contain some other particles.
In this regard, in 1920, Rutherford suggested the existence of an electrically neutral particle with a mass approximately equal to the mass of a proton.
In the early 30s. XX century Previously unknown rays were discovered, which were called beryllium radiation, since they arose when bombarded by beryllium alpha particles.
James Chadwick (1891-1974)
English experimental physicist. Works in the field of radioactivity and nuclear physics. Discovered the neutron
In 1932, the English scientist James Chadwick (a student of Rutherford), using experiments carried out in a cloud chamber, proved that beryllium radiation is a stream of electrically neutral particles whose mass is approximately equal to the mass of a proton. The absence of an electric charge in the particles under study followed, in particular, from the fact that they did not deviate in either an electric or magnetic field. And the mass of particles was estimated by their interaction with other particles.
These particles were called neutrons. Accurate measurements showed that the mass of the neutron is 1.0086649 a. e.m., i.e. slightly larger than the mass of a proton. In many cases, the mass of a neutron (as well as the mass of a proton) is considered equal to 1 a. e.m. Therefore, a unit is placed at the top before the neutron symbol. Zero at the bottom means no electrical charge.
Questions
- What conclusion was made based on the photograph of particle tracks in a cloud chamber (see Fig. 161)?
- What is the other name and symbol for the nucleus of a hydrogen atom? What is its mass and charge?
- What assumption (regarding the composition of nuclei) did the results of experiments on the interaction of α-particles with the nuclei of atoms of various elements allow one to make?
- What contradiction does the assumption that atomic nuclei consist only of protons lead to? Explain this with an example.
- How was it proven that neutrons have no electric charge? How was their mass estimated?
- How is a neutron designated, what is its mass compared to the mass of a proton?
Exercise 47
Consider a recording of the nuclear reaction of the interaction of nitrogen and helium nuclei, resulting in the formation of oxygen and hydrogen nuclei. Compare the total charge of the interacting nuclei with the total charge of the nuclei formed as a result of this interaction. Draw a conclusion about whether the law of conservation of electric charge is satisfied in this reaction.Electrons
The concept of atom arose in the ancient world to designate particles of matter. Translated from Greek, atom means “indivisible.”
The Irish physicist Stoney, based on experiments, came to the conclusion that electricity is carried by the smallest particles existing in the atoms of all chemical elements. In 1891, Stoney proposed to call these particles electrons, which means “amber” in Greek. A few years after the electron got its name, the English physicist Joseph Thomson and the French physicist Jean Perrin proved that electrons carry a negative charge. This is the smallest negative charge, which in chemistry is taken as one (-1). Thomson even managed to determine the speed of the electron (the speed of the electron in the orbit is inversely proportional to the orbit number n. The radii of the orbits increase in proportion to the square of the orbit number. In the first orbit of the hydrogen atom (n=1; Z=1) the speed is ≈ 2.2·106 m/ s, that is, about a hundred times less than the speed of light c = 3·108 m/s) and the mass of the electron (it is almost 2000 times less than the mass of the hydrogen atom).
State of electrons in an atom
The state of an electron in an atom is understood as a set of information about the energy of a particular electron and the space in which it is located. An electron in an atom does not have a trajectory of motion, i.e. we can only talk about the probability of finding it in the space around the nucleus.
It can be located in any part of this space surrounding the nucleus, and the totality of its various positions is considered as an electron cloud with a certain negative charge density. Figuratively, this can be imagined this way: if it were possible to photograph the position of an electron in an atom after hundredths or millionths of a second, as in a photo finish, then the electron in such photographs would be represented as dots. If countless such photographs were superimposed, the picture would be of an electron cloud with the greatest density where there would be the most of these points.
The space around the atomic nucleus in which an electron is most likely to be found is called an orbital. It contains approximately 90% electronic cloud, and this means that about 90% of the time the electron is in this part of space. They are distinguished by shape 4 currently known types of orbitals, which are designated by Latin letters s, p, d and f. A graphical representation of some forms of electron orbitals is presented in the figure.
The most important characteristic of the motion of an electron in a certain orbital is energy of its connection with the nucleus. Electrons with similar energy values form a single electron layer, or energy level. Energy levels are numbered starting from the nucleus - 1, 2, 3, 4, 5, 6 and 7.
The integer n, indicating the number of the energy level, is called the principal quantum number. It characterizes the energy of electrons occupying a given energy level. Electrons of the first energy level, closest to the nucleus, have the lowest energy. Compared to electrons of the first level, electrons of subsequent levels will be characterized by a large supply of energy. Consequently, the electrons of the outer level are least tightly bound to the atomic nucleus.
The largest number of electrons at an energy level is determined by the formula:
N = 2n 2 ,
where N is the maximum number of electrons; n is the level number, or the main quantum number. Consequently, the first energy level closest to the nucleus can contain no more than two electrons; on the second - no more than 8; on the third - no more than 18; on the fourth - no more than 32.
Starting from the second energy level (n = 2), each of the levels is divided into sublevels (sublayers), slightly different from each other in the binding energy with the nucleus. The number of sublevels is equal to the value of the main quantum number: the first energy level has one sublevel; the second - two; third - three; fourth - four sublevels. The sublevels, in turn, are formed by orbitals. Each valuen corresponds to the number of orbitals equal to n.
Sublevels are usually denoted by Latin letters, as well as the shape of the orbitals of which they consist: s, p, d, f.
Protons and Neutrons
An atom of any chemical element is comparable to a tiny solar system. Therefore, this model of the atom, proposed by E. Rutherford, is called planetary.
The atomic nucleus, in which the entire mass of the atom is concentrated, consists of particles of two types - protons and neutrons.
Protons have a charge equal to the charge of electrons, but opposite in sign (+1), and a mass equal to the mass of a hydrogen atom (it is taken as one in chemistry). Neutrons carry no charge, they are neutral and have a mass equal to the mass of a proton.
Protons and neutrons together are called nucleons (from the Latin nucleus - nucleus). The sum of the number of protons and neutrons in an atom is called the mass number. For example, the mass number of an aluminum atom is:
13 + 14 = 27
number of protons 13, number of neutrons 14, mass number 27
Since the mass of the electron, which is negligibly small, can be neglected, it is obvious that the entire mass of the atom is concentrated in the nucleus. Electrons are designated e - .
Since the atom electrically neutral, then it is also obvious that the number of protons and electrons in an atom is the same. It is equal to the serial number of the chemical element assigned to it in the Periodic Table. The mass of an atom consists of the mass of protons and neutrons. Knowing the atomic number of the element (Z), i.e. the number of protons, and the mass number (A), equal to the sum of the numbers of protons and neutrons, you can find the number of neutrons (N) using the formula:
N = A - Z
For example, the number of neutrons in an iron atom is:
56 — 26 = 30
Isotopes
Varieties of atoms of the same element that have the same nuclear charge but different mass numbers are called isotopes. Chemical elements found in nature are a mixture of isotopes. Thus, carbon has three isotopes with masses 12, 13, 14; oxygen - three isotopes with masses 16, 17, 18, etc. The relative atomic mass of a chemical element usually given in the Periodic Table is the average value of the atomic masses of a natural mixture of isotopes of a given element, taking into account their relative abundance in nature. The chemical properties of isotopes of most chemical elements are exactly the same. However, hydrogen isotopes vary greatly in properties due to the dramatic multiple increase in their relative atomic mass; they are even given individual names and chemical symbols.
Elements of the first period
Diagram of the electronic structure of the hydrogen atom:
Diagrams of the electronic structure of atoms show the distribution of electrons across electronic layers (energy levels).
Graphic electronic formula of the hydrogen atom (shows the distribution of electrons by energy levels and sublevels):
Graphic electronic formulas of atoms show the distribution of electrons not only among levels and sublevels, but also among orbitals.
In a helium atom, the first electron layer is complete - it has 2 electrons. Hydrogen and helium are s-elements; The s-orbital of these atoms is filled with electrons.
For all elements of the second period the first electronic layer is filled, and electrons fill the s- and p-orbitals of the second electron layer in accordance with the principle of least energy (first s and then p) and the Pauli and Hund rules.
In the neon atom, the second electron layer is complete - it has 8 electrons.
For atoms of elements of the third period, the first and second electronic layers are completed, so the third electronic layer is filled, in which electrons can occupy the 3s-, 3p- and 3d-sublevels.
The magnesium atom completes its 3s electron orbital. Na and Mg are s-elements.
In aluminum and subsequent elements, the 3p sublevel is filled with electrons.
Elements of the third period have unfilled 3d orbitals.
All elements from Al to Ar are p-elements. The s- and p-elements form the main subgroups in the Periodic Table.
Elements of the fourth - seventh periods
A fourth electron layer appears in potassium and calcium atoms, and the 4s sublevel is filled, since it has lower energy than the 3d sublevel.
K, Ca - s-elements included in the main subgroups. For atoms from Sc to Zn, the 3d sublevel is filled with electrons. These are 3d elements. They are included in secondary subgroups, their outermost electronic layer is filled, and they are classified as transition elements.
Pay attention to the structure of the electronic shells of chromium and copper atoms. In them, one electron “fails” from the 4s to the 3d sublevel, which is explained by the greater energy stability of the resulting electronic configurations 3d 5 and 3d 10:
In the zinc atom, the third electron layer is complete - all sublevels 3s, 3p and 3d are filled in it, with a total of 18 electrons. In the elements following zinc, the fourth electron layer, the 4p sublevel, continues to be filled.
Elements from Ga to Kr are p-elements.
The krypton atom has an outer layer (fourth) that is complete and has 8 electrons. But there can be a total of 32 electrons in the fourth electron layer; the krypton atom still has unfilled 4d and 4f sublevels. For elements of the fifth period, sublevels are being filled in the following order: 5s - 4d - 5p. And there are also exceptions related to “ failure» electrons, y 41 Nb, 42 Mo, 44 Ru, 45 Rh, 46 Pd, 47 Ag.
In the sixth and seventh periods, f-elements appear, i.e., elements in which the 4f- and 5f-sublevels of the third outside electronic layer are filled, respectively.
4f elements are called lanthanides.
5f elements are called actinides.
The order of filling electronic sublevels in the atoms of elements of the sixth period: 55 Cs and 56 Ba - 6s elements; 57 La ... 6s 2 5d x - 5d element; 58 Ce - 71 Lu - 4f elements; 72 Hf - 80 Hg - 5d elements; 81 T1 - 86 Rn - 6d elements. But here, too, there are elements in which the order of filling the electronic orbitals is “violated,” which, for example, is associated with the greater energy stability of half and fully filled f-sublevels, i.e. nf 7 and nf 14. Depending on which sublevel of the atom is filled with electrons last, all elements are divided into four electron families, or blocks:
- s-elements. The s-sublevel of the outer level of the atom is filled with electrons; s-elements include hydrogen, helium and elements of the main subgroups of groups I and II.
- p-elements. The p-sublevel of the outer level of the atom is filled with electrons; p-elements include elements of the main subgroups of groups III-VIII.
- d-elements. The d-sublevel of the pre-external level of the atom is filled with electrons; d-elements include elements of secondary subgroups of groups I-VIII, i.e. elements of plug-in decades of large periods located between s- and p-elements. They are also called transition elements.
- f-elements. The f-sublevel of the third outer level of the atom is filled with electrons; these include lanthanides and antinoids.
The Swiss physicist W. Pauli in 1925 established that in an atom in one orbital there can be no more than two electrons having opposite (antiparallel) spins (translated from English as “spindle”), i.e., having such properties that conditionally can be imagined as the rotation of an electron around its imaginary axis: clockwise or counterclockwise.
This principle is called Pauli principle. If there is one electron in the orbital, then it is called unpaired; if there are two, then these are paired electrons, i.e. electrons with opposite spins. The figure shows a diagram of the division of energy levels into sublevels and the order in which they are filled.
Very often, the structure of the electronic shells of atoms is depicted using energy or quantum cells - so-called graphical electronic formulas are written. For this notation, the following notation is used: each quantum cell is designated by a cell that corresponds to one orbital; Each electron is indicated by an arrow corresponding to the spin direction. When writing a graphical electronic formula, you should remember two rules: Pauli's principle and F. Hund's rule, according to which electrons occupy free cells first one at a time and have the same spin value, and only then pair, but the spins, according to the Pauli principle, will already be oppositely directed.
Hund's rule and Pauli's principle
Hund's rule- a rule of quantum chemistry that determines the order of filling the orbitals of a certain sublayer and is formulated as follows: the total value of the spin quantum number of electrons of a given sublayer must be maximum. Formulated by Friedrich Hund in 1925.
This means that in each of the orbitals of the sublayer, one electron is filled first, and only after the unfilled orbitals are exhausted, a second electron is added to this orbital. In this case, in one orbital there are two electrons with half-integer spins of the opposite sign, which pair (form a two-electron cloud) and, as a result, the total spin of the orbital becomes equal to zero.
Another wording: Lower in energy lies the atomic term for which two conditions are satisfied.
- Multiplicity is maximum
- When the multiplicities coincide, the total orbital momentum L is maximum.
Let us analyze this rule using the example of filling p-sublevel orbitals p-elements of the second period (that is, from boron to neon (in the diagram below, horizontal lines indicate orbitals, vertical arrows indicate electrons, and the direction of the arrow indicates the spin orientation).
Klechkovsky's rule
Klechkovsky's rule - as the total number of electrons in atoms increases (with an increase in the charges of their nuclei, or the serial numbers of chemical elements), atomic orbitals are populated in such a way that the appearance of electrons in an orbital with a higher energy depends only on the main quantum number n and does not depend on all other quantum numbers numbers, including from l. Physically, this means that in a hydrogen-like atom (in the absence of interelectron repulsion), the orbital energy of an electron is determined only by the spatial distance of the electron charge density from the nucleus and does not depend on the characteristics of its motion in the field of the nucleus.
The empirical Klechkovsky rule and the ordering scheme that follows from it are somewhat contradictory to the real energy sequence of atomic orbitals only in two similar cases: for atoms Cr, Cu, Nb, Mo, Ru, Rh, Pd, Ag, Pt, Au, there is a “failure” of an electron with s -sublevel of the outer layer is replaced by the d-sublevel of the previous layer, which leads to an energetically more stable state of the atom, namely: after filling orbital 6 with two electrons s
DEFINITION
Hydrogen- the first element of the Periodic Table. Designation - H. Located in the first period, group I, subgroup A.
Refers to non-metals. The charge of the nucleus is 1. The atomic weight can vary: 1, 2, 3, which is due to the presence of deuterium and tritium isotopes.
Electronic structure of the hydrogen atom
A hydrogen atom has a positively charged nucleus (+1), 1 proton and one electron. Since hydrogen has the simplest atomic structure of all the elements in the Periodic Table, it is well studied. In 1913, Niels Bohr proposed a scheme for the structure of the hydrogen atom, according to which the positively charged nucleus is located in the center, and an electron moves around it in a single orbital (Fig. 1). In accordance with this scheme, he derived the emission spectrum of this chemical element. Which was later proven using quantum mechanical calculations of the Schrödinger equation (1925-1930).
Rice. 1. Scheme of the structure of the hydrogen atom.
The electronic configuration of a hydrogen atom will look like this:
Hydrogen belongs to the s-element family. The energy diagram of the hydrogen atom looks like this:
The only electron that hydrogen has is a valence one, because participates in the formation of chemical bonds. As a result of interaction, hydrogen can either lose an electron, i.e. to be its donor, and to accept it, i.e. be an acceptor. In these cases, the atom turns into either a positively or negatively charged ion (H + /H -):
H 0 +e →H — .
Examples of problem solving
EXAMPLE 1
EXAMPLE 2
| Exercise | List the number of protons and neutrons contained in the nuclei of nitrogen (atomic number 14), silicon (atomic number 28), and barium (atomic number 137). |
| Solution | The number of protons in the nucleus of an atom of a chemical element is determined by its serial number in the Periodic Table, and the number of neutrons is the difference between the mass number (M) and the charge of the nucleus (Z). Nitrogen: n(N)= M -Z = 14-7 = 7. Silicon: n(Si)= M -Z = 28-14 = 14. Barium: n (Ba)= M -Z = 137-56 = 81. |
| Answer | The number of protons in the nitrogen nucleus is 7, neutrons - 7; in the nucleus of a silicon atom there are 14 protons and 14 neutrons; In the nucleus of a barium atom there are 56 protons and 81 neutrons. |
