The principle of the electronic structure of atoms of chemical elements. The electronic configuration of the atom. The electronic structure of the atom

Chemicals are the things that make up the world around us.

The properties of each chemical substance are divided into two types: these are chemical, which characterize its ability to form other substances, and physical, which are objectively observed and can be considered in isolation from chemical transformations. So, for example, the physical properties of a substance are its state of aggregation (solid, liquid or gaseous), thermal conductivity, heat capacity, solubility in various media (water, alcohol, etc.), density, color, taste, etc.

The transformation of some chemical substances into other substances is called chemical phenomena or chemical reactions. It should be noted that there are also physical phenomena, which, obviously, are accompanied by a change in any physical properties of a substance without its transformation into other substances. Physical phenomena, for example, include the melting of ice, the freezing or evaporation of water, etc.

The fact that during any process a chemical phenomenon takes place can be concluded by observing the characteristic signs of chemical reactions, such as a change in color, the formation of a precipitate, the evolution of gas, the evolution of heat and / or light.

So, for example, a conclusion about the course of chemical reactions can be made by observing:

The formation of sediment when boiling water, called scale in everyday life;

The release of heat and light during the burning of a fire;

Changing the color of a slice of a fresh apple in the air;

The formation of gas bubbles during the fermentation of dough, etc.

The smallest particles of matter, which in the process of chemical reactions practically do not undergo changes, but only in a new way are connected to each other, are called atoms.

The very idea of ​​the existence of such units of matter arose back in ancient Greece in the minds of ancient philosophers, which actually explains the origin of the term "atom", since "atomos" literally translated from Greek means "indivisible".

However, contrary to the idea of ​​the ancient Greek philosophers, atoms are not the absolute minimum of matter, i.e. themselves have a complex structure.

Each atom consists of the so-called subatomic particles - protons, neutrons and electrons, denoted respectively by the symbols p + , n o and e - . The superscript in the notation used indicates that the proton has a unit positive charge, the electron has a unit negative charge, and the neutron has no charge.

As for the qualitative structure of the atom, each atom has all the protons and neutrons concentrated in the so-called nucleus, around which the electrons form an electron shell.

The proton and neutron have practically the same masses, i.e. m p ≈ m n , and the electron mass is almost 2000 times less than the mass of each of them, i.e. m p / m e ≈ m n / m e ≈ 2000.

Since the fundamental property of an atom is its electrical neutrality, and the charge of one electron is equal to the charge of one proton, it can be concluded from this that the number of electrons in any atom is equal to the number of protons.

So, for example, the table below shows the possible composition of atoms:

The type of atoms with the same nuclear charge, i.e. with the same number of protons in their nuclei is called a chemical element. Thus, from the table above, we can conclude that atom1 and atom2 belong to one chemical element, and atom3 and atom4 belong to another chemical element.

Each chemical element has its own name and individual symbol, which is read in a certain way. So, for example, the simplest chemical element, the atoms of which contain only one proton in the nucleus, has the name "hydrogen" and is denoted by the symbol "H", which is read as "ash", and the chemical element with a nuclear charge of +7 (i.e. containing 7 protons) - "nitrogen", has the symbol "N", which is read as "en".

As you can see from the table above, the atoms of one chemical element can differ in the number of neutrons in the nuclei.

Atoms belonging to the same chemical element, but having a different number of neutrons and, as a result, mass, are called isotopes.

So, for example, the chemical element hydrogen has three isotopes - 1 H, 2 H and 3 H. The indices 1, 2 and 3 above the H symbol mean the total number of neutrons and protons. Those. knowing that hydrogen is a chemical element, which is characterized by the fact that there is one proton in the nuclei of its atoms, we can conclude that there are no neutrons at all in the 1 H isotope (1-1 = 0), in the 2 H isotope - 1 neutron (2-1=1) and in the isotope 3 H - two neutrons (3-1=2). Since, as already mentioned, a neutron and a proton have the same masses, and the mass of an electron is negligible compared to them, this means that the 2 H isotope is almost twice as heavy as the 1 H isotope, and the 3 H isotope is even three times as heavy. . In connection with such a large spread in the masses of hydrogen isotopes, the 2 H and 3 H isotopes were even given separate individual names and symbols, which is not typical of any other chemical element. The 2 H isotope was named deuterium and given the symbol D, and the 3 H isotope was given the name tritium and given the symbol T.

If we take the mass of the proton and neutron as unity, and neglect the mass of the electron, in fact, the upper left index, in addition to the total number of protons and neutrons in the atom, can be considered its mass, and therefore this index is called the mass number and is denoted by the symbol A. Since the charge of the nucleus of any protons correspond to the atom, and the charge of each proton is conditionally considered equal to +1, the number of protons in the nucleus is called the charge number (Z). Denoting the number of neutrons in an atom with the letter N, mathematically the relationship between mass number, charge number and number of neutrons can be expressed as:

According to modern concepts, the electron has a dual (particle-wave) nature. It has the properties of both a particle and a wave. Like a particle, an electron has a mass and a charge, but at the same time, the flow of electrons, like a wave, is characterized by the ability to diffraction.

To describe the state of an electron in an atom, the concepts of quantum mechanics are used, according to which the electron does not have a specific trajectory of motion and can be located at any point in space, but with different probabilities.

The region of space around the nucleus where an electron is most likely to be found is called the atomic orbital.

An atomic orbital can have a different shape, size and orientation. An atomic orbital is also called an electron cloud.

Graphically, one atomic orbital is usually denoted as a square cell:

Quantum mechanics has an extremely complex mathematical apparatus, therefore, within the framework of a school chemistry course, only the consequences of quantum mechanical theory are considered.

According to these consequences, any atomic orbital and an electron located on it are completely characterized by 4 quantum numbers.

• The main quantum number - n - determines the total energy of an electron in a given orbital. The range of values ​​of the main quantum number is all natural numbers, i.e. n = 1,2,3,4, 5 etc.
• The orbital quantum number - l - characterizes the shape of the atomic orbital and can take any integer values ​​from 0 to n-1, where n, recall, is the main quantum number.

Orbitals with l = 0 are called s-orbitals. s-orbitals are spherical and do not have a direction in space:

Orbitals with l = 1 are called p-orbitals. These orbitals have the shape of a three-dimensional figure eight, i.e. the shape obtained by rotating the figure eight around the axis of symmetry, and outwardly resemble a dumbbell:

Orbitals with l = 2 are called d-orbitals, and with l = 3 – f-orbitals. Their structure is much more complex.

3) Magnetic quantum number - m l - determines the spatial orientation of a particular atomic orbital and expresses the projection of the orbital angular momentum on the direction of the magnetic field. The magnetic quantum number m l corresponds to the orientation of the orbital relative to the direction of the external magnetic field strength vector and can take any integer values ​​from –l to +l, including 0, i.e. the total number of possible values ​​is (2l+1). So, for example, for l = 0 ml = 0 (one value), for l = 1 ml = -1, 0, +1 (three values), for l = 2 ml = -2, -1, 0, +1 , +2 (five values ​​of the magnetic quantum number), etc.

So, for example, p-orbitals, i.e. orbitals with an orbital quantum number l = 1, having the shape of a “three-dimensional figure eight”, correspond to three values ​​of the magnetic quantum number (-1, 0, +1), which, in turn, corresponds to three directions in space perpendicular to each other.

4) The spin quantum number (or simply spin) - m s - can be conditionally considered responsible for the direction of rotation of an electron in an atom, it can take on values. Electrons with different spins are indicated by vertical arrows pointing in different directions: ↓ and .

The set of all orbitals in an atom that have the same value of the principal quantum number is called the energy level or electron shell. Any arbitrary energy level with some number n consists of n 2 orbitals.

The set of orbitals with the same values ​​of the principal quantum number and the orbital quantum number is an energy sublevel.

Each energy level, which corresponds to the main quantum number n, contains n sublevels. In turn, each energy sublevel with an orbital quantum number l consists of (2l+1) orbitals. Thus, the s-sublayer consists of one s-orbital, the p-sublayer - three p-orbitals, the d-sublayer - five d-orbitals, and the f-sublayer - seven f-orbitals. Since, as already mentioned, one atomic orbital is often denoted by one square cell, the s-, p-, d- and f-sublevels can be graphically depicted as follows:

Each orbital corresponds to an individual strictly defined set of three quantum numbers n, l and m l .

The distribution of electrons in orbitals is called the electronic configuration.

The filling of atomic orbitals with electrons occurs in accordance with three conditions:

• The principle of minimum energy: Electrons fill orbitals starting from the lowest energy sublevel. The sequence of sublevels in order of increasing energy is as follows: 1s<2s<2p<3s<3p<4s≤3d<4p<5s≤4d<5p<6s…;

In order to make it easier to remember this sequence of filling electronic sublevels, the following graphic illustration is very convenient:

• Pauli principle: Each orbital can hold at most two electrons.

If there is one electron in the orbital, then it is called unpaired, and if there are two, then they are called an electron pair.

• Hund's rule: the most stable state of an atom is one in which, within one sublevel, the atom has the maximum possible number of unpaired electrons. This most stable state of the atom is called the ground state.

In fact, the above means that, for example, the placement of the 1st, 2nd, 3rd and 4th electrons on three orbitals of the p-sublevel will be carried out as follows:

The filling of atomic orbitals from hydrogen, which has a charge number of 1, to krypton (Kr) with a charge number of 36, will be carried out as follows:

A similar representation of the order in which atomic orbitals are filled is called an energy diagram. Based on the electronic diagrams of individual elements, you can write down their so-called electronic formulas (configurations). So, for example, an element with 15 protons and, as a result, 15 electrons, i.e. phosphorus (P) will have the following energy diagram:

When translated into an electronic formula, the phosphorus atom will take the form:

15 P = 1s 2 2s 2 2p 6 3s 2 3p 3

Normal-sized digits to the left of the sublevel symbol show the number of the energy level, and superscripts to the right of the sublevel symbol show the number of electrons in the corresponding sublevel.

Below are the electronic formulas of the first 36 elements of D.I. Mendeleev.

As already mentioned, in their ground state, electrons in atomic orbitals are arranged according to the principle of least energy. Nevertheless, in the presence of empty p-orbitals in the ground state of an atom, often, when excess energy is imparted to it, the atom can be transferred to the so-called excited state. So, for example, a boron atom in its ground state has an electronic configuration and an energy diagram of the following form:

And in the excited state (*), i.e. when imparting some energy to the boron atom, its electronic configuration and energy diagram will look like this:

Depending on which sublevel in the atom is filled last, chemical elements are divided into s, p, d or f.

Finding s, p, d and f-elements in the table D.I. Mendeleev:

• s-elements have the last s-sublevel to be filled. These elements include elements of the main (on the left in the table cell) subgroups of groups I and II.
• For p-elements, the p-sublevel is filled. The p-elements include the last six elements of each period, except for the first and seventh, as well as elements of the main subgroups of III-VIII groups.
• d-elements are located between s- and p-elements in large periods.
• The f-elements are called lanthanides and actinides. They are placed at the bottom of the table by D.I. Mendeleev.

Since the nuclei of reacting atoms remain unchanged during chemical reactions (with the exception of radioactive transformations), the chemical properties of atoms depend on the structure of their electron shells. Theory electronic structure of the atom based on the apparatus of quantum mechanics. Thus, the structure of the energy levels of an atom can be obtained on the basis of quantum mechanical calculations of the probabilities of finding electrons in the space around the atomic nucleus ( rice. 4.5).

Rice. 4.5. Scheme of division of energy levels into sublevels

The fundamentals of the theory of the electronic structure of an atom are reduced to the following provisions: the state of each electron in an atom is characterized by four quantum numbers: the main quantum number n = 1, 2, 3,; orbital (azimuth) l=0,1,2, n–1; magnetic m l = –l, –1,0,1,  l; spin m s = -1/2, 1/2 .

According to Pauli principle, in the same atom there cannot be two electrons that have the same set of four quantum numbers n,l,m l , m s; sets of electrons with the same principal quantum numbers n form electron layers, or energy levels of the atom, numbered from the nucleus and denoted as K, L, M, N, O, P, Q,  moreover, in the energy layer with the given value n can be no more than 2n 2 electrons. Sets of electrons with the same quantum numbers n And l,   form sublevels, denoted as they move away from the core as s, p, d, f.

The probabilistic finding of the position of an electron in the space around the atomic nucleus corresponds to the Heisenberg uncertainty principle. According to quantum mechanical concepts, an electron in an atom does not have a specific trajectory of motion and can be located in any part of the space around the nucleus, and its various positions are considered as an electron cloud with a certain negative charge density. The space around the nucleus, in which the electron is most likely to be found, is called orbital. It contains about 90% of the electron cloud. Each sublevel 1s, 2s, 2p etc. corresponds to a certain number of orbitals of a certain shape. For example, 1s- And 2s- Orbitals are spherical and 2p-orbitals ( 2p x , 2p y , 2p z-orbitals) are oriented in mutually perpendicular directions and have the shape of a dumbbell ( rice. 4.6).

Rice. 4.6. Shape and orientation of electron orbitals.

During chemical reactions, the atomic nucleus does not undergo changes, only the electron shells of atoms change, the structure of which explains many properties of chemical elements. Based on the theory of the electronic structure of the atom, the deep physical meaning of Mendeleev's periodic law of chemical elements was established and the theory of chemical bonding was created.

The theoretical substantiation of the periodic system of chemical elements includes data on the structure of the atom, confirming the existence of a relationship between the periodicity of changes in the properties of chemical elements and the periodic repetition of similar types of electronic configurations of their atoms.

In the light of the doctrine of the structure of the atom, Mendeleev's division of all elements into seven periods becomes justified: the number of the period corresponds to the number of energy levels of atoms filled with electrons. In short periods, with an increase in the positive charge of the nuclei of atoms, the number of electrons in the outer level increases (from 1 to 2 in the first period, and from 1 to 8 in the second and third periods), which explains the change in the properties of the elements: at the beginning of the period (except for the first) there is alkali metal, then there is a gradual weakening of the metallic properties and an increase in non-metallic ones. This regularity can be traced for the elements of the second period in table 4.2.

Table 4.2.

In large periods, with an increase in the charge of nuclei, the filling of levels with electrons is more difficult, which explains the more complex change in the properties of elements compared to elements of small periods.

The same nature of the properties of chemical elements in subgroups is explained by the similar structure of the external energy level, as shown in tab. 4.3 illustrating the sequence of electron filling of energy levels for subgroups of alkali metals.

Table 4.3.

The group number, as a rule, indicates the number of electrons in an atom that can participate in the formation of chemical bonds. This is the physical meaning of the group number. In four places in the periodic table, the elements are not in ascending order of atomic masses: Ar And K,co And Ni,Te And I,Th And Pa. These deviations were considered shortcomings of the periodic table of chemical elements. The doctrine of the structure of the atom explained these deviations. Experimental determination of the nuclear charges showed that the arrangement of these elements corresponds to an increase in the charges of their nuclei. In addition, the experimental determination of the charges of atomic nuclei made it possible to determine the number of elements between hydrogen and uranium, as well as the number of lanthanides. Now all places in the periodic system are filled in the interval from Z=1 before Z=114, however, the periodic table is not complete, the discovery of new transuranium elements is possible.

Algorithm for compiling the electronic formula of an element:

1. Determine the number of electrons in an atom using the Periodic Table of Chemical Elements D.I. Mendeleev.

2. By the number of the period in which the element is located, determine the number of energy levels; the number of electrons in the last electronic level corresponds to the group number.

3. Divide the levels into sublevels and orbitals and fill them with electrons in accordance with the rules for filling orbitals:

It must be remembered that the first level has a maximum of 2 electrons. 1s2, on the second - a maximum of 8 (two s and six R: 2s 2 2p 6), on the third - a maximum of 18 (two s, six p, and ten d: 3s 2 3p 6 3d 10).

• Principal quantum number n should be minimal.
• Filled in first s- sublevel, then p-, d-b f- sublevels.
• Electrons fill orbitals in ascending order of orbital energy (Klechkovsky's rule).
• Within the sublevel, electrons first occupy free orbitals one at a time, and only after that they form pairs (Hund's rule).
• There cannot be more than two electrons in one orbital (Pauli principle).

Examples.

1. Compose the electronic formula of nitrogen. Nitrogen is number 7 on the periodic table.

2. Compose the electronic formula of argon. In the periodic table, argon is at number 18.

1s 2 2s 2 2p 6 3s 2 3p 6.

3. Compose the electronic formula of chromium. In the periodic table, chromium is number 24.

1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5

Energy diagram of zinc.

4. Compose the electronic formula of zinc. In the periodic table, zinc is number 30.

1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10

Note that part of the electronic formula, namely 1s 2 2s 2 2p 6 3s 2 3p 6 is the electronic formula of argon.

The electronic formula of zinc can be represented as.

The location of electrons on energy shells or levels is recorded using electronic formulas of chemical elements. Electronic formulas or configurations help to represent the structure of an element's atom.

The structure of the atom

The atoms of all elements consist of a positively charged nucleus and negatively charged electrons that are located around the nucleus.

The electrons are at different energy levels. The farther an electron is from the nucleus, the more energy it has. The size of the energy level is determined by the size of the atomic orbit or orbital cloud. This is the space in which the electron moves.

Rice. 1. The general structure of the atom.

Orbitals can have different geometric configurations:

• s-orbitals- spherical;
• p-, d and f-orbitals- dumbbell-shaped, lying in different planes.

At the first energy level of any atom, there is always an s-orbital with two electrons (an exception is hydrogen). Starting from the second level, the s- and p-orbitals are at the same level.

Rice. 2. s-, p-, d and f-orbitals.

Orbitals exist regardless of the location of electrons on them and can be filled or vacant.

Formula entry

Electronic configurations of atoms of chemical elements are written according to the following principles:

• each energy level corresponds to a serial number, denoted by an Arabic numeral;
• the number is followed by a letter denoting the orbital;
• a superscript is written above the letter, corresponding to the number of electrons in the orbital.

Recording examples:

Electronic configuration of an atom is a formula showing the arrangement of electrons in an atom by levels and sublevels. After studying the article, you will find out where and how electrons are located, get acquainted with quantum numbers and be able to build the electronic configuration of an atom by its number, at the end of the article there is a table of elements.

Why study the electronic configuration of elements?

Atoms are like a constructor: there are a certain number of parts, they differ from each other, but two parts of the same type are exactly the same. But this constructor is much more interesting than the plastic one, and here's why. The configuration changes depending on who is nearby. For example, oxygen next to hydrogen maybe turn into water, next to sodium into gas, and being next to iron completely turns it into rust. To answer the question why this happens and to predict the behavior of an atom next to another, it is necessary to study the electronic configuration, which will be discussed below.

How many electrons are in an atom?

An atom consists of a nucleus and electrons revolving around it, the nucleus consists of protons and neutrons. In the neutral state, each atom has the same number of electrons as the number of protons in its nucleus. The number of protons was designated by the serial number of the element, for example, sulfur has 16 protons - the 16th element of the periodic system. Gold has 79 protons - the 79th element of the periodic table. Accordingly, there are 16 electrons in sulfur in the neutral state, and 79 electrons in gold.

Where to look for an electron?

Observing the behavior of an electron, certain patterns were derived, they are described by quantum numbers, there are four of them in total:

• Principal quantum number
• Orbital quantum number
• Magnetic quantum number
• Spin quantum number

Orbital

Further, instead of the word orbit, we will use the term "orbital", the orbital is the wave function of the electron, roughly - this is the area in which the electron spends 90% of the time.

N - level
L - shell
M l - orbital number
M s - the first or second electron in the orbital

Orbital quantum number l

As a result of the study of the electron cloud, it was found that depending on the level of energy, the cloud takes four main forms: a ball, dumbbells and the other two, more complex. In order of increasing energy, these forms are called s-,p-,d- and f-shell. Each of these shells can have 1 (on s), 3 (on p), 5 (on d) and 7 (on f) orbitals. The orbital quantum number is the shell on which the orbitals are located. The orbital quantum number for s, p, d and f orbitals, respectively, takes the values ​​0,1,2 or 3.

On the s-shell one orbital (L=0) - two electrons
There are three orbitals on the p-shell (L=1) - six electrons
There are five orbitals on the d-shell (L=2) - ten electrons
There are seven orbitals (L=3) on the f-shell - fourteen electrons

Magnetic quantum number m l

There are three orbitals on the p-shell, they are denoted by numbers from -L to +L, that is, for the p-shell (L=1) there are orbitals "-1", "0" and "1". The magnetic quantum number is denoted by the letter m l .

Inside the shell, it is easier for electrons to be located in different orbitals, so the first electrons fill one for each orbital, and then its pair is added to each.

Consider a d-shell:
The d-shell corresponds to the value L=2, that is, five orbitals (-2,-1,0,1 and 2), the first five electrons fill the shell, taking the values ​​M l =-2,M l =-1,M l =0 , M l =1, M l =2.

Spin quantum number m s

Spin is the direction of rotation of an electron around its axis, there are two directions, so the spin quantum number has two values: +1/2 and -1/2. Only two electrons with opposite spins can be on the same energy sublevel. The spin quantum number is denoted m s

Principal quantum number n

The main quantum number is the energy level, at the moment seven energy levels are known, each is denoted by an Arabic numeral: 1,2,3,...7. The number of shells at each level is equal to the level number: there is one shell on the first level, two on the second, and so on.

Electron number

So, any electron can be described by four quantum numbers, the combination of these numbers is unique for each position of the electron, let's take the first electron, the lowest energy level is N=1, one shell is located on the first level, the first shell at any level has the shape of a ball (s -shell), i.e. L=0, the magnetic quantum number can take only one value, M l =0 and the spin will be equal to +1/2. If we take the fifth electron (in whatever atom it is), then the main quantum numbers for it will be: N=2, L=1, M=-1, spin 1/2.