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 has sublevel 2 electrons that come off easier 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.
For the correct answer to each of tasks 1-8, 12-16, 20, 21, 27-29, 1 point is given.
Tasks 9–11, 17–19, 22–26 are considered completed correctly if the sequence of numbers is indicated correctly. For a complete correct answer in tasks 9–11, 17–19, 22–26, 2 points are given; if one mistake is made - 1 point; for an incorrect answer (more than one error) or lack thereof – 0 points.
Theory on assignment:
1) F 2) S 3) I 4) Na 5) Mg
Determine which atoms of the indicated elements in the ground state are missing one electron before the outer electron layer is completed.
1
The eight-electron shell corresponds to the shell of an inert gas. For each of the substances in the period in which they are found there corresponds an inert gas, for fluorine neon, for sulfur argon, for iodine xenon, for sodium and magnesium argon, but of the listed elements, only fluorine and iodine lack one electron to reach the eight-electron shell, since they are in the seventh group.
To complete the task, use the following series of chemical elements. The answer in the task is a sequence of three numbers, under which the chemical elements in this row are indicated.
1) Be 2) H 3) N 4) K 5) C
Determine which atoms of the indicated elements in the ground state contain the same number of unpaired electrons.
1
4 Be Beryllium: 1s 2 2s 2
7 N Nitrogen: 1s 2 2s 2 2p 3
Number of unpaired electrons - 1
6 C Carbon: 1s 2 2s 2 2p 2
| 1s 2 | 2s 2 | 2p 3 | ||
| ↓ | ↓ | |||
Number of unpaired electrons - 2
From this it is obvious that for hydrogen and potassium the number of unpaired electrons is the same.
To complete the task, use the following series of chemical elements. The answer in the task is a sequence of three numbers, under which the chemical elements in this row are indicated.
1) Ge 2) Fe 3) Sn 4) Pb 5) Mn
Determine which atoms of the elements indicated in the series have valence electrons in both the s- and d-sublevels.
1
To solve this task, it is necessary to describe the upper electronic level of the elements:
- 32 Ge Germanium: 3d 10 4s 2 4p 2
- 26 Fe Iron: 3d 6 4s 2
- 50 Sn Tin: 4d 10 5s 2 5p 2
- 82 Pb Lead: 4f 14 5d 10 6s 2 6p 2
- 25 Mn Manganese: 3d 5 4s 2
In iron and manganese, valence electrons are located in the s- and d-sublevels.
To complete the task, use the following series of chemical elements. The answer in the task is a sequence of three numbers, under which the chemical elements in this row are indicated.
1) Br 2) Si 3) Mg 4) C 5) Al
Determine which atoms of the elements indicated in the series in the excited state have the electronic formula of the external energy level ns 1 np 3
1
For a non-excited state, the electronic formula is ns 1 np 3 will represent ns 2 np 2, it is precisely the elements of this configuration that we need. Let's write down the upper electronic level of the elements (or simply find the elements of the fourth group):
- 35 Br Bromine: 3d 10 4s 2 4p 5
- 14 Si Silicon: 3s 2 3p 2
- 12 Mg Magnesium: 3s 2
- 6 C Carbon: 1s 2 2s 2 2p 2
- 13 Al Aluminum: 3s 2 3p 1
For silicon and carbon, the upper energy level coincides with the desired one
To complete the task, use the following series of chemical elements. The answer in the task is a sequence of three numbers, under which the chemical elements in this row are indicated.
1) Si 2) F 3) Al 4) S 5) Li
how to determine the number of unpaired electrons in an atom and got the best answer
Answer from Rafael ahmetov[guru]
Using Klechkovsky's rule, write the electronic formula. This can be easily determined using the electronic formula. For example, the electronic formula of carbon is 1s2 2s2 2p2, we see that there are 2 electrons in the s-orbitals, i.e. they are paired. There are 2 electrons in p-orbitals, but there are three 2-p orbitals. This means that according to Hund’s rule, 2 electrons will occupy 2 different p-orbitals, and carbon has 2 unpaired electrons. Reasoning similarly, we see that the nitrogen atom has 1s2 2s2 2p3 - 3 unpaired electrons. Oxygen has 1s2 2s2 2p4 - there are 4 electrons in p-orbitals. 3 electrons are located one at a time in different p-orbitals, and there is no separate place for the fourth. Therefore, it pairs with one of the three, while two remain unpaired. Similarly, fluorine 1s2 2s2 2p5 has one unpaired electron, and neon 1s2 2s2 2p6 has no unpaired electrons.
In exactly the same way, we need to consider both the d- and f-orbitals (if they are involved in the electronic formula, and do not forget that there are five d-orbitals and seven f-orbitals.
Answer from Vadim Belenetsky[guru]
You don’t have to describe any element and then it will be clear whether there are unpaired electrons or not. For example, aluminum has a charge of +13. and the distribution by levels is 2.8.3. It is already clear that the p-electron in the last layer is unpaired. And check all elements in the same way.
Answer from Eenat Lezgintsev[newbie]
Vadim, can you give us more details?
Answer from Egor Ershov[newbie]
The number of unpaired electrons is equal to the number of the group in which the element is located
Answer from 3 answers[guru]
Hello! Here is a selection of topics with answers to your question: how to determine the number of unpaired electrons in an atom
Indicate the quantum numbers (n, l, m(l), m(s)) of the electron that is the last in the filling order, and determine the number
what is there to think? the last one will be the 5p electron.
n = 5 (principal number = level number)
