As a result of ionization, atoms are formed. Ionization of atoms in strong electric fields. See what “ionization” is in other dictionaries
Ionization energy is the main characteristic of an atom. It is this that determines the nature and strength that an atom is capable of forming. The reducing properties of a (simple) substance also depend on this characteristic.
The concept of “ionization energy” is sometimes replaced by the concept of “first ionization potential” (I1), meaning the smallest energy that is needed for an electron to move away from a free atom when it is in a state of energy called the lowest.
In particular, for a hydrogen atom this is the name given to the energy required to remove an electron from a proton. For atoms with several electrons there is the concept of a second, third, etc. ionization potentials.
Ionization energy is a sum, one term of which is the energy of the electron, and the other is the energy of the system.
In chemistry, the energy of a hydrogen atom is denoted by the symbol “Ea”, and the sum of the potential energy of the system and the energy of the electron can be expressed by the formula: Ea= E+T= -Z.e/2.R.
From this expression it is clear that the stability of the system is directly related to the charge of the nucleus and the distance between it and the electron. The smaller this distance, the stronger the charge of the nucleus, the stronger they attract, the more stable and stable the system, the more energy must be spent on breaking this bond.
Obviously, the stability of systems can be compared by the level of energy spent to destroy the connection: the greater the energy, the more stable the system.
Atomic ionization energy (the force required to break bonds in a hydrogen atom) was calculated experimentally. Today its value is known exactly: 13.6 eV (electronvolt). Later, scientists, also through a series of experiments, were able to calculate the energy required to break the atom-electron bond in systems consisting of a single electron and a nucleus with a charge twice that of a hydrogen atom. It has been experimentally established that in this case 54.4 electron volts are required.
The well-known laws of electrostatics state that the ionization energy required to break the bond between opposite charges (Z and e), provided that they are located at a distance R, is fixed (determined) by the following equation: T=Z.e/R.
This energy is proportional to the magnitude of the charges and, accordingly, is inversely related to the distance. This is quite natural: the stronger the charges, the stronger the forces connecting them, the more powerful the force required to destroy the connection between them. The same applies to the distance: the smaller it is, the stronger the ionization energy, the more force will have to be applied to destroy the bond.
This reasoning explains why a system of atoms with a strong nuclear charge is more stable and requires more energy to remove an electron.
The question immediately arises: “If only twice as strong, why does the ionization energy required to remove an electron increase not two, but four times? Why is it equal to twice the charge squared (54.4/13.6 = 4)? ".
This contradiction can be explained quite simply. If the charges Z and e in the system are in a relatively mutual state of immobility, then the energy (T) is proportional to the charge Z, and they increase proportionally.
But in a system where an electron with charge e rotates around a nucleus with charge Z, and Z increases, the radius of rotation R decreases proportionally: the electron is attracted to the nucleus with greater force.
The conclusion is obvious. The ionization energy is affected by the charge of the nucleus, the distance (radially) from the nucleus to the highest point of the charge density of the outer electron; the repulsive force between outer electrons and a measure of the penetrating power of an electron.
An important energy parameter for studying chemical processes is the ionization energy of an atom. In the case of a hydrogen atom, this is the energy that must be expended in order to remove an electron from a proton.
It is equal to the sum of the potential energy of the system and the kinetic energy of the electron.
E a = E+T= -Z . e/2. R, (2.7)
where E a is the energy of the hydrogen atom.
From formula (2.7) it follows that a decrease in the distance between the electron and the nucleus and an increase in the charge of the nucleus mean an increase in the force of attraction of the electron to the nucleus. That is, more energy will be required to remove an electron from the nucleus. The more energy required to break this bond, the more stable the system.
Therefore, if breaking a bond (separating an electron from the nucleus) in one system requires more energy than in another, then the first system is more stable.
The ionization energy of an atom, the energy required to break bonds in a hydrogen atom, has been determined experimentally. It is equal to 13.6 eV (electron volts). The energy required to remove an electron from the nucleus in an atom consisting of one electron and a nucleus, the charge of which is twice the charge of the nucleus of a hydrogen atom, was also determined experimentally. In this case, it is necessary to expend four times more energy (54.4 eV).
As is known from electrostatics, energy ( T), necessary to break the bond between opposite charges ( Z And e), located at a distance from each other R, is determined by the equality
It is proportional to the size of the charges and inversely proportional to the distance between them. This correlation is quite understandable: the larger the charges, the stronger their attraction to each other, therefore, more energy is required to break the bond between them. And the smaller the distance between them, the more energy will have to be spent on breaking the connection. Thanks to this, it becomes clear why an atomic system, where the nuclear charge is twice as large as the nuclear charge in a hydrogen atom, is more stable and requires more energy to remove an electron.
ELECTRON AFFINITY particles (molecules, atoms, ions), min. energy A required to remove an electron from the corresponding negative. ion to infinity. For particle X S. to e. refers to the process:
S. to e. equal to the ionization energy E negative. ion X - (first ionization potential U 1, measured in eV). By analogy with the ionization potential, a distinction is made between the first and second solar energy, as well as vertical and adiabatic solar energy. polyatomic particle. Thermodynamic definition of S. to e. - standard enthalpy of the solution (1) at abs. zero temperature:
AN A (N A ~Avogadro’s constant).
Reliable experiments. data on S. to e. atoms and molecules to sulfur. 60s 20th century practically did not exist. Currently, the use of equilibrium methods of production and research is denied. ions made it possible to obtain the first S. to e. for most elements periodic. systems and several hundreds of org. and non-org. molecules. Naib. promising methods for determining S. to e.-photoelectron spectroscopy (accuracy + 0.01 eV) and mass spectrometry. study of the equilibrium of ion-molecular reactions. Quantum Mech. S.'s calculations to e. are similar to calculations of ionization potentials. The best accuracy for polyatomic molecules is 0.05-0.1 eV.
The largest S. to e. possess halogen atoms. For a number of elements S. to e. close to zero or less than zero. The latter means that for a given element the stable value is negative. ion does not exist. In table Table 1 shows the values of S. to e. atoms obtained by photoelectron spectroscopy (work by W. Lineberger and co-workers).
ELECTRONEGATIVITY, a quantity characterizing the ability of an atom to polarize covalent bonds. If in a diatomic molecule A - B the electrons forming the bond are attracted to atom B more strongly than to atom A, then atom B is considered more electronegative than A.
L. Pauling proposed (1932) for quantities. electronegativity characteristics use thermochemical. data on the energy of bonds A-A, B - B and A - B - respectively. E AA, E bb and E AB. The energy is hypothetical purely covalent bond A - B (E cov) is assumed to be equal to the arithmetic mean. or geometric mean the values of E AA and E BB. If the electronegativity of atoms A and B are different, then the A - B bond ceases to be purely covalent and the bond energy E AB will become greater than E covalent by the amount
The greater the difference in electronegativity of atoms A and B, the greater the value Using empirical. f-lu 
(a factor of 0.208 arises when converting energy values from kcal/mol to eV) and taking an arbitrary electronegativity value of 2.1 for the hydrogen atom, Pauling obtained a convenient relative scale. numerical values electronegativity, some of which are given in table. Naib. The lightest of the halogens, F, is electronegative; the least heavy are the heavy alkali metals.
For quantities. descriptions of electronegativity, in addition to thermochemical. data, data on the geometry of molecules (for example, Sanderson's method), spectral characteristics (for example, Gordy's method) are also used.
ATOMIC RADIUS, effective characteristics of atoms, allowing one to approximately estimate the interatomic (internuclear) distance in molecules and crystals. According to ideas quantum mechanics, atoms do not have clear boundaries, but the probability of finding an electron associated with a given nucleus at a certain distance from that nucleus decreases rapidly with increasing distance. Therefore, a certain radius is assigned to the atom, believing that the vast majority of the electron density (90-98%) is contained in the sphere of this radius. Atomic radii are very small values, on the order of 0.1 nm, but even small differences in their sizes can affect the structure of crystals built from them, the equilibrium configuration of molecules, etc. Experimental data show that in many In cases, the shortest distance between two atoms is indeed approximately equal to the sum of the corresponding atomic radii (the so-called principle of additivity of atomic radii). Depending on the type of bond between atoms, metallic, ionic, covalent and van der Waals atomic radii are distinguished.
Metallic radius is equal to half the shortest distance between atoms in a crystalline. metal structure. Its value depends on the coordination. numbers K (number of nearest neighbors of an atom in the structure). The most common structures are metals with K = 12. If we take the value of the atomic radii in such crystals as 1, then the atomic radii of metals with K equal to 8, 6 and 4 will be respectively. 0.98, 0.96 and 0.88. The proximity of the values of atomic radii decomp. metals - a necessary (although not sufficient) condition for the fact that these metals form solid solutions substitutions. Thus, liquid K and Li (radii 0.236 and 0.155 nm, respectively) usually do not mix, and K with Rb and Cs form a continuous series of solid solutions (radii Rb and Cs, respectively, 0.248 and 0.268 nm). Additivity of metallic atomic radii allows one to predict crystalline parameters with moderate accuracy. intermetallic gratings connections.
Ionic radii are used to approximate estimates of the shortest internuclear distances in ionic crystals, assuming that these distances are equal to the sum of the corresponding ionic radii of atoms. There are several systems of ionic radii values that differ for individual ions, but lead to approximately the same internuclear distances in ionic crystals. Ionic radii were first determined in the 1920s. 20th century V. M. Goldshmidt, who relied on refractometric. the values of the radii F - and O 2-, equal respectively. 0.133 and 0.132 nm. In L. Pauling’s system, the radius of the O 2- ion is taken as a basis, equal to 0.140 nm, in the widespread system of N.V. Belov and G.B. Bokiy, the radius of the same ion is taken equal to 0.136 nm, in K. Shannon’s system -0.121 nm (K = 2).
The covalent radius is equal to half the length of a single chemical. X-X connections, where X is a non-metal atom. For halogens, the covalent atomic radius is half the internuclear distance in the X 2 molecule, for S and Se - in X 8, for S-in crystal diamond The covalent radii of F, Cl, Br, I, S, Se and C are equal, respectively. 0.064, 0.099, 0.114, 0.133, 0.104, 0.117 and 0.077 nm. The covalent radius of hydrogen is taken to be 0.030 nm, although half the length N-N connections in the H2 molecule is 0.037 nm. Using the rule of additivity of atomic radii, bond lengths in polyatomic molecules are predicted. For example, the lengths of the C-H, C-F and C-C1 bonds should be 0.107, 0.141 and 0.176 nm, respectively, and they are indeed approximately equal to the specified values in many. org. molecules that do not contain multiple carbon-carbon bonds; otherwise, the corresponding internuclear distances decrease.
Van der Waals radii determine the effective sizes of noble gas atoms. It is also believed that these radii are equal to half the internuclear distance between the nearest identical atoms that are not chemically bonded to each other. communication, i.e. belonging to different molecules, for example. in molecular crystals. The values of van der Waals radii are found, using the principle of additivity of atomic radii, from the shortest contacts of neighboring molecules in crystals. On average they are ~0.08 nm larger than the covalent radii. Knowledge of van der Waals radii allows one to determine the conformation of molecules and their packing in molecular crystals. Energetically favorable conformations of molecules are usually those in which the overlap of van der Waals radii of valence-unbonded atoms is small. Van der Waals spheres of valence-bonded atoms within one molecule overlap. Ext. the outline of the overlapping spheres determines the shape of the molecule. Molecular crystals obey the principle of close packing, according to which molecules, modeled by their “van der Waals fringing,” are arranged so that the “protrusions” of one molecule fit into the “cavities” of another. Using these ideas, one can interpret the crystallographic. data, and in some cases predict the structure of molecular crystals.
Ticket 6.
Chemical bond.
The formation of molecules, molecular ions, ions, crystalline, amorphous and other substances from atoms is accompanied by a decrease in energy compared to non-interacting atoms. In this case, the minimum energy corresponds to a certain arrangement of atoms relative to each other, which corresponds to a significant redistribution of electron density. The forces that hold atoms together in new formations have received the general name “chemical bond”. The most important types of chemical bonds: ionic, covalent, metallic, hydrogen, intermolecular.
According to the electronic valence theory, a chemical bond arises due to the redistribution of electrons in valence orbitals, resulting in the formation of a stable electronic configuration noble gas (octet) due to the formation of ions (W. Kossel) or the formation of shared electron pairs (G. Lewis).
A chemical bond is characterized by energy and length. A measure of the strength of a bond is the energy spent on breaking the bond, or the gain in energy when forming a compound from individual atoms (Eb). Thus, 435 kJmol √1 is spent on breaking the H√H bond, and 1648 kJmol √1 is spent on the atomization of methane CH 4 √ 1648 kJmol √1, in this case E C√H = 1648: 4 = 412 kJ. Bond length (nm) √ distance between nuclei in a particular connection. Typically, the bond length and its energy are antithetical: the longer the bond length, the lower its energy.
A chemical bond is usually represented by lines connecting interacting atoms; each stroke is equivalent to a generalized pair of electrons. In compounds containing more than two atoms, an important characteristic is the bond angle formed by the chemical bonds in the molecule and reflecting its geometry.
The polarity of a molecule is determined by the difference in electronegativity of the atoms forming a two-center bond, the geometry of the molecule, as well as the presence of lone electron pairs, since part of the electron density in the molecule can be localized not in the direction of the bonds. The polarity of a bond is expressed through its ionic component, that is, through the displacement of an electron pair to a more electronegative atom. The polarity of a bond can be expressed through its dipole moment m, equal to the product of the elementary charge and the length of the dipole *) m = e l. The polarity of a molecule is expressed through its dipole moment, which is equal to vector sum all dipole moments of the bonds of the molecule.
*) Dipole is a system of two equal but opposite charges located at a unit distance from each other. Dipole moment is measured in coulomb meters (Cm) or debyes (D); 1D = 0.33310 √29 Klm.
All these factors should be taken into account. For example, for a linear molecule CO 2 m = 0, but for SO 2 m = 1.79 D due to its angular structure. Dipole moments of NF 3 and NH 3 with the same hybridization of the nitrogen atom (sp 3), approximately the same polarity of the N√F and N√H bonds (OEO N = 3; OEO F = 4; OEO H = 2.1) and similar molecular geometry differ significantly, since the dipole moment of the lone pair of nitrogen electrons during vector addition in the case of NH 3 increases the m of the molecule, and in the case of NF 3 it decreases it.
Ionization energy(E ion) is called energy expended in removing an electron from an atom and transforming the atom into a positively charged ion.
Experimentally, the ionization of atoms is carried out in an electric field by measuring the potential difference at which ionization occurs. This potential difference is called ionization potential(J). The unit of measurement for ionization potential is eV/atom, and the unit for ionization energy is kJ/mol; the transition from one value to another is carried out according to the relationship:
E ion = 96.5 J
The removal of the first electron from an atom is characterized by the first ionization potential (J 1), the second by the second (J 2), etc. Successive ionization potentials increase (Table 1), since each subsequent electron must be removed from an ion with a positive charge increasing by one. From the table 1 shows that in lithium a sharp increase in the ionization potential is observed for J2, in beryllium - for J3, in boron - for J4, etc. A sharp increase in J occurs when the removal of outer electrons ends and the next electron is at the pre-outer energy level.
Table 1
Ionization potentials of atoms (eV/atom) of elements of the second period
| Element | J 1 | J2 | J 3 | J 4 | J5 | J 6 | J 7 | J 8 |
| Lithium | 5,39 | 75,6 | 122,4 | – | – | – | – | – |
| Beryllium | 9,32 | 18,2 | 158,3 | 217,7 | – | – | – | – |
| Bor | 8,30 | 25,1 | 37,9 | 259,3 | 340,1 | – | – | – |
| Carbon | 11,26 | 24,4 | 47,9 | 64,5 | 392,0 | 489,8 | – | – |
| Nitrogen | 14,53 | 29,6 | 47,5 | 77,4 | 97,9 | 551,9 | 666,8 | – |
| Oxygen | 13,60 | 35,1 | 54,9 | 77,4 | 113,9 | 138,1 | 739,1 | 871,1 |
| Fluorine | 17,40 | 35,0 | 62,7 | 87,2 | 114,2 | 157,1 | 185,1 | 953,6 |
| Neon | 21,60 | 41,1 | 63,0 | 97,0 | 126,3 | 157,9 |
Ionization potential is an indicator of the “metallicity” of an element: the lower it is, the easier it is for an electron to detach from an atom and the more strongly the metallic properties of the element should be expressed. For the elements with which periods begin (lithium, sodium, potassium, etc.), the first ionization potential is 4–5 eV/atom, and these elements are typical metals. For other metals, J 1 values are higher, but not more than 10 eV/atom, and for non-metals, usually more than 10 eV/atom: nitrogen 14.53 eV/atom, oxygen 13.60 eV/atom, etc.
The first ionization potentials increase in periods and decrease in groups (Fig. 14), which indicates an increase in non-metallic properties in periods and metallic ones in groups. Therefore, non-metals are in the upper right part, and metals are in the lower left part of the periodic table. The boundary between metals and non-metals is “blurred”, because Most elements have amphoteric (dual) properties. However, such a conventional boundary can be drawn; it is shown in the long (18-cell) form of the periodic table, which is available here in the classroom and in the reference book.
Rice. 14. Dependence of ionization potential
from the atomic number of elements of the first – fifth periods.
Example 10. The ionization potential of sodium is 5.14 eV/atom, and that of carbon is 11.26 eV/atom. What is their ionization energy?
Solution. 1) E ion (Na) = 5.14 96.5 = 496.0 kJ/mol
2) E ion (C) = 11.26·96.5 = 1086.6 kJ/mol
The practical significance of this relationship is that, knowing μ, which is relatively easy to measure, one can determine D,
which is quite difficult to determine directly.
Ambipolar diffusion
Both electrons and ions diffuse in the gas discharge plasma. The diffusion process appears to be as follows. Electrons, which have higher mobility, diffuse faster than ions. Due to this, an electric field is created between the electrons and the lagging positive ions. This field inhibits further diffusion of electrons, and vice versa, accelerates the diffusion of ions. When the ions are pulled towards the electrons, this electric field weakens and the electrons are again separated from the ions. This process occurs continuously. This diffusion is called ambipolar diffusion, the coefficient of which is
D amb = |
D e μ and + D and μ e |
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μ e + μ and |
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where D e ,D and |
– diffusion coefficients of electrons and ions; μ e, μ and – |
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mobility of electrons and ions. |
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Since D e >> D u and μ e >> μ and , it turns out that |
D and μ e ≈ D e μ and , |
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therefore D amb ≈ 2D and . Such diffusion takes place, for example, in the positive column of a glow discharge.
1.6. Excitation and ionization of atoms and molecules
It is known that an atom consists of a positive ion and electrons, the number of which is determined by the number of the element in periodic table DI. Mendeleev. Electrons in an atom are at specific energy levels. If an electron receives some energy from the outside, it moves to more high level, which is called the excitation level.
Usually the electron is at the excitation level for a short time, about 10-8 s. When an electron receives significant energy, it moves away from the nucleus to such a great distance that it can lose connection with it and becomes free. The least associated with the nucleus are the valence electrons, which are at higher energy levels and therefore are more easily detached from the atom. The process of removing an electron from an atom is called ionization.
In Fig. Figure 1.3 shows the energy picture of the valence electron in an atom. Here W o is the ground level of the electron, W mst is the metastable level
nal level, W 1,W 2 – excitation levels (first, second, etc.).
Part I. Chapter 1. Electronic and ionic processes in a gas discharge
Rice. 1.3. Energy picture of an electron in an atom
W ′ = 0 is the state when the electron loses its connection with the atom. The value W and = W ′ − W o is
ionization energy. The values of these levels for some gases are given in table. 1.3.
A metastable level is characterized by the fact that electron transitions to and from it are prohibited. This level is filled by the so-called exchange interaction, when an electron from outside lands on the W mst level, and the excess
electron leaves the atom. Metastable levels play an important role in the processes occurring in gas-discharge plasma, because on normal level The electron is excited for 10-8 s, and at the metastable level – 10-2 ÷ 10-3 s.
Table 1.3
Energy, eV |
CO2 |
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W revenge |
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The process of excitation of atomic particles also determines ionization through the so-called phenomenon of diffusion of resonant radiation. This phenomenon is that an excited atom, returning to a normal state, emits a quantum of light, which excites the next atom, and so on. The diffusion region of resonant radiation is determined by the photon mean free path λ ν, which depends
sieves on the density of atomic particles n. So, at n= 1016 cm-3 λ ν =10-2 ÷ 1
see. The phenomenon of diffusion of resonant radiation is also determined by the presence of metastable levels.
Stepwise ionization can occur according to different schemes: a) the first electron or photon excites the neutral
neutron particle, and the second electron or photon imparts additional energy to the valence electron, causing the ionization of this neutral particle;
Part I. Chapter 1. Electronic and ionic processes in a gas discharge
atom, and at this moment the excited atom goes into a normal state and emits a quantum of light, which increases the energy
c) finally, two excited atoms find themselves close to each other. In this case, one of them goes into a normal state and emits a quantum of light, which ionizes the second atom.
It should be noted that stepwise ionization becomes effective when the concentration of fast electrons (with energies close to
to W and ), photons and excited atoms is quite large. This is
occurs when ionization becomes sufficiently intense. In turn, photons incident on atoms and molecules can also produce excitation and ionization (direct or stepwise). The source of photons in a gas discharge is the radiation of an electron avalanche.
1.6.1. Excitation and ionization of molecules
For molecular gases, it is necessary to take into account the possibility of excitation of the molecules themselves, which, unlike atoms, perform rotational and vibrational movements. These movements are also quantized. Energy of the shock at rotational movement is 10-3÷ 10-1 eV, and with oscillatory motion – 10-2 ÷ 1 eV.
During an elastic collision of an electron with an atom, the electron loses
a significant part of your energy |
W=2 |
≈ 10 |
− 4 W . When a |
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When an electron interacts with a molecule, the electron excites rotational and vibrational motion of the molecules. In the latter case, the electron loses particularly significant energy up to 10-1 ÷ 1 eV. Therefore the excitement oscillatory movements molecules is an effective mechanism for extracting energy from an electron. In the presence of such a mechanism, the acceleration of the electron is hampered, and a stronger field is required so that the electron can gain sufficient energy for ionization. Therefore, the breakdown of a molecular gas requires a higher voltage than the breakdown of an atomic (inert) gas at the same interelectrode distance and equal pressure. This is demonstrated by the data in Table. 1.4, where the values of λ t, S t and U pr atom are compared
nal and molecular gases at atmospheric pressure and d = 1.3 cm.
Part I. Chapter 1. Electronic and ionic processes in a gas discharge
Table 1.4 |
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Characteristic |
Name of gas |
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S t 10 − 16, cm2 |
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U pr, kV |
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From the table 1.4 it is clear that although the transport cross sections S t for molecular
polar gases and argon are comparable, but the breakdown voltage of argon is significantly lower.
1.7. Thermal ionization
At high temperatures, gas ionization can occur due to an increase in the kinetic energy of atomic particles, called thermal ionization. Thus, for Na, K, Cs vapors, thermal ionization is significant at a temperature of several thousand degrees, and for air at a temperature of about 104 degrees. The probability of thermal ionization increases with increasing temperature and decreasing ionization potential of atoms (molecules). At ordinary temperatures, thermal ionization is insignificant and can practically only have an effect when an arc discharge develops.
However, it should be noted that back in 1951, Hornbeck and Molnar discovered that when monoenergetic electrons are passed through cold inert gases, ions are formed at an electron energy sufficient only to excite, but not to ionize, atoms. This process was called associative ionization.
Associative ionization sometimes plays an important role in the propagation of ionization waves and spark discharges in places where there are still very few electrons. Excited atoms are formed there as a result of the absorption of light quanta emerging from already ionized regions. In moderately heated air, at temperatures of 4000–8000 K, the molecules are sufficiently dissociated, but there are still too few electrons for the development of an avalanche. The main ionization mechanism is a reaction in which unexcited N and O atoms participate.
Associative ionization proceeds according to the following scheme N + O + 2. 8 eV ↔ NO + + q. The missing energy of 2.8 eV is obtained from the kinetic energy of the relative motion of atoms.
IONIZATION - the transformation of atoms and molecules into ions. The degree of ionization is the ratio of the number of ions to the number of neutral particles per unit volume. Large encyclopedic dictionary
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