Electronic configuration of an 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 by the atomic number of the 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
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
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.
The arrangement of electrons on energy shells or levels is written using electronic formulas of chemical elements. Electronic formulas or configurations help represent the atomic structure of an element.
Atomic structure
The atoms of all elements consist of a positively charged nucleus and negatively charged electrons, which are located around the nucleus.
Electrons are at different energy levels. The further an electron is from the nucleus, the more energy it has. The size of the energy level is determined by the size of the atomic orbital or orbital cloud. This is the space in which the electron moves.
Rice. 1. General structure of the atom.
- Orbitals can have different geometric configurations: s-orbitals
- - spherical; p-, d- and f-orbitals
- dumbbell-shaped, lying in different planes.
The first energy level of any atom always contains an s-orbital with two electrons (the exception is hydrogen). Starting from the second level, the s- and p-orbitals are at the same level.
Rice. 2. s-, p-, d and f-orbitals.
Orbitals exist regardless of the presence of electrons in them and can be filled or vacant.
Writing a formula
- Electronic configurations of atoms of chemical elements are written according to the following principles:
- each energy level has a corresponding serial number, indicated by an Arabic numeral;
- the number is followed by a letter indicating the orbital;
A superscript is written above the letter, corresponding to the number of electrons in the orbital.
- Recording examples:
calcium -
- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 ;
oxygen -
- 1s 2 2s 2 2p 4 ;
carbon -
1s 2 2s 2 2p 2 .
Let's take Na as an example. Sodium is in the first group, in the third period, at number 11. This means that the sodium atom has a positively charged nucleus (contains 11 protons), around which 11 electrons are located at three energy levels. There is one electron in the outer level.
Recall that the first energy level contains an s orbital with two electrons, and the second contains s and p orbitals. All that remains is to fill out the levels and get the full record:
11 Na) 2) 8) 1 or 1s 2 2s 2 2p 6 3s 1 .
For convenience, special tables of electronic formulas of the element have been created. In a long periodic table, formulas are also listed in each element cell.
Rice. 3. Table of electronic formulas.
For brevity, elements whose electronic formula coincides with the beginning of the element's formula are written in square brackets. For example, the electronic formula of magnesium is 3s 2, neon is 1s 2 2s 2 2p 6. Therefore, the full formula of magnesium is 1s 2 2s 2 2p 6 3s 2. 4.6. Total ratings received: 195.
- First we write down the chemical sign. element, where at the bottom left of the sign we indicate its serial number.
- Next, by the number of the period (from which the element) we determine the number of energy levels and draw such a number of arcs next to the sign of the chemical element.
- Then, according to the group number, the number of electrons in the outer level is written under the arc.
- At the 1st level, the maximum possible is 2, at the second there are already 8, at the third - as many as 18. We begin to put numbers under the corresponding arcs.
- The number of electrons at the penultimate level must be calculated as follows: the number of electrons already assigned is subtracted from the element’s serial number.
- It remains to turn our diagram into an electronic formula:
- We write the chemical element and its serial number. The number shows the number of electrons in the atom.
- Let's make a formula. To do this, you need to find out the number of energy levels; the basis for the determination is the period number of the element.
- We divide the levels into sub-levels.
Below you can see an example of how to correctly compose electronic formulas of chemical elements.
- first we fill the s-sublevel, and then the p-, d- b f-sublevels;
- according to Klechkovsky's rule, electrons fill orbitals in order of increasing energy of these orbitals;
- according to Hund's rule, electrons within one sublevel occupy free orbitals one at a time and then form pairs;
- According to the Pauli principle, there are no more than 2 electrons in one orbital.
The electronic formula of a chemical element shows how many electron layers and how many electrons are contained in the atom and how they are distributed among the layers.
To compose the electronic formula of a chemical element, you need to look at the periodic table and use the information obtained for this element. The atomic number of an element in the periodic table corresponds to the number of electrons in an atom. The number of electronic layers corresponds to the period number, the number of electrons in the last electronic layer corresponds to the group number.
It must be remembered that the first layer contains a maximum of 2 electrons 1s2, the second - a maximum of 8 (two s and six p: 2s2 2p6), the third - a maximum of 18 (two s, six p, and ten d: 3s2 3p6 3d10).
For example, the electronic formula of carbon: C 1s2 2s2 2p2 (serial number 6, period number 2, group number 4).
Electronic formula for sodium: Na 1s2 2s2 2p6 3s1 (serial number 11, period number 3, group number 1).
To check whether the electronic formula is written correctly, you can look at the website www.alhimikov.net.
At first glance, compiling an electronic formula for chemical elements may seem like a rather complicated task, but everything will become clear if you adhere to the following scheme:
- first we write the orbitals
- We insert numbers in front of the orbitals that indicate the number of the energy level. Don't forget the formula for determining the maximum number of electrons at the energy level: N=2n2
How can you find out the number of energy levels? Just look at the periodic table: this number is equal to the number of the period in which the element is located.
- Above the orbital icon we write a number that indicates the number of electrons that are in this orbital.
For example, the electronic formula for scandium will look like this.
The task of compiling an electronic formula for a chemical element is not the easiest.
So, the algorithm for compiling electronic formulas of elements is as follows:
Here are the electronic formulas of some chemical elements:
You need to create electronic formulas of chemical elements in this way: you need to look at the number of the element in the periodic table, thus finding out how many electrons it has. Then you need to find out the number of levels, which is equal to the period. Then the sublevels are written and filled in:
First of all, you need to determine the number of atoms according to the periodic table.
To compile the electronic formula, you will need the Mendeleev periodic system. Find your chemical element there and look at the period - it will be equal to the number of energy levels. The group number will correspond numerically to the number of electrons in the last level. The number of an element will be quantitatively equal to the number of its electrons. You also clearly need to know that the first level has a maximum of 2 electrons, the second - 8, and the third - 18.
These are the main points. In addition, on the Internet (including our website) you can find information with a ready-made electronic formula for each element, so you can test yourself.
Compiling electronic formulas of chemical elements is a very complex process; you can’t do it without special tables, and you need to use a whole bunch of formulas. Briefly, to compile you need to go through these stages:
It is necessary to draw up an orbital diagram in which there will be a concept of how electrons differ from each other. The diagram highlights orbitals and electrons.
Electrons are filled in levels, from bottom to top, and have several sublevels.
So first we find out the total number of electrons of a given atom.
We fill out the formula according to a certain scheme and write it down - this will be the electronic formula.
For example, for Nitrogen this formula looks like this, first we deal with electrons:
And write down the formula:
To understand the principle of compiling the electronic formula of a chemical element, first you need to determine the total number of electrons in an atom by the number in the periodic table. After this, you need to determine the number of energy levels, taking as a basis the number of the period in which the element is located.
The levels are then broken down into sublevels, which are filled with electrons based on the Principle of Least Energy.
You can check the correctness of your reasoning by looking, for example, here.
By composing the electronic formula of a chemical element, you can find out how many electrons and electron layers are in a particular atom, as well as the order of their distribution among the layers.
First, we determine the atomic number of the element according to the periodic table; it corresponds to the number of electrons. The number of electron layers indicates the period number, and the number of electrons in the last layer of the atom corresponds to the group number.
Composition of the atom.
An atom is made up of atomic nucleus And electron shell.
The nucleus of an atom consists of protons ( p+) and neutrons ( n 0). Most hydrogen atoms have a nucleus consisting of one proton.
Number of protons N(p+) is equal to the nuclear charge ( Z) and the ordinal number of the element in the natural series of elements (and in the periodic table of elements).
N(p +) = Z
Sum of neutrons N(n 0), denoted simply by the letter N, and number of protons Z called mass number and is designated by the letter A.
A = Z + N
The electron shell of an atom consists of electrons moving around the nucleus ( e -).
Number of electrons N(e-) in the electron shell of a neutral atom is equal to the number of protons Z at its core.
The mass of a proton is approximately equal to the mass of a neutron and 1840 times the mass of an electron, so the mass of an atom is almost equal to the mass of the nucleus.
The shape of the atom is spherical. The radius of the nucleus is approximately 100,000 times smaller than the radius of the atom.
Chemical element- type of atoms (collection of atoms) with the same nuclear charge (with the same number of protons in the nucleus).
Isotope- a collection of atoms of the same element with the same number of neutrons in the nucleus (or a type of atom with the same number of protons and the same number of neutrons in the nucleus).
Different isotopes differ from each other in the number of neutrons in the nuclei of their atoms.
Designation of an individual atom or isotope: (E - element symbol), for example: .
Structure of the electron shell of an atom
Atomic orbital- state of an electron in an atom. The symbol for the orbital is . Each orbital has a corresponding electron cloud.
Orbitals of real atoms in the ground (unexcited) state are of four types: s, p, d And f.
Electronic cloud- the part of space in which an electron can be found with a probability of 90 (or more) percent.
Note: sometimes the concepts of “atomic orbital” and “electron cloud” are not distinguished, calling both “atomic orbital”.
The electron shell of an atom is layered. Electronic layer formed by electron clouds of the same size. The orbitals of one layer form electronic ("energy") level, their energies are the same for the hydrogen atom, but different for other atoms.
Orbitals of the same type are grouped into electronic (energy) sublevels:
s-sublevel (consists of one s-orbitals), symbol - .
p-sublevel (consists of three p
d-sublevel (consists of five d-orbitals), symbol - .
f-sublevel (consists of seven f-orbitals), symbol - .
The energies of orbitals of the same sublevel are the same.
When designating sublevels, the number of the layer (electronic level) is added to the sublevel symbol, for example: 2 s, 3p, 5d means s-sublevel of the second level, p- sublevel of the third level, d-sublevel of the fifth level.
The total number of sublevels at one level is equal to the level number n. The total number of orbitals at one level is equal to n 2. Accordingly, the total number of clouds in one layer is also equal to n 2 .
Designations: - free orbital (without electrons), - orbital with an unpaired electron, - orbital with an electron pair (with two electrons).
The order in which electrons fill the orbitals of an atom is determined by three laws of nature (the formulations are given in simplified terms):
1. The principle of least energy - electrons fill the orbitals in order of increasing energy of the orbitals.
2. The Pauli principle - there cannot be more than two electrons in one orbital.
3. Hund's rule - within a sublevel, electrons first fill empty orbitals (one at a time), and only after that they form electron pairs.
The total number of electrons in the electronic level (or electron layer) is 2 n 2 .
The distribution of sublevels by energy is expressed as follows (in order of increasing energy):
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p ...
This sequence is clearly expressed by an energy diagram:
The distribution of an atom's electrons across levels, sublevels, and orbitals (electronic configuration of an atom) can be depicted as an electron formula, an energy diagram, or, more simply, as a diagram of electron layers ("electron diagram").
Examples of the electronic structure of atoms:
Valence electrons- electrons of an atom that can take part in the formation of chemical bonds. For any atom, these are all the outer electrons plus those pre-outer electrons whose energy is greater than that of the outer ones. For example: the Ca atom has 4 outer electrons s 2, they are also valence; the Fe atom has 4 outer electrons s 2 but he has 3 d 6, therefore the iron atom has 8 valence electrons. Valence electronic formula of the calcium atom is 4 s 2, and iron atoms - 4 s 2 3d 6 .
Periodic table of chemical elements by D. I. Mendeleev
(natural system of chemical elements)
Periodic law of chemical elements(modern formulation): the properties of chemical elements, as well as simple and complex substances formed by them, are periodically dependent on the value of the charge of atomic nuclei.
Periodic table- graphic expression of the periodic law.
Natural series of chemical elements- a series of chemical elements arranged according to the increasing number of protons in the nuclei of their atoms, or, what is the same, according to the increasing charges of the nuclei of these atoms. The atomic number of an element in this series is equal to the number of protons in the nucleus of any atom of this element.
The table of chemical elements is constructed by "cutting" the natural series of chemical elements into periods(horizontal rows of the table) and groupings (vertical columns of the table) of elements with a similar electronic structure of atoms.
Depending on the way elements are grouped, the table may be long-period(elements with the same number and type of valence electrons are collected into groups) and short period(elements with the same number of valence electrons are collected in groups).
The short-period table groups are divided into subgroups ( main And side), coinciding with the groups of the long-period table.
All atoms of elements of the same period have the same number of electron layers, equal to the period number.
Number of elements in periods: 2, 8, 8, 18, 18, 32, 32. Most of the elements of the eighth period were obtained artificially; the last elements of this period have not yet been synthesized. All periods except the first begin with an alkali metal-forming element (Li, Na, K, etc.) and end with a noble gas-forming element (He, Ne, Ar, Kr, etc.).
In the short-period table there are eight groups, each of which is divided into two subgroups (main and secondary), in the long-period table there are sixteen groups, which are numbered in Roman numerals with the letters A or B, for example: IA, IIIB, VIA, VIIB. Group IA of the long-period table corresponds to the main subgroup of the first group of the short-period table; group VIIB - secondary subgroup of the seventh group: the rest - similarly.
The characteristics of chemical elements naturally change in groups and periods.
In periods (with increasing serial number)
- nuclear charge increases
- the number of outer electrons increases,
- the radius of atoms decreases,
- the strength of the bond between electrons and the nucleus increases (ionization energy),
- electronegativity increases,
- the oxidizing properties of simple substances are enhanced ("non-metallicity"),
- the reducing properties of simple substances weaken ("metallicity"),
- weakens the basic character of hydroxides and corresponding oxides,
- the acidic character of hydroxides and corresponding oxides increases.
In groups (with increasing serial number)
- nuclear charge increases
- the radius of atoms increases (only in A-groups),
- the strength of the bond between electrons and the nucleus decreases (ionization energy; only in A-groups),
- electronegativity decreases (only in A-groups),
- the oxidizing properties of simple substances weaken ("non-metallicity"; only in A-groups),
- the reducing properties of simple substances are enhanced ("metallicity"; only in A-groups),
- the basic character of hydroxides and corresponding oxides increases (only in A-groups),
- weakens the acidic character of hydroxides and corresponding oxides (only in A-groups),
- the stability of hydrogen compounds decreases (their reducing activity increases; only in A-groups).
Tasks and tests on the topic "Topic 9. "Structure of the atom. Periodic law and periodic system of chemical elements by D. I. Mendeleev (PSHE) "."
- Periodic law - Periodic law and structure of atoms grades 8–9
You must know: the laws of filling orbitals with electrons (the principle of least energy, the Pauli principle, Hund's rule), the structure of the periodic table of elements.You must be able to: determine the composition of an atom by the position of the element in the periodic table, and, conversely, find an element in the periodic system, knowing its composition; depict the structure diagram, electronic configuration of an atom, ion, and, conversely, determine the position of a chemical element in the PSCE from the diagram and electronic configuration; characterize the element and the substances it forms according to its position in the PSCE; determine changes in the radius of atoms, properties of chemical elements and the substances they form within one period and one main subgroup of the periodic system.
Example 1. Determine the number of orbitals in the third electron level. What are these orbitals?
To determine the number of orbitals, we use the formula N orbitals = n 2 where n- level number. N orbitals = 3 2 = 9. One 3 s-, three 3 p- and five 3 d-orbitals.Example 2. Determine which element's atom has electronic formula 1 s 2 2s 2 2p 6 3s 2 3p 1 .
In order to determine what element it is, you need to find out its atomic number, which is equal to the total number of electrons of the atom. In this case: 2 + 2 + 6 + 2 + 1 = 13. This is aluminum.After making sure that everything you need has been learned, proceed to completing the tasks. We wish you success.
Recommended reading:- O. S. Gabrielyan and others. Chemistry 11th grade. M., Bustard, 2002;
- G. E. Rudzitis, F. G. Feldman. Chemistry 11th grade. M., Education, 2001.