The structure of the electron shells of atoms. Electron shell of an atom Eight electrons

We found out that the heart of an atom is its nucleus. It has electrons around it. They cannot be stationary, as they would immediately fall onto the core.

At the beginning of the XX century. The planetary model of the structure of the atom was adopted, according to which electrons move around a very small positive nucleus, just as the planets revolve around the Sun. Further research showed that the structure of the atom is much more complicated. The problem of the structure of the atom remains relevant for modern science.

Elementary particles, an atom, a molecule - all these are objects of the microworld, which is not observed by us. It has different laws than in the macrocosm, whose objects we can observe either directly or with the help of instruments (microscope, telescope, etc.). Therefore, discussing further the structure of the electron shells of atoms, we will understand that we create our own representation (model), which largely corresponds to modern views, although it is not exactly the same as that of a chemist. Our model is simplified.

Electrons, moving around the nucleus of an atom, together form its electron shell. The number of electrons in the shell of an atom is equal, as you already know, to the number of protons in the nucleus of an atom, it corresponds to the ordinal, or atomic, number of the element in the table of D. I. Mendeleev. So, the electron shell of the hydrogen atom consists of one electron, chlorine - seventeen, gold - seventy-nine.

How do electrons move? Chaotically, like midges around a burning light bulb? Or in some particular order? It turns out that in a certain order.

The electrons in an atom differ in their energy. As experiments show, some of them are attracted to the nucleus more strongly, others - weaker. The main reason for this is the different removal of electrons from the nucleus of an atom. The closer the electrons are to the nucleus, the stronger they are bound to it and the more difficult it is to pull them out of the electron shell, but the farther they are from the nuclei, the easier it is to tear them off. It is obvious that as the distance from the nucleus of the atom increases, the energy reserve of the electron (E) increases (Fig. 38).

Rice. 38.
The maximum number of electrons in the energy level

Electrons moving near the nucleus, as it were, block (shield) the nucleus from other electrons, which are attracted to the nucleus weaker and move at a greater distance from it. This is how electron layers are formed in the electron shell of an atom. Each electron layer consists of electrons with similar energy values,

Therefore, the electronic layers are also called energy levels. Further, we will say so: "The electron is at a certain energy level."

The number of energy levels filled with electrons in an atom is equal to the number of the period in the table of D. I. Mendeleev, in which the chemical element is located. This means that the electron shell of atoms of the 1st period contains one energy level, the 2nd period - two, the 3rd - three, etc. For example, in a nitrogen atom it consists of two energy levels, and in a magnesium atom - of three :

The maximum (largest) number of electrons in the energy level can be determined by the formula: 2n 2 , where n is the number of the level. Therefore, the first energy level is filled when there are two electrons on it (2 × 1 2 = 2); the second - in the presence of eight electrons (2 × 2 2 \u003d 8); the third - eighteen (2 × 3 2 \u003d 18), etc. In the chemistry course of grades 8-9, we will consider elements of only the first three periods, therefore we will not meet with the completed third energy level in atoms.

The number of electrons in the outer energy level of the electron shell of the atom for the chemical elements of the main subgroups is equal to the group number.

Now we can draw up diagrams of the structure of the electron shells of atoms, guided by the plan:

determine the total number of electrons on the shell by the element's serial number;
determine the number of energy levels filled with electrons in the electron shell by the number of the period;
determine the number of electrons at each energy level (at the 1st - no more than two; at the 2nd - no more than eight; at the outer level, the number of electrons is equal to the group number - for elements of the main subgroups).

The nucleus of a hydrogen atom has a charge of +1, i.e. it contains only one proton, respectively, only one electron at a single energy level:

This is written using the electronic formula as follows:

The next element of the 1st period is helium. The nucleus of a helium atom has a charge of +2. It already has two electrons at the first energy level:

At the first energy level, only two electrons can fit and no more - it is completely completed. That is why the 1st period of the table of D. I. Mendeleev consists of two elements.

The lithium atom, an element of the 2nd period, has another energy level, to which the third electron will “go”:

In the beryllium atom, one more electron "enters" the second level:

The boron atom has three electrons on the outer level, and the carbon atom has four electrons... the fluorine atom has seven electrons, the neon atom has eight electrons:

The second level can only hold eight electrons and is therefore complete for neon.

The sodium atom, an element of the 3rd period, has a third energy level (note that the atom of the 3rd period element contains three energy levels!), And it has one electron:

Please note: sodium is an element of group I, it has one electron at the external energy level!

Obviously, it will not be difficult to write down the structure of the energy levels for the sulfur atom, the VIA element of group 3 of the 3rd period:

Completes the 3rd period argon:

The atoms of the elements of the 4th period, of course, have a fourth level, in which the potassium atom has one electron, and the calcium atom has two electrons.

Now that we have become acquainted with simplified ideas about the structure of atoms of elements of the 1st and 2nd periods of the Periodic system of D. I. Mendeleev, we can make refinements that bring us closer to a more correct view of the structure of the atom.

Let's start with an analogy. Just as a fast moving needle of a sewing machine, piercing a fabric, embroiders a pattern on it, so an electron moving immeasurably faster in space around an atomic nucleus “embroiders”, only not a flat, but a three-dimensional pattern of an electron cloud. Since the speed of the electron is hundreds of thousands of times greater than the speed of the sewing needle, they talk about the probability of finding an electron in a particular place in space. Let's assume that we managed, as in a sports photo finish, to establish the position of an electron in some place near the nucleus and mark this position with a dot. If such a “photo-finish” is done hundreds, thousands of times, then an electron cloud model will be obtained.

Sometimes electron clouds are called orbitals. We will do the same. Depending on the energy, electron clouds, or orbitals, differ in size. It is clear that the smaller the energy reserve of an electron, the stronger it is attracted to the nucleus and the smaller its orbital.

Electron clouds (orbitals) can have different shapes. Each energy level in an atom begins with an s-orbital, which has a spherical shape. At the second and subsequent levels, dumbbell-shaped p-orbitals appear after one s-orbital (Fig. 39). There are three such orbitals. Each orbital is occupied by no more than two electrons. Therefore, there can be only two of them on the s-orbital, and six on three p-orbitals.

Rice. 39.
Shapes of s- and p-orbitals (electron clouds)

By using Arabic numerals for the level, and designating the orbitals with the letters s and p, and the number of electrons in a given orbital with an Arabic numeral at the top right above the letter, we can represent the structure of atoms with more complete electronic formulas.

Let's write down the electronic formulas of atoms of the 1st and 2nd periods:

If the elements have external energy levels similar in structure, then the properties of these elements are similar. For example, argon and neon contain eight electrons at the outer level, and therefore they are inert, that is, they almost do not enter into chemical reactions. In their free form, argon and neon are gases whose molecules are monatomic. The atoms of lithium, sodium and potassium contain one electron at the external level and have similar properties, therefore they are placed in the same group of the Periodic Table of D. I. Mendeleev.

Let's make a generalization: the same structure of external energy levels is periodically repeated, therefore, the properties of chemical elements are periodically repeated. This pattern is reflected in the name of the Periodic system of chemical elements of D. I. Mendeleev.

Keywords and phrases

Electrons in atoms are located at energy levels.
The first energy level can contain only two electrons, the second - eight. Such levels are called complete.
The number of filled energy levels is equal to the number of the period in which the element is located.
The number of electrons at the outer level of an atom of a chemical element is equal to the number of its group (for elements of the main subgroups).
The properties of chemical elements are periodically repeated, since the structure of the external energy levels of their atoms is periodically repeated.

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Questions and tasks

An atom is the smallest particle of matter, consisting of a nucleus and electrons. The structure of the electron shells of atoms is determined by the position of the element in the Periodic system of chemical elements of D. I. Mendeleev.

Electron and electron shell of an atom

An atom, which is generally neutral, consists of a positively charged nucleus and a negatively charged electron shell (electron cloud), while the total positive and negative charges are equal in absolute value. When calculating the relative atomic mass, the mass of electrons is not taken into account, since it is negligible and 1840 times less than the mass of a proton or neutron.

Rice. 1. Atom.

An electron is a completely unique particle that has a dual nature: it has both the properties of a wave and a particle. They are constantly moving around the nucleus.

The space around the nucleus, where the probability of finding an electron is most likely, is called the electron orbital, or electron cloud. This space has a specific shape, which is denoted by the letters s-, p-, d-, and f-. The S-electron orbital has a spherical shape, the p-orbital has the shape of a dumbbell or a volume eight, the shapes of the d- and f-orbitals are much more complicated.

Rice. 2. Shapes of electronic orbitals.

Around the nucleus, electrons are located on electron layers. Each layer is characterized by its distance from the nucleus and its energy, which is why the electron layers are often referred to as electronic energy levels. The closer the level is to the nucleus, the lower the energy of the electrons in it. One element differs from another in the number of protons in the nucleus of an atom and, accordingly, in the number of electrons. Therefore, the number of electrons in the electron shell of a neutral atom is equal to the number of protons contained in the nucleus of this atom. Each next element has one more proton in the nucleus, and one more electron in the electron shell.

The newly entering electron occupies the orbital with the lowest energy. However, the maximum number of electrons per level is determined by the formula:

where N is the maximum number of electrons and n is the energy level number.

The first level can have only 2 electrons, the second - 8 electrons, the third - 18 electrons, and the fourth level - 32 electrons. At the outer level of an atom, there cannot be more than 8 electrons: as soon as the number of electrons reaches 8, the next level, more distant from the nucleus, begins to fill.

The structure of the electron shells of atoms

Each element is in a certain period. A period is a horizontal set of elements arranged in ascending order of the charge of the nuclei of their atoms, which begins with an alkali metal and ends with an inert gas. The first three periods in the table are small, and the next, starting from the fourth period, are large, consisting of two rows. The number of the period in which the element is located has a physical meaning. It means how many electronic energy levels there are in an atom of any element of a given period. So, the element chlorine Cl is in period 3, that is, its electron shell has three electron layers. Chlorine is in the VII group of the table, and in the main subgroup. The main subgroup is the column within each group that starts with periods 1 or 2.

Thus, the state of the electron shells of the chlorine atom is as follows: the serial number of the chlorine element is 17, which means that the atom has 17 protons in the nucleus, and 17 electrons in the electron shell. At level 1, there can only be 2 electrons, at level 3 - 7 electrons, since chlorine is in the main subgroup of group VII. Then at the 2nd level is: 17-2-7=8 electrons.

Independent work in chemistry The structure of the electron shells of atoms for students in grade 8 with answers. Independent work consists of 4 options, each with 3 tasks.

1 option

	Element	Electronic formula

2. Write the electronic formulas of the elements oxygen and sodium. Specify for each element:

a) the maximum number of electrons in the outer energy level of atoms of any element is equal to the group number,
b) the maximum number of electrons in the second energy level is eight,
c) the total number of electrons in the atoms of any element is equal to the atomic number of the element.

Option 2

1. Fill the table. Determine the element and its electronic formula.

Distribution of electrons by energy levels	Element	Electronic formula

What elements have atoms that have similar properties? Why?

2. Write the electronic formulas of the elements carbon and argon. Specify for each element:

a) the total number of energy levels in an atom,
b) the number of filled energy levels in an atom,
c) the number of electrons in the outer energy level.

3. Choose the correct statements:

a) the number of energy levels in the atoms of elements is equal to the number of the period,
b) the total number of electrons in an atom of a chemical element is equal to the group number,
c) the number of electrons at the outer level of atoms of the elements of one group of the main subgroup is the same.

3 option

1. Fill the table. Determine the element and its electronic formula.

Distribution of electrons by energy levels	Element	Electronic formula

What elements have atoms that have similar properties? Why?

2. Write the electronic formulas for the elements chlorine and boron. Specify for each element:

a) the total number of energy levels in an atom,
b) the number of filled energy levels in an atom,
c) the number of electrons in the outer energy level.

3. Choose the correct statements:

a) atoms of elements of the same period contain the same number of energy levels,
b) the maximum number of electrons per s-orbital is equal to two,
c) atoms of chemical elements with the same number of energy levels have similar properties.

4 option

1. Fill the table. Determine the element and its electronic formula.

Distribution of electrons by energy levels	Element	Electronic formula

What elements have atoms that have similar properties? Why?

2. Write the electronic formulas for the elements aluminum and neon. Specify for each element:

a) the total number of energy levels in an atom,
b) the number of filled energy levels in an atom,
c) the number of electrons in the outer energy level.

3. Choose the correct statements:
a) all energy levels can contain up to eight electrons,
b) isotopes of one chemical element have the same electronic formulas,
c) the maximum number of electrons per R-orbital is six.

Answers independent work in chemistry The structure of the electron shells of atoms
1 option
1.
1) B - 1s 2 2s 2 2p 1
2) H - 1s 1
3) Al - 1s 2 2s 2 2p 6 3s 2 3p 1
B and Al have similar properties, since the atoms of these elements have three electrons at the external energy level.
2.
O - 1s 2 2s 2 2p 4
a) 2,
b) 1,
at 6;
Na - 1s 2 2s 2 2p 6 3s 1 ,
a) 3,
b) 2,
in 1.
3. b, c.
Option 2
1.
1) F - 1s 2 2s 2 2p 5
2) Na - 1s 2 2s 2 2p 6 3s 1
3) Li - 1s 2 2s 1
Na and Li have similar properties, since these elements have one electron each at the external energy level.
2. C - 1s 2 2s 2 2p 2
a) 2,
b) 1,
at 4;
Ar - 1s 2 2s 2 2p 6 3s 2 3p 6
a) 3,
b) 2,
at 8.
3. a, c.
3 option
1.
1) P - 1s 2 2s 2 2p 6 3s 2 3p 3
2) N - 1s 2 2s 2 2p 3
3) Not - 1s 2
P and N have similar properties, since these elements have five electrons at the external energy level.
2. Cl - 1s 2 2s 2 2p 6 3s 2 3p 5
a) 3,
b) 2,
at 7;
B - 1s 2 2s 2 2p 1
a) 2,
b) 1,
in 3.
3. a, b.
4 option
1.
1) Mg - 1s 2 2s 2 2p 6 3s 2
2) C - 1s 2 2s 2 2p 2
3) Be - 1s 2 2s 2
Be and Mg have similar properties, since these elements have two electrons at the external energy level.
2.
Al - 1s 2 2s 2 2p 6 3s 2 3p 1
a) 3,
b) 2,
in 3;
Ne - 1s 2 2s 2 2p 6 ,
a) 2,
b) 2,
at 8.
3. b, c.

The number of electrons in an atom is equal to the charge of the nucleus. The charge of the nucleus is the ordinal number of the element in the Periodic system. Therefore, the atoms of each next chemical element in the Periodic Table have one electron more than the previous one.

When describing the electronic structure of an atom, they indicate how its electrons are distributed over energy levels. Electrons first occupy levels with lower energy, then with higher ones. So, the first energy level is filled first, if there are still electrons, then the second, third, etc. The number of energy levels in atoms is determined by the number of the period in which the chemical element to which the atom belongs is located.

The first energy level can have only two electrons. Therefore, in the first period there are only two chemical elements - hydrogen and helium. When at some level only the maximum possible number of electrons is located, then we say that this level is completed. So the first energy level is completed for all elements except hydrogen.

The elements of the second period gradually fill the second energy level. The second energy level can have a maximum of 8 electrons. Therefore, there are eight chemical elements in the second period.

The third energy level can have a maximum of 18 electrons. However, in the third period this level is external. No outer level can have more than 8 electrons. Therefore, in the third period, the third energy level is filled only up to 8 electrons inclusive, and, consequently, the third period, as well as the second, contains only 8 chemical elements.

In the fourth period, the third energy level is no longer external, therefore, up to 18 electrons are filled, inclusive. For the first two elements of the 4th period (K, Ca), the external energy level is filled. So for potassium, one electron goes to it, and for calcium, 2. Then, for elements from scandium (Sc) to zinc (Zn), the third energy level is filled, and 2 electrons remain on the outer one. After zinc with gallium (Ga), the fourth energy level is again filled up to 8 electrons in krypton (Kr).

In general, the maximum number of electrons in each energy level is determined by the formula 2n2, where n is the number of the level. So, if the level is the second, then 2 * 2 2 = 8, and if the 3rd, then 2 * 3 2 = 18.

Electrons with the highest energy determine the chemical properties of atoms, and are called valence. In the main subgroups, the electrons of the outer level are valence, and their number is determined by the group number. That is why the properties of the elements of one subgroup are similar.

The properties of atoms depend on the number of valence electrons. Metals have few of them, while non-metals have a lot of them.

Atoms, originally considered indivisible, are complex systems.

An atom consists of a nucleus and an electron shell

Electron shell - a set of electrons moving around the nucleus

The nuclei of atoms are positively charged, they consist of protons (positively charged particles) p + and neutrons (having no charge) no

The atom as a whole is electrically neutral, the number of electrons e– is equal to the number of protons p+, equal to the ordinal number of the element in the periodic table.

The figure shows a planetary model of an atom, according to which electrons move in stationary circular orbits. It is very illustrative, but does not reflect the essence, because in reality the laws of the microworld obey classical mechanics, but quantum mechanics, which takes into account the wave properties of the electron.

According to quantum mechanics, an electron in an atom does not move along certain trajectories, but can be in any parts of the nuclear space, however probability its location in different parts of this space is not the same.

The space around the nucleus, in which the probability of finding an electron is large enough, is called an orbital. (not to be confused with an orbit!) or an electron cloud.

That is, the electron does not have the concept of "trajectory", the electrons do not move either in circular orbits or in any other. The biggest difficulty of quantum mechanics lies in the fact that it is impossible to imagine, we are all used to the phenomena of the macrocosm, which obeys classical mechanics, where any moving particle has its own trajectory.

So, the electron has a complex movement, it can be located anywhere in space near the nucleus, but with different probabilities. Let's now consider those parts of space where the probability of finding an electron is high enough - orbitals - their shapes and the sequence of filling orbitals with electrons.

Imagine a three-dimensional coordinate system, in the center of which is the nucleus of an atom.

First, the 1s orbital is filled, it is located closest to the nucleus and has the shape of a sphere.

The designation of any orbital consists of a number and a Latin letter. The number shows the energy level, and the letter shows the shape of the orbital.

The 1s orbital has the lowest energy and the electrons in this orbital have the lowest energy.

This orbital can contain no more than two electrons. The electrons of hydrogen and helium atoms (the first two elements) are in this orbital.

Electronic configuration of hydrogen: 1s 1

Electronic configuration of helium: 1s 2

The superscript shows the number of electrons in that orbital.

The next element is lithium, it has 3 electrons, two of which are located in 1s orbitals, but where is the third electron located?

It occupies the next most energetic orbital, the 2s orbital. It also has the shape of a sphere, but with a larger radius (the 1s orbital is inside the 2s orbital).

The electrons in this orbital have more energy compared to the 1s orbital, because they are located farther from the nucleus. There can also be a maximum of 2 electrons in this orbital.
Electronic configuration of lithium: 1s 2 2s 1
Electronic configuration of beryllium: 1s 2 2s 2

The next element, boron, already has 5 electrons, and the fifth electron will fill the orbital, which has even more energy - the 2p orbital. P-orbitals have the shape of a dumbbell or figure eight and are located along the coordinate axes perpendicular to each other.

Each p-orbital can hold no more than two electrons, so three p-orbitals can hold no more than six. The valence electrons of the next six elements fill p-orbitals, so they are referred to as p-elements.

The electronic configuration of the boron atom: 1s 2 2s 2 2p 1
The electronic configuration of the carbon atom: 1s 2 2s 2 2p 2
The electronic configuration of the nitrogen atom: 1s 2 2s 2 2p 3
Electronic configuration of the oxygen atom: 1s 2 2s 2 2р 4
The electronic configuration of the fluorine atom: 1s 2 2s 2 2p 5
Electronic configuration of the neon atom: 1s 2 2s 2 2p 6

Graphically, the electronic formulas of these atoms are shown below:

A square is an orbital or a quantum cell, an electron is indicated by an arrow, the direction of the arrow is a special characteristic of the electron's motion - spin (it can be simplified as the rotation of an electron around its axis clockwise and counterclockwise). You need to know that there cannot be two electrons with the same spins on the same orbital (two arrows in the same direction cannot be drawn in one square!). That's what it is W. Pauli exclusion principle: “In an atom there cannot even be two electrons in which all four quantum numbers would be the same”

There is one more rule Gund's rule), along which electrons are settled in orbitals of the same energy, first one by one, and only when each such orbital already contains one electron, the filling of these orbitals with second electrons begins. When an orbital is populated by two electrons, these electrons are called paired.

The neon atom has a completed outer level of eight electrons (2 s-electrons + 6 p-electrons = 8 electrons in the second energy level), this configuration is energetically favorable, and all other atoms strive to acquire it. That is why the elements of group 8 A - the noble gases - are so chemically inert.

The next element is sodium, serial number 11, the first element of the third period, it has one more energy level - the third. The eleventh electron will populate the next highest energy orbital -3s orbital.

The electronic configuration of the sodium atom: 1s 2 2s 2 2p 6 3s 1

Next, the orbitals of the elements of the third period are filled, first the 3s sublevel with two electrons is filled, and then the 3p sublevel with six electrons (similar to the second period) to the noble gas argon, which, like neon, has a completed eight-electron external level. Electronic configuration of the argon atom (18 electrons): 1s 2 2s 2 2p 6 3s 2 3p 6

The fourth period begins with the element potassium (atomic number 19), the last outer electron of which is located in the 4s orbital. The 20th electron of calcium also fills the 4s orbital.

Calcium is followed by a series of 10 d-elements, starting with scandium (atomic number 21) and ending with zinc (atomic number 30). The electrons of these atoms fill 3d orbitals, the appearance of which is shown in the figure below.

So let's sum it up: