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 color change, precipitation, gas evolution, heat and / or light evolution.
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 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 assigned separate individual names and symbols, which is not typical for 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 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, with l = 0 m l = 0 (one value), with l = 1 m l = -1, 0, +1 (three values), with l = 2 m l = -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 numbers 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.
| period | Item No. | symbol | title | electronic formula |
| I | 1 | H | hydrogen | 1s 1 |
| 2 | He | helium | 1s2 | |
| II | 3 | Li | lithium | 1s2 2s1 |
| 4 | Be | beryllium | 1s2 2s2 | |
| 5 | B | boron | 1s 2 2s 2 2p 1 | |
| 6 | C | carbon | 1s 2 2s 2 2p 2 | |
| 7 | N | nitrogen | 1s 2 2s 2 2p 3 | |
| 8 | O | oxygen | 1s 2 2s 2 2p 4 | |
| 9 | F | fluorine | 1s 2 2s 2 2p 5 | |
| 10 | Ne | neon | 1s 2 2s 2 2p 6 | |
| III | 11 | Na | sodium | 1s 2 2s 2 2p 6 3s 1 |
| 12 | mg | magnesium | 1s 2 2s 2 2p 6 3s 2 | |
| 13 | Al | aluminum | 1s 2 2s 2 2p 6 3s 2 3p 1 | |
| 14 | Si | silicon | 1s 2 2s 2 2p 6 3s 2 3p 2 | |
| 15 | P | phosphorus | 1s 2 2s 2 2p 6 3s 2 3p 3 | |
| 16 | S | sulfur | 1s 2 2s 2 2p 6 3s 2 3p 4 | |
| 17 | Cl | chlorine | 1s 2 2s 2 2p 6 3s 2 3p 5 | |
| 18 | Ar | argon | 1s 2 2s 2 2p 6 3s 2 3p 6 | |
| IV | 19 | K | potassium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 |
| 20 | Ca | calcium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 | |
| 21 | sc | scandium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1 | |
| 22 | Ti | titanium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 2 | |
| 23 | V | vanadium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 3 | |
| 24 | Cr | chromium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 s on the d sublevel | |
| 25 | Mn | manganese | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 5 | |
| 26 | Fe | iron | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 | |
| 27 | co | cobalt | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 7 | |
| 28 | Ni | nickel | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 8 | |
| 29 | Cu | copper | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 10 s on the d sublevel | |
| 30 | Zn | zinc | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 | |
| 31 | Ga | gallium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 1 | |
| 32 | Ge | germanium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 2 | |
| 33 | As | arsenic | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 3 | |
| 34 | Se | selenium | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 4 | |
| 35 | Br | bromine | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 5 | |
| 36 | kr | krypton | 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 |
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:
5 B = 1s 2 2s 2 2p 1
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:
5 B* = 1s 2 2s 1 2p 2
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.
Atom(from the Greek atomos - indivisible) - a single-nuclear, chemically indivisible particle of a chemical element, a carrier of the properties of a substance. Substances are made up of atoms. The atom itself consists of a positively charged nucleus and a negatively charged electron cloud. In general, the atom is electrically neutral. The size of an atom is completely determined by the size of its electron cloud, since the size of the nucleus is negligible compared to the size of the electron cloud. The core is made up of Z positively charged protons (proton charge corresponds to +1 in arbitrary units) and N neutrons that do not carry a charge (the number of neutrons can be equal to or slightly more or less than protons). Protons and neutrons are called nucleons, that is, particles of the nucleus. Thus, the charge of the nucleus is determined only by the number of protons and is equal to the serial number of the element in the periodic table. The positive charge of the nucleus is compensated by negatively charged electrons (electron charge -1 in arbitrary units), which form an electron cloud. The number of electrons is equal to the number of protons. The masses of protons and neutrons are equal (1 and 1 amu, respectively). The mass of an atom is mainly determined by the mass of its nucleus, since the mass of an electron is about 1836 times less than the mass of a proton and a neutron and is rarely taken into account in calculations. The exact number of neutrons can be found by the difference between the mass of an atom and the number of protons ( N=A-Z). The type of atoms of any chemical element with a nucleus consisting of a strictly defined number of protons (Z) and neutrons (N) is called a nuclide (these can be either different elements with the same total number of nucleons (isobars) or neutrons (isotones), or one chemical element - one number of protons, but a different number of neutrons (isomers)).
Since almost the entire mass is concentrated in the nucleus of the atom, but its dimensions are negligible compared to the total volume of the atom, the nucleus is conditionally taken as a material point resting in the center of the atom, and the atom itself is considered as a system of electrons. In a chemical reaction, the nucleus of an atom is not affected (except for nuclear reactions), as are the internal electronic levels, but only the electrons of the outer electron shell are involved. For this reason, it is necessary to know the properties of an electron and the rules for the formation of electron shells of atoms.
Electron properties
Before studying the properties of the electron and the rules for the formation of electronic levels, it is necessary to touch upon the history of the formation of ideas about the structure of the atom. We will not consider the full history of the formation of the atomic structure, but will dwell only on the most relevant and most "correct" ideas that can most clearly show how the electrons are located in the atom. The presence of atoms as elementary constituents of matter was first suggested by the ancient Greek philosophers (if you start to divide any body in half, half in half, and so on, then this process will not be able to continue indefinitely; we will stop at a particle that we can no longer divide - this and there will be an atom). After that, the history of the structure of the atom went through a difficult path and different ideas, such as the indivisibility of the atom, the Thomson model of the atom, and others. The model of the atom proposed by Ernest Rutherford in 1911 turned out to be the closest. He compared the atom to the solar system, where the nucleus of the atom acted as the sun, and the electrons moved around it like planets. Placing electrons in stationary orbits was a very important step in understanding the structure of the atom. However, such a planetary model of the structure of the atom was in conflict with classical mechanics. The fact is that when an electron moved in orbit, it had to lose potential energy and eventually "fall" onto the nucleus, and the atom had to cease to exist. Such a paradox was eliminated by the introduction of postulates by Niels Bohr. According to these postulates, the electron moved in stationary orbits around the nucleus and under normal conditions did not absorb or emit energy. The postulates show that the laws of classical mechanics are not suitable for describing the atom. This model of the atom is called the Bohr-Rutherford model. The continuation of the planetary structure of the atom is the quantum mechanical model of the atom, according to which we will consider the electron.
An electron is a quasi-particle, showing wave-particle duality: it is both a particle (corpuscle) and a wave at the same time. The properties of a particle include the mass of an electron and its charge, and the wave properties - the ability to diffraction and interference. The connection between the wave and corpuscular properties of an electron is reflected in the de Broglie equation:
λ = h m v , (\displaystyle \lambda =(\frac (h)(mv)),)where λ (\displaystyle \lambda )
- wavelength, - particle mass, - particle velocity, - Planck's constant = 6.63 10 -34 J s.
For an electron, it is impossible to calculate the trajectory of its movement, we can only talk about the probability of finding an electron in one place or another around the nucleus. For this reason, they are not talking about the orbits of the electron around the nucleus, but about the orbitals - the space around the nucleus, in which probability finding an electron exceeds 95%. For an electron, it is impossible to accurately measure both the coordinate and the velocity at the same time (Heisenberg's uncertainty principle).
Δ x ∗ m ∗ Δ v > ℏ 2 (\displaystyle \Delta x*m*\Delta v>(\frac (\hbar )(2)))where ∆ x (\displaystyle \Delta x)
- uncertainty of the electron coordinate, ∆ v (\displaystyle \Delta v)
- speed measurement error, ħ=h/2π=1.05 10 -34 J s
The more accurately we measure the electron's coordinate, the greater the error in measuring its velocity, and vice versa: the more accurately we know the electron's velocity, the greater the uncertainty in its coordinate.
The presence of wave properties of an electron allows us to apply the Schrödinger wave equation to it.
∂ 2 Ψ ∂ x 2 + ∂ 2 Ψ ∂ y 2 + ∂ 2 Ψ ∂ z 2 + 8 π 2 m h (E − V) Ψ = 0 (\displaystyle (\frac ((\partial )^(2)\Psi )(\partial x^(2)))+(\frac ((\partial )^(2)\Psi )(\partial y^(2)))+(\frac ((\partial )^(2) \Psi )(\partial z^(2)))+(\frac (8(\pi ^(2))m)(h))\left(E-V\right)\Psi =0)
where is the total energy of the electron, the potential energy of the electron, the physical meaning of the function Ψ (\displaystyle \psi )
- square root of the probability of finding an electron in space with coordinates x, y and z(the kernel is considered the origin).
The presented equation is written for a one-electron system. For systems containing more than one electron, the principle of description remains the same, but the equation takes on a more complex form. The graphical solution of the Schrödinger equation is the geometry of atomic orbitals. So, the s-orbital has the shape of a ball, the p-orbital has the shape of a figure-eight with a "knot" at the origin (on the nucleus, where the probability of finding an electron tends to zero).
In the framework of modern quantum mechanical theory, an electron is described by a set of quantum numbers: n
, l
, m l
, s
and m s
. According to the Pauli principle, one atom cannot have two electrons with a completely identical set of all quantum numbers.
Principal quantum number n
determines the energy level of an electron, that is, at what electronic level the given electron is located. The principal quantum number can only take integer values greater than 0: n
=1;2;3... Maximum value n
for a particular atom of an element corresponds to the number of the period in which the element is located in the periodic table of D. I. Mendeleev.
Orbital (additional) quantum number l
determines the geometry of the electron cloud. Can take integer values from 0 to n
-one. For the values of the additional quantum number l
letter designation is used:
| meaning l | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|
| letter designation | s | p | d | f | g |
S-orbital is spherical, p-orbital is figure-eight. The remaining orbitals have a very complex structure, such as the d-orbital shown in the figure.
Electrons in levels and orbitals are not arranged randomly, but according to the Klechkovsky rule, according to which the filling of electrons occurs according to the principle of least energy, that is, in ascending order of the sum of the principal and orbital quantum numbers n +l . In the case when the sum for the two filling options is the same, the lowest energy level is initially filled (for example: when n =3 a l =2 and n =4 a l =1 will initially fill level 3). Magnetic quantum number m l determines the location of the orbital in space and can take an integer value from -l before +l , including 0. Only one value is possible for the s-orbital m l =0. For the p-orbital, there are already three values -1, 0 and +1, that is, the p-orbital can be located along three coordinate axes x, y and z.
arrangement of orbitals depending on the value m l
The electron has its own angular momentum - the spin, denoted by the quantum number s . The electron spin is a constant value and equal to 1/2. The phenomenon of spin can be conditionally represented as a movement around its own axis. Initially, the electron spin was equated with the planet's motion around its own axis, but such a comparison is erroneous. Spin is a purely quantum phenomenon that has no analogues in classical mechanics.
Atom- the smallest particle of a substance that is chemically indivisible. In the 20th century, the complex structure of the atom was elucidated. Atoms are made up of positively charged nuclei and a shell formed by negatively charged electrons. The total charge of a free atom is zero, since the charges of the nucleus and electron shell balance each other. In this case, the charge of the nucleus is equal to the number of the element in the periodic table ( atomic number) and is equal to the total number of electrons (electron charge is −1).
The atomic nucleus is made up of positively charged protons and neutral particles - neutrons that have no charge. The generalized characteristics of elementary particles in the composition of an atom can be presented in the form of a table:
The number of protons is equal to the charge of the nucleus, therefore, equal to the atomic number. To find the number of neutrons in an atom, it is necessary to subtract the nuclear charge (the number of protons) from the atomic mass (the sum of the masses of protons and neutrons).
For example, in the sodium atom 23 Na, the number of protons is p = 11, and the number of neutrons is n = 23 − 11 = 12
The number of neutrons in atoms of the same element can be different. Such atoms are called isotopes .
The electron shell of the atom also has a complex structure. Electrons are located on energy levels (electronic layers).
The level number characterizes the electron energy. This is due to the fact that elementary particles can transmit and receive energy not in arbitrarily small quantities, but in certain portions - quanta. The higher the level, the more energy the electron has. Since the lower the energy of the system, the more stable it is (compare the low stability of a stone on top of a mountain with high potential energy, and the stable position of the same stone on the plain below, when its energy is much lower), the levels with low electron energy are filled first and only then - high.
The maximum number of electrons that a level can hold can be calculated using the formula:
N \u003d 2n 2, where N is the maximum number of electrons in the level,
n - level number.
Then for the first level N = 2 1 2 = 2,
for the second N = 2 2 2 = 8, etc.
The number of electrons at the outer level for the elements of the main (A) subgroups is equal to the group number.
In most modern periodic tables, the arrangement of electrons by levels is indicated in the cell with the element. Very important understand that the levels are read upwards, which corresponds to their energy. Therefore, a column of numbers in a cell with sodium:
1
8
2
at the 1st level - 2 electrons,
at the 2nd level - 8 electrons,
at the 3rd level - 1 electron
Be careful, a very common mistake!
The distribution of electrons over levels can be represented as a diagram:
11 Na)))
2 8 1
If the periodic table does not indicate the distribution of electrons by levels, you can be guided by:
- the maximum number of electrons: at the 1st level, no more than 2 e - ,
on the 2nd - 8 e - ,
at the external level - 8 e − ; - the number of electrons in the outer level (for the first 20 elements, it is the same as the group number)
Then for sodium the course of reasoning will be as follows:
- The total number of electrons is 11, therefore, the first level is filled and contains 2 e − ;
- The third, outer level contains 1 e − (I group)
- The second level contains the remaining electrons: 11 − (2 + 1) = 8 (completely filled)
* For a clearer distinction between a free atom and an atom in a compound, a number of authors propose using the term "atom" only to refer to a free (neutral) atom, and to refer to all atoms, including those in compounds, they propose the term "atomic particles". Time will tell how the fate of these terms will turn out. From our point of view, an atom, by definition, is a particle, therefore, the expression "atomic particles" can be considered as a tautology ("butter oil").
2. Task. Calculation of the amount of substance of one of the reaction products, if the mass of the starting substance is known.
Example:
What amount of hydrogen substance will be released during the interaction of zinc with hydrochloric acid weighing 146 g?
Solution:
- We write the reaction equation: Zn + 2HCl \u003d ZnCl 2 + H 2
- Find the molar mass of hydrochloric acid: M (HCl) \u003d 1 + 35.5 \u003d 36.5 (g / mol)
(we look at the molar mass of each element, numerically equal to the relative atomic mass, in the periodic table under the sign of the element and round it up to integers, except for chlorine, which is taken as 35.5) - Find the amount of hydrochloric acid substance: n (HCl) \u003d m / M \u003d 146 g / 36.5 g / mol \u003d 4 mol
- We write the available data above the reaction equation, and under the equation - the number of moles according to the equation (equal to the coefficient in front of the substance):
4 mol x mol
Zn + 2HCl \u003d ZnCl 2 + H 2
2 mol 1 mol - We make a proportion:
4 mol - x mole
2 mol - 1 mol
(or with explanation:
from 4 moles of hydrochloric acid you get x mole of hydrogen
and out of 2 mol - 1 mol) - We find x:
x= 4 mol 1 mol / 2 mol = 2 mol
Answer: 2 mol.
The concept of an atom arose in the ancient world to designate the particles of matter. In Greek, atom means "indivisible".
Electrons
The Irish physicist Stoney, on the basis of experiments, came to the conclusion that electricity is carried by the smallest particles that exist in the atoms of all chemical elements. In $1891$, Stoney proposed to call these particles electrons, which in Greek means "amber".
A few years after the electron got its name, English physicist Joseph Thomson and French physicist Jean Perrin proved that electrons carry a negative charge. This is the smallest negative charge, which in chemistry is taken as the unit $(–1)$. Thomson even managed to determine the speed of the electron (it is equal to the speed of light - $300,000$ km/s) and the mass of the electron (it is $1836$ times less than the mass of the hydrogen atom).
Thomson and Perrin connected the poles of a current source with two metal plates - a cathode and an anode, soldered into a glass tube, from which air was evacuated. When a voltage of about 10 thousand volts was applied to the electrode plates, a luminous discharge flashed in the tube, and particles flew from the cathode (negative pole) to the anode (positive pole), which scientists first called cathode rays, and then found out that it was a stream of electrons. Electrons, hitting special substances applied, for example, to a TV screen, cause a glow.
The conclusion was made: electrons escape from the atoms of the material from which the cathode is made.
Free electrons or their flux can also be obtained in other ways, for example, by heating a metal wire or by falling light on metals formed by elements of the main subgroup of group I of the periodic table (for example, cesium).
The state of electrons in an atom
The state of an electron in an atom is understood as a set of information about energy specific electron in space in which it is located. We already know that an electron in an atom does not have a trajectory of motion, i.e. can only talk about probabilities 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 as follows: if it were possible to photograph the position of an electron in an atom in hundredths or millionths of a second, as in a photo finish, then the electron in such photographs would be represented as a point. Overlaying countless such photographs would result in a picture of an electron cloud with the highest density where there are most of these points.
The figure shows a "cut" of such an electron density in a hydrogen atom passing through the nucleus, and a sphere is bounded by a dashed line, inside which the probability of finding an electron is $90%$. The contour closest to the nucleus covers the region of space in which the probability of finding an electron is $10%$, the probability of finding an electron inside the second contour from the nucleus is $20%$, inside the third one - $≈30%$, etc. There is some uncertainty in the state of the electron. To characterize this special state, the German physicist W. Heisenberg introduced the concept of uncertainty principle, i.e. showed that it is impossible to determine simultaneously and exactly the energy and location of the electron. The more accurately the energy of an electron is determined, the more uncertain its position, and vice versa, having determined the position, it is impossible to determine the energy of the electron. The electron detection probability region has no clear boundaries. However, it is possible to single out the space where the probability of finding an electron is maximum.
The space around the atomic nucleus, in which the electron is most likely to be found, is called the orbital.
It contains approximately $90%$ of the electron cloud, which means that about $90%$ of the time the electron is in this part of space. According to the form, $4$ of currently known types of orbitals are distinguished, which are denoted by the Latin letters $s, p, d$ and $f$. A graphic representation of some forms of electronic orbitals is shown in the figure.
The most important characteristic of the motion of an electron in a certain orbit is the energy of its connection with the nucleus. Electrons with similar energy values form a single electronic layer, or energy level. Energy levels are numbered starting from the nucleus: $1, 2, 3, 4, 5, 6$ and $7$.
An integer $n$ denoting the number of the energy level is called the principal quantum number.
It characterizes the energy of electrons occupying a given energy level. The electrons of the first energy level, closest to the nucleus, have the lowest energy. Compared with the electrons of the first level, the electrons of the next levels are characterized by a large amount of energy. Consequently, the electrons of the outer level are the least strongly bound to the nucleus of the atom.
The number of energy levels (electronic layers) in an atom is equal to the number of the period in the system of D. I. Mendeleev, to which the chemical element belongs: the atoms of the elements of the first period have one energy level; the second period - two; seventh period - seven.
The largest number of electrons in the energy level is determined by the formula:
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 $32$. And how, in turn, are the energy levels (electronic layers) arranged?
Starting from the second energy level $(n = 2)$, each of the levels is subdivided into sublevels (sublayers), slightly different from each other by 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 sub level; the second - two; third - three; the fourth is four. Sublevels, in turn, are formed by orbitals.
Each value of $n$ corresponds to the number of orbitals equal to $n^2$. According to the data presented in the table, it is possible to trace the relationship between the principal quantum number $n$ and the number of sublevels, the type and number of orbitals, and the maximum number of electrons per sublevel and level.
Principal quantum number, types and number of orbitals, maximum number of electrons at sublevels and levels.
| Energy level $(n)$ | Number of sublevels equal to $n$ | Orbital type | Number of orbitals | Maximum number of electrons | ||
| in sublevel | in level equal to $n^2$ | in sublevel | at a level equal to $n^2$ | |||
| $K(n=1)$ | $1$ | $1s$ | $1$ | $1$ | $2$ | $2$ |
| $L(n=2)$ | $2$ | $2s$ | $1$ | $4$ | $2$ | $8$ |
| $2p$ | $3$ | $6$ | ||||
| $M(n=3)$ | $3$ | $3s$ | $1$ | $9$ | $2$ | $18$ |
| $3p$ | $3$ | $6$ | ||||
| $3d$ | $5$ | $10$ | ||||
| $N(n=4)$ | $4$ | $4s$ | $1$ | $16$ | $2$ | $32$ |
| $4p$ | $3$ | $6$ | ||||
| $4d$ | $5$ | $10$ | ||||
| $4f$ | $7$ | $14$ | ||||
It is customary to designate sublevels in Latin letters, as well as the shape of the orbitals of which they consist: $s, p, d, f$. So:
- $s$-sublevel - the first sublevel of each energy level closest to the atomic nucleus, consists of one $s$-orbital;
- $p$-sublevel - the second sublevel of each, except for the first, energy level, consists of three $p$-orbitals;
- $d$-sublevel - the third sublevel of each, starting from the third energy level, consists of five $d$-orbitals;
- The $f$-sublevel of each, starting from the fourth energy level, consists of seven $f$-orbitals.
atom nucleus
But not only electrons are part of atoms. Physicist Henri Becquerel discovered that a natural mineral containing uranium salt also emits unknown radiation, illuminating photographic films that are closed from light. This phenomenon has been called radioactivity.
There are three types of radioactive rays:
- $α$-rays, which consist of $α$-particles having a charge $2$ times greater than the charge of an electron, but with a positive sign, and a mass $4$ times greater than the mass of a hydrogen atom;
- $β$-rays are a stream of electrons;
- $γ$-rays are electromagnetic waves with a negligible mass that do not carry an electric charge.
Consequently, the atom has a complex structure - it consists of a positively charged nucleus and electrons.
How is the atom arranged?
In 1910 in Cambridge, near London, Ernest Rutherford with his students and colleagues studied the scattering of $α$ particles passing through thin gold foil and falling on a screen. Alpha particles usually deviated from the original direction by only one degree, confirming, it would seem, the uniformity and uniformity of the properties of gold atoms. And suddenly the researchers noticed that some $α$-particles abruptly changed the direction of their path, as if running into some kind of obstacle.
By placing the screen in front of the foil, Rutherford was able to detect even those rare cases when $α$-particles, reflected from gold atoms, flew in the opposite direction.
Calculations showed that the observed phenomena could occur if the entire mass of the atom and all its positive charge were concentrated in a tiny central nucleus. The radius of the nucleus, as it turned out, is 100,000 times smaller than the radius of the entire atom, that area in which there are electrons that have a negative charge. If we apply a figurative comparison, then the entire volume of the atom can be likened to the Luzhniki stadium, and the nucleus can be likened to a soccer ball located in the center of the field.
An atom of any chemical element is comparable to a tiny solar system. Therefore, such a model of the atom, proposed by Rutherford, is called planetary.
Protons and neutrons
It turns out that the tiny 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 accepted in chemistry as a unit). Protons are denoted by $↙(1)↖(1)p$ (or $р+$). Neutrons do not carry a charge, they are neutral and have a mass equal to the mass of a proton, i.e. $1$. Neutrons are denoted by $↙(0)↖(1)n$ (or $n^0$).
Protons and neutrons are collectively called nucleons(from lat. nucleus- nucleus).
The sum of the number of protons and neutrons in an atom is called mass number. For example, the mass number of an aluminum atom:
Since the mass of the electron, which is negligible, can be neglected, it is obvious that the entire mass of the atom is concentrated in the nucleus. Electrons are denoted as follows: $e↖(-)$.
Since the atom is electrically neutral, it is also obvious that that the number of protons and electrons in an atom is the same. It is equal to the atomic number of the chemical element assigned to it in the Periodic Table. For example, the nucleus of an iron atom contains $26$ protons, and $26$ electrons revolve around the nucleus. And how to determine the number of neutrons?
As you know, the mass of an atom is the sum of the mass of protons and neutrons. Knowing the ordinal 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:
For example, the number of neutrons in an iron atom is:
$56 – 26 = 30$.
The table shows the main characteristics of elementary particles.
Basic characteristics of elementary particles.
isotopes
Varieties of atoms of the same element that have the same nuclear charge but different mass numbers are called isotopes.
Word isotope consists of two Greek words: isos- the same and topos- place, means "occupying one place" (cell) in the Periodic system of elements.
Chemical elements found in nature are a mixture of isotopes. Thus, carbon has three isotopes with a mass of $12, 13, 14$; oxygen - three isotopes with a mass of $16, 17, 18$, etc.
Usually given in the Periodic system, the relative atomic mass of a chemical element 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, therefore, the values of atomic masses are quite often fractional. For example, natural chlorine atoms are a mixture of two isotopes - $35$ (there are $75%$ in nature) and $37$ (there are $25%$); therefore, the relative atomic mass of chlorine is $35.5$. Isotopes of chlorine are written as follows:
$↖(35)↙(17)(Cl)$ and $↖(37)↙(17)(Cl)$
The chemical properties of chlorine isotopes are exactly the same as the isotopes of most chemical elements, such as potassium, argon:
$↖(39)↙(19)(K)$ and $↖(40)↙(19)(K)$, $↖(39)↙(18)(Ar)$ and $↖(40)↙(18 )(Ar)$
However, hydrogen isotopes differ greatly in properties due to the dramatic fold increase in their relative atomic mass; they were even given individual names and chemical signs: protium - $↖(1)↙(1)(H)$; deuterium - $↖(2)↙(1)(H)$, or $↖(2)↙(1)(D)$; tritium - $↖(3)↙(1)(H)$, or $↖(3)↙(1)(T)$.
Now it is possible to give a modern, more rigorous and scientific definition of a chemical element.
A chemical element is a collection of atoms with the same nuclear charge.
The structure of the electron shells of atoms of the elements of the first four periods
Consider the mapping of the electronic configurations of the atoms of the elements by the periods of the system of D. I. Mendeleev.
Elements of the first period.
Schemes of the electronic structure of atoms show the distribution of electrons over electronic layers (energy levels).
The electronic formulas of atoms show the distribution of electrons over energy levels and sublevels.
Graphic electronic formulas of atoms show the distribution of electrons not only in levels and sublevels, but also in orbitals.
In a helium atom, the first electron layer is complete - it has $2$ electrons.
Hydrogen and helium are $s$-elements, these atoms have $s$-orbitals filled with electrons.
Elements of the second period.
For all elements of the second period, the first electron layer is filled, and the electrons fill the $s-$ and $p$ orbitals of the second electron layer in accordance with the principle of least energy (first $s$, then $p$) and the rules of Pauli and Hund.
In the neon atom, the second electron layer is complete - it has $8$ electrons.
Elements of the third period.
For atoms of elements of the third period, the first and second electron layers are completed, so the third electron layer is filled, in which electrons can occupy 3s-, 3p- and 3d-sublevels.
The structure of the electron shells of atoms of the elements of the third period.
A $3.5$-electron orbital is completed at the magnesium atom. $Na$ and $Mg$ are $s$-elements.
For aluminum and subsequent elements, the $3d$ sublevel is filled with electrons.
| $↙(18)(Ar)$ Argon |  |
$1s^2(2)s^2(2)p^6(3)s^2(3)p^6$ |  |
In an argon atom, the outer layer (the third electron layer) has $8$ electrons. As the outer layer is completed, but in total, in the third electron layer, as you already know, there can be 18 electrons, which means that the elements of the third period have $3d$-orbitals left unfilled.
All elements from $Al$ to $Ar$ - $p$ -elements.
$s-$ and $r$ -elements form main subgroups in the Periodic system.
Elements of the fourth period.
Potassium and calcium atoms have a fourth electron layer, the $4s$-sublevel is filled, because it has less energy than the $3d$-sublevel. To simplify the graphical electronic formulas of the atoms of the elements of the fourth period:
- we denote conditionally the graphic electronic formula of argon as follows: $Ar$;
- we will not depict the sublevels that are not filled for these atoms.
$K, Ca$ - $s$ -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 side subgroups, their pre-external electron layer is filled, they are referred to transition elements.
Pay attention to the structure of the electron shells of chromium and copper atoms. In them, one electron "falls" from the $4s-$ to the $3d$ sublevel, which is explained by the greater energy stability of the resulting $3d^5$ and $3d^(10)$ electronic configurations:
$↙(24)(Cr)$ $1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(4) 4s^(2)…$ 
$↙(29)(Cu)$ $1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(9)4s^(2)…$ 
| Element symbol, serial number, name | Diagram of the electronic structure | Electronic formula | Graphic electronic formula |
| $↙(19)(K)$ Potassium |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^1$ | |
| $↙(20)(C)$ Calcium |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^2$ | |
| $↙(21)(Sc)$ Scandium |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^1(3)d^1$ or $1s^2(2)s^2(2)p ^6(3)p^6(3)d^1(4)s^1$ |  |
| $↙(22)(Ti)$ Titanium |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^2(3)d^2$ or $1s^2(2)s^2(2)p ^6(3)p^6(3)d^2(4)s^2$ |  |
| $↙(23)(V)$ Vanadium |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^2(3)d^3$ or $1s^2(2)s^2(2)p ^6(3)p^6(3)d^3(4)s^2$ |  |
| $↙(24)(Cr)$ Chrome |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^1(3)d^5$ or $1s^2(2)s^2(2)p ^6(3)p^6(3)d^5(4)s^1$ |  |
| $↙(29)(Сu)$ Chromium |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^1(3)d^(10)$ or $1s^2(2)s^2(2 )p^6(3)p^6(3)d^(10)(4)s^1$ |  |
| $↙(30)(Zn)$ Zinc |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^2(3)d^(10)$ or $1s^2(2)s^2(2 )p^6(3)p^6(3)d^(10)(4)s^2$ |  |
| $↙(31)(Ga)$ Gallium |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^2(3)d^(10)4p^(1)$ or $1s^2(2) s^2(2)p^6(3)p^6(3)d^(10)(4)s^(2)4p^(1)$ |  |
| $↙(36)(Kr)$ Krypton |  |
$1s^2(2)s^2(2)p^6(3)p^6(4)s^2(3)d^(10)4p^6$ or $1s^2(2)s^ 2(2)p^6(3)p^6(3)d^(10)(4)s^(2)4p^6$ |  |
In the zinc atom, the third electron layer is complete - all the $3s, 3p$ and $3d$ sublevels are filled in it, in total there are $18$ of electrons on them.
In the elements following zinc, the fourth electron layer, the $4p$-sublevel, continues to be filled. Elements from $Ga$ to $Kr$ - $r$ -elements.
The outer (fourth) layer of a krypton atom is completed, it has $8$ of electrons. But just in the fourth electron layer, as you know, there can be $32$ of electrons; the krypton atom still has $4d-$ and $4f$-sublevels unfilled.
The elements of the fifth period are filling the sublevels in the following order: $5s → 4d → 5р$. And there are also exceptions related to the "failure" of electrons, for $↙(41)Nb$, $↙(42)Mo$, $↙(44)Ru$, $↙(45)Rh$, $↙(46) Pd$, $↙(47)Ag$. $f$ appear in the sixth and seventh periods -elements, i.e. elements whose $4f-$ and $5f$-sublevels of the third outside electronic layer are being filled, respectively.
$4f$ -elements called lanthanides.
$5f$ -elements called actinides.
The order of filling of electronic sublevels in the atoms of elements of the sixth period: $↙(55)Cs$ and $↙(56)Ba$ - $6s$-elements; $↙(57)La ... 6s^(2)5d^(1)$ - $5d$-element; $↙(58)Ce$ – $↙(71)Lu - 4f$-elements; $↙(72)Hf$ – $↙(80)Hg - 5d$-elements; $↙(81)Т1$ – $↙(86)Rn - 6d$-elements. But here, too, there are elements in which the order of filling of electron orbitals is violated, which, for example, is associated with greater energy stability of half and completely filled $f$-sublevels, i.e. $nf^7$ and $nf^(14)$.
Depending on which sublevel of the atom is filled with electrons last, all elements, as you already understood, are divided into four electronic 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;
- $r$ -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 preexternal level of the atom is filled with electrons; $d$-elements include elements of secondary subgroups of groups I–VIII, i.e. elements of intercalated decades of large periods located between $s-$ and $p-$elements. They are also called transition elements;
- $f$ -elements;$f-$sublevel of the third level of the atom outside is filled with electrons; these include lanthanides and actinides.
The electronic configuration of the atom. Ground and excited states of atoms
The Swiss physicist W. Pauli in $1925$ established that An atom can have at most two electrons in one orbital. having opposite (antiparallel) spins (translated from English as a spindle), i.e. possessing such properties that can be conditionally imagined as the rotation of an electron around its imaginary axis clockwise or counterclockwise. This principle is called the Pauli principle.
If there is one electron in an orbital, then it is called unpaired, if two, then this paired electrons, i.e. electrons with opposite spins.
The figure shows a diagram of the division of energy levels into sublevels.
$s-$ Orbital, as you already know, has a spherical shape. The hydrogen atom electron $(n = 1)$ is located on this orbital and is unpaired. According to this his electronic formula, or electronic configuration, is written like this: $1s^1$. In electronic formulas, the number of the energy level is indicated by the number in front of the letter $ (1 ...) $, the sublevel (orbital type) is denoted by the Latin letter, and the number that is written to the right of the letter (as an exponent) shows the number of electrons in the sublevel.
For a helium atom He, which has two paired electrons in the same $s-$orbital, this formula is: $1s^2$. The electron shell of the helium atom is complete and very stable. Helium is a noble gas. The second energy level $(n = 2)$ has four orbitals, one $s$ and three $p$. Second-level $s$-orbital electrons ($2s$-orbitals) have a higher energy, because are at a greater distance from the nucleus than the electrons of the $1s$-orbital $(n = 2)$. In general, for each value of $n$ there is one $s-$orbital, but with a corresponding amount of electron energy on it and, therefore, with a corresponding diameter, growing as the value of $n$.$s-$Orbital increases, as you already know , has a spherical shape. The hydrogen atom electron $(n = 1)$ is located on this orbital and is unpaired. Therefore, its electronic formula, or electronic configuration, is written as follows: $1s^1$. In electronic formulas, the number of the energy level is indicated by the number in front of the letter $ (1 ...) $, the sublevel (orbital type) is denoted by the Latin letter, and the number that is written to the right of the letter (as an exponent) shows the number of electrons in the sublevel.
For a helium atom $He$, which has two paired electrons in the same $s-$orbital, this formula is: $1s^2$. The electron shell of the helium atom is complete and very stable. Helium is a noble gas. The second energy level $(n = 2)$ has four orbitals, one $s$ and three $p$. Electrons of $s-$orbitals of the second level ($2s$-orbitals) have a higher energy, because are at a greater distance from the nucleus than the electrons of the $1s$-orbital $(n = 2)$. In general, for each value of $n$ there is one $s-$orbital, but with a corresponding amount of electron energy on it and, therefore, with a corresponding diameter, growing as the value of $n$ increases. 
$r-$ Orbital It has the shape of a dumbbell, or volume eight. All three $p$-orbitals are located in the atom mutually perpendicularly along the spatial coordinates drawn through the nucleus of the atom. It should be emphasized again that each energy level (electronic layer), starting from $n= 2$, has three $p$-orbitals. As the value of $n$ increases, the electrons occupy $p$-orbitals located at large distances from the nucleus and directed along the $x, y, z$ axes.
For elements of the second period $(n = 2)$, first one $s$-orbital is filled, and then three $p$-orbitals; electronic formula $Li: 1s^(2)2s^(1)$. The $2s^1$ electron is less bound to the atomic nucleus, so a lithium atom can easily give it away (as you probably remember, this process is called oxidation), turning into a lithium ion $Li^+$.
In the beryllium atom Be, the fourth electron is also placed in the $2s$ orbital: $1s^(2)2s^(2)$. The two outer electrons of the beryllium atom are easily detached - $B^0$ is oxidized into the $Be^(2+)$ cation.
The fifth electron of the boron atom occupies the $2p$-orbital: $1s^(2)2s^(2)2p^(1)$. Next, the $2p$-orbitals of the $C, N, O, F$ atoms are filled, which ends with the neon noble gas: $1s^(2)2s^(2)2p^(6)$.
For elements of the third period, $3s-$ and $3p$-orbitals are filled, respectively. Five $d$-orbitals of the third level remain free:
$↙(11)Na 1s^(2)2s^(2)2p^(6)3s^(1)$,
$↙(17)Cl 1s^(2)2s^(2)2p^(6)3s^(2)3p^(5)$,
$↙(18)Ar 1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)$.
Sometimes, in diagrams depicting the distribution of electrons in atoms, only the number of electrons at each energy level is indicated, i.e. write abbreviated electronic formulas of atoms of chemical elements, in contrast to the above full electronic formulas, for example:
$↙(11)Na 2, 8, 1;$ $↙(17)Cl 2, 8, 7;$ $↙(18)Ar 2, 8, 8$.
For elements of large periods (fourth and fifth), the first two electrons occupy respectively $4s-$ and $5s$-orbitals: $↙(19)K 2, 8, 8, 1;$ $↙(38)Sr 2, 8, 18, 8, 2$. Starting from the third element of each large period, the next ten electrons will go to the previous $3d-$ and $4d-$orbitals, respectively (for elements of secondary subgroups): $↙(23)V 2, 8, 11, 2;$ $↙( 26)Fr 2, 8, 14, 2;$ $↙(40)Zr 2, 8, 18, 10, 2;$ $↙(43)Tc 2, 8, 18, 13, 2$. As a rule, when the previous $d$-sublevel is filled, the outer (respectively $4p-$ and $5p-$) $p-$sublevel will start to be filled: $↙(33)As 2, 8, 18, 5;$ $ ↙(52)Te 2, 8, 18, 18, 6$.
For elements of large periods - the sixth and incomplete seventh - electronic levels and sublevels are filled with electrons, as a rule, as follows: the first two electrons enter the outer $s-$sublevel: $↙(56)Ba 2, 8, 18, 18, 8, 2;$ $↙(87)Fr 2, 8, 18, 32, 18, 8, 1$; the next one electron (for $La$ and $Ca$) to the previous $d$-sublevel: $↙(57)La 2, 8, 18, 18, 9, 2$ and $↙(89)Ac 2, 8, 18, 32, 18, 9, 2$.
Then the next $14$ electrons will enter the third energy level from the outside, the $4f$ and $5f$ orbitals of the lantonides and actinides, respectively: $↙(64)Gd 2, 8, 18, 25, 9, 2;$ $↙(92 )U 2, 8, 18, 32, 21, 9, 2$.
Then the second energy level from the outside ($d$-sublevel) will begin to build up again for the elements of side subgroups: $↙(73)Ta 2, 8, 18, 32, 11, 2;$ $↙(104)Rf 2, 8, 18 , 32, 32, 10, 2$. And, finally, only after the $d$-sublevel is completely filled with ten electrons, the $p$-sublevel will be filled again: $↙(86)Rn 2, 8, 18, 32, 18, 8$.
Very often, the structure of the electron shells of atoms is depicted using energy or quantum cells - they write down the so-called graphic electronic formulas. For this record, the following notation is used: each quantum cell is denoted by a cell that corresponds to one orbital; each electron is indicated by an arrow corresponding to the direction of the spin. When writing a graphical electronic formula, two rules should be remembered: Pauli principle, according to which a cell (orbital) can have no more than two electrons, but with antiparallel spins, and F. Hund's rule, according to which electrons occupy free cells first one at a time and at the same time have the same spin value, and only then pair, but the spins, according to the Pauli principle, will already be oppositely directed.
Atom is an electrically neutral particle consisting of a positively charged nucleus and negatively charged electrons.The structure of atomic nuclei
Nuclei of atoms consist of elementary particles of two types: protons(p) and neutrons(n). The sum of protons and neutrons in the nucleus of one atom is called nucleon number:,
where BUT- nucleon number, N- number of neutrons, Z is the number of protons.
Protons have a positive charge (+1), neutrons have no charge (0), electrons have a negative charge (-1). The masses of a proton and a neutron are approximately the same, they are taken equal to 1. The mass of an electron is much less than the mass of a proton, therefore it is neglected in chemistry, considering that the entire mass of an atom is concentrated in its nucleus.
The number of positively charged protons in the nucleus is equal to the number of negatively charged electrons, then the atom as a whole electrically neutral.
Atoms with the same nuclear charge are chemical element.
Atoms of various elements are called nuclides.
isotopes- atoms of the same element, having a different nucleon number due to a different number of neutrons in the nucleus.
Isotopes of Hydrogen
| Name | A | Z | N |
| Protium N | 1 | 1 | 0 |
| Deuterium D | 2 | 1 | 1 |
| Tritium T | 3 | 1 | 2 |
radioactive decay
The nuclei of nuclides can decay with the formation of nuclei of other elements, as well as, or other particles. The spontaneous decay of atoms of certain elements is called radioactive yu, and such substances - radioactive and. Radioactivity is accompanied by the emission of elementary particles and electromagnetic waves - radiation G.
Nuclear decay equation- nuclear reactions- are written as follows:
The time it takes for half of the atoms of a given nuclide to decay is called half-life.
Elements that contain only radioactive isotopes are called radioactive s. These are elements 61 and 84-107.
Types of radioactive decay
1) -rozpa e. -particles are emitted, i.e. nuclei of a helium atom. In this case, the nucleon number of the isotope decreases by 4, and the charge of the nucleus decreases by 2 units, for example: 2) -rozpa e. In an unstable nucleus, a neutron turns into a proton, while the nucleus emits electrons and antineutrinos. During -decay, the nucleon number does not change, and the nuclear charge increases by 1, for example:
3) -rozpa e. An excited nucleus emits rays with a very short wavelength, while the energy of the nucleus decreases, the nucleon number and charge of the nucleus do not change, for example:
The structure of the electron shells of atoms of the elements of the first three periods
The electron has a dual nature: it can behave both as a particle and as a wave. An electron in an atom does not move along certain trajectories, but can be located in any part around the nuclear space, but the probability of its being in different parts of this space is not the same. The area around the nucleus where an electron is likely to be is called orbital Yu. Each electron in an atom is located at a certain distance from the nucleus according to its energy reserve. Electrons with more or less the same energy form energy rіvn and, or electronic layer and.
The number of energy levels filled with electrons in an atom of a given element is equal to the number of the period in which it is located.
The number of electrons in the outer energy level is equal to the group number, inwhich the element is located.
Within the same energy level, electrons can differ in shape e clouds and, or orbital and. There are such forms of orbitals:
s-the form:
p-the form:
There are also d-, f-orbitals and others with a more complex shape.
Electrons with the same shape of the electron cloud form the same energy supply and: s-, p-, d-, f-sublevels.
The number of sublevels at each energy level is equal to the number of this level.
Within the same energy sublevel, a different distribution of orbitals in space is possible. So, in a three-dimensional coordinate system for s Orbitals can only have one position:
for R-orbitals - three:
for d-orbitals - five, for f-orbitals - seven.
Orbitals represent:
s-sublevel-
p-sublevel-
d-sublevel-
An electron in the diagrams is indicated by an arrow that indicates its spin. Spin is the rotation of an electron around its axis. It is indicated by an arrow: or . Two electrons in the same orbital are written but not .
There cannot be more than two electrons in one orbital ( Pauli principle).
Principle of Least Energy th : in an atom, each electron is located so that its energy is minimal (which corresponds to its largest bond with the nucleus).
For example, distribution of electrons in the chlorine atom in:
One unpaired electron determines the valency of Chlorine in this state - I.
During the receipt of additional energy (irradiation, heating), it is possible to separate electrons (promotion). This state of the atom is called zbudzheni m. In this case, the number of unpaired electrons increases and, accordingly, the valency of the atom changes.
Excited state of the Chlorine atom in :
Accordingly, among the number of unpaired electrons, Chlorine can have valencies III, V, and VII.
