Processes of formation of excited particles during radiolysis. Chemistry The process of ionization of unpaired electrons
The discovery of radioactivity confirmed the complexity of the structure of not only atoms, but also their nuclei. In 1903, E. Rutherford and F. Soddy proposed a theory of radioactive decay, which radically changed the old views on the structure of atoms. According to this theory, radioactive elements spontaneously decay, releasing α- or β-particles and forming atoms of new elements that are chemically different from the original ones. At the same time, the stability of the mass of both the original atoms and those formed as a result of the decay process is maintained. E. Rutherford in 1919 was the first to study the artificial transformation of nuclei. During the bombardment of nitrogen atoms with α particles, he isolated the nuclei of hydrogen atoms (protons) and atoms of the oxygen nuclide. Such transformations are called nuclear reactions, since from the nuclei of atoms of one element the nuclei of atoms of other elements are obtained. Nuclear reactions are written using equations. Thus, the nuclear reaction discussed above can be written as follows:
The phenomenon of radioactivity can be defined using the concept of isotopes: radioactivity is the transformation of unstable nuclei of atoms of one chemical element into the nuclei of atoms of another element, which is accompanied by the release of elementary particles. The radioactivity exhibited by isotopes of elements that exist in nature is called natural radioactivity. The rate of radioactive transformations is different for different isotopes. It is characterized by a radioactive decay constant, which shows how many atoms of a radioactive nuclide decay in 1 s. It has been established that the number of atoms of a radioactive nuclide that decays per unit time is proportional to the total number of atoms of this nuclide and depends on the value of the radioactive decay constant. For example, if over a certain period half of the total number of atoms of a radioactive nuclide decayed, then in the next such period half of the remainder will decay, that is, half as much as in the previous period, etc.
The lifespan of a radioactive nuclide is characterized by its half-life, that is, the period of time during which half of the initial amount of this nuclide decays. For example, the half-life of Radon is 3.85 days, Radium - 1620 years, Uranium - 4.5 billion years. These types of radioactive transformations are known: α-decay, β-decay, spontaneous (unintentional) nuclear fission. These types of radioactive transformations are accompanied by the release of α-particles, electrons, positrons, and γ-rays. In the process of α-decay, the nucleus of an atom of a radioactive element releases the nucleus of a Helium atom, as a result of which the charge of the nucleus of an atom of the original radioactive element decreases by two units, and the mass number by four. For example, the transformation of a Radium atom to a Radon atom can be written by the equation
The nuclear reaction of β-decay, which is accompanied by the release of electrons, positrons or entrainment of orbital electrons, can also be written by the equation
where e is an electron; hν - γ-radiation quantum; ν o - antineutrino (an elementary particle whose rest mass and charge are equal to zero).
The possibility of β-decay is due to the fact that, in accordance with modern concepts, a neutron can, under certain conditions, transform into a proton, releasing an electron and an antineutrino. A proton and a neutron are two states of the same nuclear particle - the nucleon. This process can be represented by a diagram
Neutron -> Proton + Electron + Antineutrino
During the beta decay of atoms of a radioactive element, one of the neutrons that is part of the nucleus of the atom releases an electron and an antineutrino, turning into a proton. In this case, the positive charge of the nucleus increases by one. This type of radioactive decay is called electron decay (β - decay). So, if the nucleus of an atom of a radioactive element releases one α-particle, the result is the nucleus of an atom of a new element with a proton number two units less, and when a β-particle is released, the nucleus of a new atom is obtained with a proton number one greater than that of the original one. This is the essence of the Soddy-Fajans displacement law. The atomic nuclei of some unstable isotopes can release particles that have a positive charge of +1 and a mass close to the mass of an electron. This particle is called a positron. So, the possible conversion of a proton to a neutron is according to the diagram:
Proton → Neutron + Positron + Neutrino
The transformation of a proton into a neutron is observed only in the case when the instability of the nucleus is caused by the excess content of protons in it. Then one of the protons turns into a neutron, and the positron and neutrino that arise in this case fly out beyond the boundaries of the nucleus; the nuclear charge decreases by one. This type of radioactive decay is called positron -decay (β+-decay). So, due to the β-decay of the nucleus of an atom of a radioactive element, an atom of the element is obtained that is shifted one place to the right (β-decay) or to the left (β+-decay) from the original radioactive element. A decrease in the nuclear charge of a radioactive atom by one can be caused not only by β+ decay, but also by electron entrainment, as a result of which one of the electrons of the electron ball closest to the nucleus is captured by the nucleus. This electron with one of the protons of the nucleus forms a neutron: e - + p → n
The theory of the structure of the atomic nucleus was developed in the 30s of the XX century. Ukrainian scientists D.D. Ivanenko and E.M. Gapon, as well as the German scientist W. Heisenberg. According to this theory, the nuclei of atoms consist of positively charged protons and electrically neutral neutrons. The relative masses of these elementary particles are almost the same (proton mass 1.00728, neutron mass 1.00866). Protons and neutrons (nucleons) are contained in the nucleus by very strong nuclear forces. Nuclear forces act only at very small distances - on the order of 10 -15 m.
The energy that is released during the formation of a nucleus from protons and neutrons is called the binding energy of the nucleus and characterizes its stability.
Paired electrons
If there is one electron in an orbital, it is called unpaired, and if there are two, then this paired electrons.
Four quantum numbers n, l, m, m s completely characterize the energy state of an electron in an atom.
When considering the structure of the electron shell of multielectron atoms of various elements, it is necessary to take into account three main provisions:
· Pauli principle,
· principle of least energy,
Hund's rule.
According to Pauli principle An atom cannot have two electrons with the same values of all four quantum numbers.
The Pauli principle determines the maximum number of electrons in one orbital, level and sublevel. Since AO is characterized by three quantum numbers n, l, m, then the electrons of a given orbital can differ only in the spin quantum number m s. But the spin quantum number m s can only have two values + 1/2 and – 1/2. Consequently, one orbital can contain no more than two electrons with different values of spin quantum numbers.
Rice. 4.6. The maximum capacity of one orbital is 2 electrons.
The maximum number of electrons at an energy level is defined as 2 n 2 , and at the sublevel – like 2(2 l+ 1). The maximum number of electrons located at different levels and sublevels is given in Table. 4.1.
Table 4.1.
Maximum number of electrons at quantum levels and sublevels
| Energy level | Energy sublevel | Possible values of the magnetic quantum number m | Number of orbitals per | Maximum number of electrons per | ||
| sublevel | level | sublevel | level | |||
| K (n=1) | s (l=0) | |||||
| L (n=2) | s (l=0) p (l=1) | –1, 0, 1 | ||||
| M (n=3) | s (l=0) p (l=1) d (l=2) | –1, 0, 1 –2, –1, 0, 1, 2 | ||||
| N (n=4) | s (l=0) p (l=1) d (l=2) f (l=3) | –1, 0, 1 –2, –1, 0, 1, 2 –3, –2, –1, 0, 1, 2, 3 |
The sequence of filling orbitals with electrons is carried out in accordance with principle of least energy .
According to the principle of least energy, electrons fill orbitals in order of increasing energy.
The order of filling the orbitals is determined Klechkovsky's rule: the increase in energy and, accordingly, the filling of orbitals occurs in increasing order of the sum of the principal and orbital quantum numbers (n + l), and if the sum is equal (n + l) - in increasing order of the principal quantum number n.
For example, the energy of an electron at the 4s sublevel is less than at the 3 sublevel d, since in the first case the amount n+ l = 4 + 0 = 4 (recall that for s-sublevel value of orbital quantum number l= = 0), and in the second n+ l = 3 + 2= 5 ( d- sublevel, l= 2). Therefore, sublevel 4 is filled first s, and then 3 d(see Fig. 4.8).
On 3 sublevels d (n = 3, l = 2) , 4R (n = 4, l= 1) and 5 s (n = 5, l= 0) sum of values P And l are identical and equal to 5. In case of equal values of the sums n And l the sublevel with the minimum value is filled first n, i.e. sublevel 3 d.
In accordance with the Klechkovsky rule, the energy of atomic orbitals increases in the series:
1s < 2s < 2R < 3s < 3R < 4s < 3d < 4R < 5s < 4d < 5p < 6s < 5d »
"4 f < 6p < 7s….
Depending on which sublevel in the atom is filled last, all chemical elements are divided into 4 electronic family : s-, p-, d-, f-elements.
4f
4 4d
3 4s
3p
3s
1 2s
Levels Sublevels
Rice. 4.8. Energy of atomic orbitals.
Elements whose atoms are the last to fill the s-sublevel of the outer level are called s-elements . U s-valence elements are the s-electrons of the outer energy level.
U p-elements The p-sublayer of the outer layer is filled last. Their valence electrons are located on p- And s-sub-levels of the external level. U d-elements are filled in last d-sublevel of the preexternal level and valence are s-electrons of the external and d-electrons of the pre-external energy levels.
U f-elements last to be filled f-sublevel of the third outer energy level.
The order of electron placement within one sublevel is determined Hund's rule:
within a sublevel, electrons are placed in such a way that the sum of their spin quantum numbers has a maximum absolute value.
In other words, the orbitals of a given sublevel are filled first by one electron with the same value of the spin quantum number, and then by a second electron with the opposite value.
For example, if it is necessary to distribute 3 electrons in three quantum cells, then each of them will be located in a separate cell, i.e. occupy a separate orbital:
∑m s= ½ – ½ + ½ = ½.
The order of electron distribution among energy levels and sublevels in the shell of an atom is called its electronic configuration, or electronic formula. Composing electronic configuration number energy level (main quantum number) is designated by numbers 1, 2, 3, 4..., sublevel (orbital quantum number) – by letters s, p, d, f. The number of electrons in a sublevel is indicated by a number, which is written at the top of the sublevel symbol.
The electronic configuration of an atom can be depicted as the so-called electron graphic formula. This is a diagram of the arrangement of electrons in quantum cells, which are a graphical representation of an atomic orbital. Each quantum cell can contain no more than two electrons with different spin quantum numbers.
To create an electronic or electronic-graphic formula for any element, you should know:
1. Serial number of the element, i.e. the charge of its nucleus and the corresponding number of electrons in the atom.
2. The period number, which determines the number of energy levels of the atom.
3. Quantum numbers and the connection between them.
For example, a hydrogen atom with atomic number 1 has 1 electron. Hydrogen is an element of the first period, so the only electron occupies the one located in the first energy level s-orbital having the lowest energy. The electronic formula of the hydrogen atom will be:
1 N 1 s 1 .
The electronic graphic formula of hydrogen will look like:
Electronic and electron-graphic formulas of the helium atom:
2 Not 1 s 2
2 Not 1 s
reflect the completeness of the electronic shell, which determines its stability. Helium is a noble gas characterized by high chemical stability (inertness).
The lithium atom 3 Li has 3 electrons, it is an element of period II, which means that the electrons are located at 2 energy levels. Two electrons fill s- sublevel of the first energy level and the 3rd electron is located on s- sublevel of the second energy level:
3 Li 1 s 2 2s 1
Valence I
The lithium atom has an electron located at 2 s-sublevel, is less tightly bound to the nucleus than electrons of the first energy level, therefore, in chemical reactions, a lithium atom can easily give up this electron, turning into the Li + ion ( and he -electrically charged particle ). In this case, the lithium ion acquires a stable complete shell of the noble gas helium:
3 Li + 1 s 2 .
It should be noted that, the number of unpaired (single) electrons determines element valency , i.e. its ability to form chemical bonds with other elements.
Thus, a lithium atom has one unpaired electron, which determines its valency equal to one.
Electronic formula of the beryllium atom:
4 Be 1s 2 2s 2 .
Electron graphic formula of the beryllium atom:
2 Valence mainly
State is 0
Beryllium's sublevel 2 electrons are more easily removed than others. s 2, forming the Be +2 ion:
It can be noted that the helium atom and the ions of lithium 3 Li + and beryllium 4 Be +2 have the same electronic structure, i.e. are characterized isoelectronic structure.
The structure of an atom determines its radius, ionization energy, electron affinity, electronegativity and other parameters of the atom. The electronic shells of atoms determine the optical, electrical, magnetic, and most importantly the chemical properties of atoms and molecules, as well as most properties of solids.
Magnetic characteristics of the atom
The electron has its own magnetic moment, which is quantized in a direction parallel or opposite to the applied magnetic field. If two electrons occupying the same orbital have opposite spins (according to the Pauli principle), then they cancel each other. In this case we say that the electrons paired. Atoms with only paired electrons are pushed out of the magnetic field. Such atoms are called diamagnetic. Atoms that have one or more unpaired electrons are drawn into a magnetic field. They are called diamagnetic.
The magnetic moment of an atom, which characterizes the intensity of interaction of an atom with a magnetic field, is practically proportional to the number of unpaired electrons.
Features of the electronic structure of atoms of various elements are reflected in such energy characteristics as ionization energy and electron affinity.
Ionization energy
Energy (potential) of ionization of an atom E i is the minimum energy required to remove an electron from an atom to infinity according to the equation
X = X + + e −
Its values are known for atoms of all elements of the Periodic Table. For example, the ionization energy of a hydrogen atom corresponds to the transition of an electron from 1 s-energy sublevel (−1312.1 kJ/mol) to the sublevel with zero energy and is equal to +1312.1 kJ/mol.
In the change in the first ionization potentials corresponding to the removal of one electron of atoms, periodicity is clearly expressed with increasing atomic number:
When moving from left to right across a period, the ionization energy, generally speaking, gradually increases; with an increase in the atomic number within the group, it decreases. Alkali metals have the minimum first ionization potentials, and noble gases have the maximum.
For the same atom, the second, third and subsequent ionization energies always increase, since an electron has to be removed from a positively charged ion. For example, for a lithium atom, the first, second and third ionization energies are 520.3, 7298.1 and 11814.9 kJ/mol, respectively.
The sequence of electron abstraction is usually the reverse sequence of the filling of orbitals with electrons in accordance with the principle of minimum energy. However, the elements that are populated d-orbitals are exceptions - first of all, they do not lose d-, A s-electrons.
Electron affinity
Atom electron affinity A e is the ability of atoms to attach an additional electron and turn into a negative ion. A measure of electron affinity is the energy released or absorbed. Electron affinity is equal to the ionization energy of the negative ion X −:
X − = X + e −
Halogen atoms have the greatest electron affinity. For example, for a fluorine atom, the addition of an electron is accompanied by the release of 327.9 kJ/mol of energy. For a number of elements, the electron affinity is close to zero or negative, which means the absence of a stable anion for this element.
Typically, the electron affinity of atoms of various elements decreases in parallel with an increase in their ionization energy. However, there are exceptions for some pairs of elements:
| Element | Ei, kJ/mol | A e, kJ/mol |
| F | 1681 | −238 |
| Cl | 1251 | −349 |
| N | 1402 | 7 |
| P | 1012 | −71 |
| O | 1314 | −141 |
| S | 1000 | −200 |
An explanation for this can be given based on the smaller sizes of the first atoms and the greater electron-electron repulsion in them.
Electronegativity
Electronegativity characterizes the ability of an atom of a chemical element to shift an electron cloud in its direction when forming a chemical bond (towards an element with a higher electronegativity). The American physicist Mulliken proposed defining electronegativity as the arithmetic mean between the ionization potential and electron affinity:
χ = 1/2 ( Ei + A e)
The difficulty in using this method is that electron affinities are not known for all elements.
Magnetic characteristics of the atom
The electron has its own magnetic moment, which is quantized in a direction parallel or opposite to the applied magnetic field. If two electrons occupying the same orbital have oppositely directed spins (according to the Pauli principle), then they cancel each other. In this case we say that the electrons paired. Atoms with only paired electrons are pushed out of the magnetic field. Such atoms are called diamagnetic. Atoms that have one or more unpaired electrons are drawn into a magnetic field. Οʜᴎ are called diamagnetic.
The magnetic moment of an atom, which characterizes the intensity of interaction of an atom with a magnetic field, is practically proportional to the number of unpaired electrons.
Features of the electronic structure of atoms of various elements are reflected in such energy characteristics as ionization energy and electron affinity.
Energy (potential) of ionization of an atom E i is the minimum energy required to remove an electron from an atom to infinity according to the equation
X = X + + e −
Its values are known for atoms of all elements of the Periodic Table. For example, the ionization energy of a hydrogen atom corresponds to the transition of an electron from 1 s-energy sublevel (−1312.1 kJ/mol) to the sublevel with zero energy and is equal to +1312.1 kJ/mol.
In the change in the first ionization potentials corresponding to the removal of one electron of atoms, periodicity is clearly expressed with increasing atomic number:
Figure 13
When moving from left to right across a period, the ionization energy, generally speaking, gradually increases; with an increase in the atomic number within the group, it decreases. Alkali metals have the minimum first ionization potentials, and noble gases have the maximum.
For the same atom, the second, third and subsequent ionization energies always increase, since an electron has to be torn away from a positively charged ion. For example, for a lithium atom, the first, second and third ionization energies are 520.3, 7298.1 and 11814.9 kJ/mol, respectively.
The sequence of electron abstraction is usually the reverse sequence of the filling of orbitals with electrons in accordance with the principle of minimum energy. Moreover, the elements that are populated d-orbitals are exceptions - first of all, they do not lose d-, A s-electrons.
Magnetic characteristics of an atom An electron has its own magnetic moment, which is quantized in a direction parallel or opposite to the applied magnetic field. If two electrons occupying the same orbital have opposite spins... [read more]
The ionization process is expressed by the scheme: E - n En+.
The process... [read more]
Characteristics of the atom.