Electronic configuration atom is a formula showing the arrangement of electrons in an atom by levels and sublevels. After studying the article, you will learn where and how electrons are located, get acquainted with quantum numbers and be able to construct the electronic configuration of an atom by its number; at the end of the article there is a table of elements.
Why study the electronic configuration of elements?
Atoms are like a construction set: there is a certain number of parts, they differ from each other, but two parts of the same type are absolutely the same. But this construction set is much more interesting than the plastic one and here’s why. The configuration changes depending on who is nearby. For example, oxygen next to hydrogen Maybe turn into water, when near sodium it turns into gas, and when near iron it completely turns it into rust. To answer the question of why this happens and predict the behavior of an atom next to another, it is necessary to study the electronic configuration, which will be discussed below.
How many electrons are in an atom?
An atom consists of a nucleus and electrons rotating around it; the nucleus consists of protons and neutrons. In the neutral state, each atom has the number of electrons equal to the number of protons in its nucleus. The number of protons is designated serial number element, for example, sulfur, has 16 protons - the 16th element of the periodic table. Gold has 79 protons - the 79th element of the periodic table. Accordingly, sulfur has 16 electrons in the neutral state, and gold has 79 electrons.
Where to look for an electron?
By observing the behavior of the electron, certain patterns were derived; they are described by quantum numbers, there are four in total:
- Principal quantum number
- Orbital quantum number
- Magnetic quantum number
- Spin quantum number
Orbital
Further, instead of the word orbit, we will use the term “orbital”; an orbital is the wave function of an electron; roughly, it is the region in which the electron spends 90% of its time.
N - level
L - shell
M l - orbital number
M s - first or second electron in the orbital
Orbital quantum number l
As a result of studying the electron cloud, they found that depending on the energy level, the cloud takes four main forms: a ball, dumbbells and two other, more complex ones. In order of increasing energy, these forms are called the s-, p-, d- and f-shell. Each of these shells can have 1 (on s), 3 (on p), 5 (on d) and 7 (on f) orbitals. The orbital quantum number is the shell in which the orbitals are located. The orbital quantum number for the s,p,d and f orbitals takes the values 0,1,2 or 3, respectively.
There is one orbital on the s-shell (L=0) - two electrons
There are three orbitals on the p-shell (L=1) - six electrons
There are five orbitals on the d-shell (L=2) - ten electrons
There are seven orbitals on the f-shell (L=3) - fourteen electrons
Magnetic quantum number m l
There are three orbitals on the p-shell, they are designated by numbers from -L to +L, that is, for the p-shell (L=1) there are orbitals “-1”, “0” and “1”. The magnetic quantum number is denoted by the letter m l.
Inside the shell, it is easier for electrons to be located in different orbitals, so the first electrons fill one in each orbital, and then a pair of electrons is added to each one.
Consider the d-shell:
The d-shell corresponds to the value L=2, that is, five orbitals (-2,-1,0,1 and 2), the first five electrons fill the shell taking the values M l =-2, M l =-1, M l =0 , M l =1,M l =2.
Spin quantum number m s
Spin is the direction of rotation of an electron around its axis, there are two directions, so the spin quantum number has two values: +1/2 and -1/2. One energy sublevel can only contain two electrons with opposite spins. The spin quantum number is denoted m s
Principal quantum number n
The main quantum number is the energy level at this moment seven energy levels are known, each indicated by an Arabic numeral: 1,2,3,...7. The number of shells at each level is equal to the level number: there is one shell on the first level, two on the second, etc.
Electron number
So, any electron can be described by four quantum numbers, the combination of these numbers is unique for each position of the electron, take the first electron, the lowest energy level is N = 1, at the first level there is one shell, the first shell at any level has the shape of a ball (s -shell), i.e. L=0, the magnetic quantum number can take only one value, M l =0 and the spin will be equal to +1/2. If we take the fifth electron (in whatever atom it is), then the main quantum numbers for it will be: N=2, L=1, M=-1, spin 1/2.
Algorithm for composing the electronic formula of an element:
1. Determine the number of electrons in an atom using the Periodic Table of Chemical Elements D.I. Mendeleev.
2. Using the number of the period in which the element is located, determine the number of energy levels; the number of electrons in the last electronic level corresponds to the group number.
3. Divide the levels into sublevels and orbitals and fill them with electrons in accordance with the rules for filling orbitals:
It must be remembered that the first level contains a maximum of 2 electrons 1s 2, on the second - a maximum of 8 (two s and six R: 2s 2 2p 6), on the third - a maximum of 18 (two s, six p, and ten d: 3s 2 3p 6 3d 10).
- Principal quantum number n should be minimal.
- First to fill s- sublevel, then р-, d- b f- sublevels.
- Electrons fill the orbitals in order of increasing energy of the orbitals (Klechkovsky's rule).
- Within a sublevel, electrons first occupy free orbitals one by one, and only after that they form pairs (Hund’s rule).
- There cannot be more than two electrons in one orbital (Pauli principle).
Examples.
1. Let's create the electronic formula of nitrogen. IN periodic table nitrogen is at number 7.
2. Let's create the electronic formula for argon. Argon is number 18 on the periodic table.
1s 2 2s 2 2p 6 3s 2 3p 6.
3. Let's create the electronic formula of chromium. Chromium is number 24 on the periodic table.
1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5
Energy diagram of zinc.
4. Let's create the electronic formula of zinc. Zinc is number 30 on the periodic table.
1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10
Please note that part of the electronic formula, namely 1s 2 2s 2 2p 6 3s 2 3p 6, is the electronic formula of argon.
The electronic formula of zinc can be represented as:
When writing electronic formulas for atoms of elements, indicate energy levels (values of the main quantum number n in the form of numbers - 1, 2, 3, etc.), energy sublevels (orbital quantum number values l in the form of letters - s, p, d, f) and the number at the top indicate the number of electrons in a given sublevel.
The first element in the table is D.I. Mendeleev is hydrogen, therefore the charge of the nucleus of the atom N equals 1, an atom has only one electron per s-sublevel of the first level. Therefore, the electronic formula of the hydrogen atom has the form:
The second element is helium; its atom has two electrons, so the electronic formula of the helium atom is 2 Not 1s 2. The first period includes only two elements, since the first energy level is filled with electrons, which can only be occupied by 2 electrons.
The third element in order - lithium - is already in the second period, therefore, its second energy level begins to be filled with electrons (we talked about this above). The filling of the second level with electrons begins with s-sublevel, therefore the electronic formula of the lithium atom is 3 Li 1s 2 2s 1 . The beryllium atom is completed filling with electrons s-sublevel: 4 Ve 1s 2 2s 2 .
In subsequent elements of the 2nd period, the second energy level continues to be filled with electrons, only now it is filled with electrons R-sublevel: 5 IN 1s 2 2s 2 2R 1 ; 6 WITH 1s 2 2s 2 2R 2 … 10 Ne 1s 2 2s 2 2R 6 .
The neon atom completes filling with electrons R-sublevel, this element ends the second period, it has eight electrons, since s- And R-sublevels can only contain eight electrons.
The elements of the 3rd period have a similar sequence of filling the energy sublevels of the third level with electrons. The electronic formulas of the atoms of some elements of this period are as follows:
11 Na 1s 2 2s 2 2R 6 3s 1 ; 12 Mg 1s 2 2s 2 2R 6 3s 2 ; 13 Al 1s 2 2s 2 2R 6 3s 2 3p 1 ;
14 Si 1s 2 2s 2 2R 6 3s 2 3p 2 ;…; 18 Ar 1s 2 2s 2 2R 6 3s 2 3p 6 .
The third period, like the second, ends with an element (argon), which is completely filled with electrons R-sublevel, although the third level includes three sublevels ( s, R, d). According to the above order of filling energy sublevels in accordance with Klechkovsky's rules, the energy of sublevel 3 d more sublevel 4 energy s, therefore, the potassium atom next to argon and the calcium atom behind it are filled with electrons 3 s– sublevel of the fourth level:
19 TO 1s 2 2s 2 2R 6 3s 2 3p 6 4s 1 ; 20 Ca 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 .
Starting from the 21st element - scandium, sublevel 3 in the atoms of the elements begins to be filled with electrons d. The electronic formulas of the atoms of these elements are:
21 Sc 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 3d 1 ; 22 Ti 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 3d 2 .
In the atoms of the 24th element (chromium) and the 29th element (copper), a phenomenon called “leakage” or “failure” of an electron is observed: an electron from the outer 4 s– sublevel “falls” by 3 d– sublevel, completing filling it halfway (for chromium) or completely (for copper), which contributes to greater stability of the atom:
24 Cr 1s 2 2s 2 2R 6 3s 2 3p 6 4s 1 3d 5 (instead of...4 s 2 3d 4) and
29 Cu 1s 2 2s 2 2R 6 3s 2 3p 6 4s 1 3d 10 (instead of...4 s 2 3d 9).
Starting from the 31st element - gallium, the filling of the 4th level with electrons continues, now - R– sublevel:
31 Ga 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 3d 10 4p 1 …; 36 Kr 1s 2 2s 2 2R 6 3s 2 3p 6 4s 2 3d 10 4p 6 .
This element ends the fourth period, which already includes 18 elements.
A similar order of filling energy sublevels with electrons occurs in the atoms of elements of the 5th period. For the first two (rubidium and strontium) it is filled s– sublevel of the 5th level, for the next ten elements (from yttrium to cadmium) is filled d– sublevel of the 4th level; The period is completed by six elements (from indium to xenon), the atoms of which are filled with electrons R– sublevel of the external, fifth level. There are also 18 elements in a period.
For elements of the sixth period, this order of filling is violated. At the beginning of the period, as usual, there are two elements whose atoms are filled with electrons s– sublevel of the external, sixth, level. The next element behind them, lanthanum, begins to fill with electrons d– sublevel of the previous level, i.e. 5 d. This completes the filling with electrons 5 d-sublevel stops and the next 14 elements - from cerium to lutetium - begin to fill f-sublevel of the 4th level. These elements are all included in one cell of the table, and below is an expanded row of these elements, called lanthanides.
Starting from the 72nd element - hafnium - to the 80th element - mercury, filling with electrons continues 5 d-sublevel, and the period ends, as usual, with six elements (from thallium to radon), the atoms of which are filled with electrons R– sublevel of the external, sixth, level. This is the largest period, including 32 elements.
In the atoms of the elements of the seventh, incomplete, period, the same order of filling sublevels is visible as described above. We let students write the electronic formulas of atoms of elements of the 5th – 7th periods themselves, taking into account everything said above.
Note:In some textbooks a different order of writing the electronic formulas of atoms of elements is allowed: not in the order of their filling, but in accordance with the number of electrons given in the table on each energy level. For example, the electronic formula of the arsenic atom may look like: As 1s 2 2s 2 2R 6 3s 2 3p 6 3d 10 4s 2 4p 3 .
