Valence possibilities of the phosphorus atom. Valence possibilities of atoms of elements in chemical compounds

Concept valence comes from the Latin word “valentia” and was known back in the mid-19th century. The first “extensive” mention of valency was in the works of J. Dalton, who argued that all substances consist of atoms connected to each other in certain proportions. Then, Frankland introduced the very concept of valency, which was further developed in the works of Kekule, who spoke about the relationship between valency and chemical bonding, A.M. Butlerov, who in his theory of the structure of organic compounds linked valency with the reactivity of a particular chemical compound and D.I. Mendeleev (in the Periodic Table of Chemical Elements, the highest valence of an element is determined by the group number).

DEFINITION

Valence is the number of covalent bonds that an atom can form when combined with a covalent bond.

The valence of an element is determined by the number of unpaired electrons in an atom, since they take part in the formation of chemical bonds between atoms in the molecules of compounds.

The ground state of an atom (state with minimum energy) is characterized by the electronic configuration of the atom, which corresponds to the position of the element in the Periodic Table. An excited state is a new energy state of an atom, with a new distribution of electrons within the valence level.

Electronic configurations of electrons in an atom can be depicted not only in the form of electronic formulas, but also using electron graphic formulas (energy, quantum cells). Each cell denotes an orbital, an arrow indicates an electron, the direction of the arrow (up or down) indicates the spin of the electron, a free cell represents a free orbital that an electron can occupy when excited. If there are 2 electrons in a cell, such electrons are called paired, if there is 1 electron, they are called unpaired. For example:

6 C 1s 2 2s 2 2p 2

The orbitals are filled as follows: first, one electron with the same spins, and then a second electron with opposite spins. Since the 2p sublevel has three orbitals with the same energy, each of the two electrons occupied one orbital. One orbital remained free.

Determination of the valence of an element using electronic graphic formulas

The valency of an element can be determined by electron-graphical formulas for the electronic configurations of electrons in an atom. Let's consider two atoms - nitrogen and phosphorus.

7 N 1s 2 2s 2 2p 3

Because The valency of an element is determined by the number of unpaired electrons, therefore, the valence of nitrogen is III. Since the nitrogen atom has no empty orbitals, an excited state is not possible for this element. However, III is not the maximum valence of nitrogen, the maximum valency of nitrogen is V and is determined by the group number. Therefore, it should be remembered that using electronic graphic formulas it is not always possible to determine the highest valence, as well as all the valences characteristic of this element.

15 P 1s 2 2s 2 2p 6 3s 2 3p 3

In the ground state, the phosphorus atom has 3 unpaired electrons, therefore, the valence of phosphorus is III. However, in the phosphorus atom there are free d-orbitals, therefore electrons located on the 2s sublevel are able to pair up and occupy vacant orbitals of the d-sublevel, i.e. go into an excited state.

Now the phosphorus atom has 5 unpaired electrons, therefore phosphorus also has a valence of V.

Elements having multiple valence values

Elements of groups IVA – VIIA can have several valency values, and, as a rule, the valency changes in steps of 2 units. This phenomenon is due to the fact that electrons participate in pairs in the formation of a chemical bond.

Unlike the elements of the main subgroups, the elements of the B-subgroups in most compounds do not exhibit a higher valency equal to the group number, for example, copper and gold. In general, transition elements exhibit a wide variety of chemical properties, which is explained by a large range of valences.

Let us consider the electronic graphic formulas of the elements and establish why the elements have different valences (Fig. 1).

Tasks: determine the valence possibilities of As and Cl atoms in the ground and excited states.

Lecture 3. Who is capable of what or Valence capabilities of atoms.

1. Structure of the Periodic Table

Each of those present in the audience has a bright personality and special talent. In the same way, the elements gathered together in the Periodic Table, although sometimes similar to one another, still have their own characteristics: strengths and weaknesses.

Let's start with the fact that there are a lot of elements - and it would be nice for us to call them somehow so as not to get confused. Let's collect elements with similar properties into groups -

electronic analogues.

To avoid confusion, let’s first “add up” the f-elements in two rows: lanthanides and actinides.

Then we arrange the groups so that the elements of the first group have 1 valence electron,

elements of the second group have 2 valence electrons, etc.

We will get 8 groups, in each of which subgroups are formed: one will contain s- or p-elements, and the other will contain d-elements.

For example, group 1A: H, Li, Na, K, Rb, Cs, Fr and group 1B: Cu, Ag, Au, Rg

Let's assemble the Periodic Table from the groups. Since a period is the time between two repeating events, the distance between two adjacent electronic analogues (the horizontal row of the Periodic System) will also be called a period.

Finally, let's name the groups

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

We will name the side subgroups by their first element: “copper subgroup”, “zinc subgroup”.

Let's try to find metals in our system.

It turns out that if you draw a diagonal from boron B to astatine At, then the metals of the main subgroups occupy the lower left corner, and nonmetals occupy the upper right. We will call such metals intransitive, i.e. intransition elements are metals of the main subgroups.

All elements of side subgroups and f-elements – transition elements, or transition metals.

Considering that in nature there are negligible amounts (or none at all) of elements with Z > 92,

Let's call such elements transuranium.

Now we can actually start.

2. Valence capabilities of atoms.

So our question for today is: how do atoms form molecules and why do these molecules

don't fall apart?

It is logical to assume that if atoms stick together, then something connects them.

Let's call this state chemical bond. Since the structure of the atom is for us

is not a secret, we will focus on the simplest possible explanation:

Chemical bond– a special type of interaction between atoms in chemicals

compounds, based on the interaction of positively charged atomic nuclei

one element with negatively charged electrons of another element.

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

Drawing an analogy with the law of universal gravitation, the nucleus of an atom, like a black hole, tries

attract any electron that falls within its sphere of attraction.

Types of chemical bonds. Covalent bond.

As you know, any animal is looking for a mate. And the electron is no exception: in order

To form a strong chemical bond, you need a pair of electrons with opposite spins.

Let there be 2 atoms - A and B, which interact with each other.

Depending on the method of interaction, electrons can be either “in phase”

(same sign of the wave functions e 1 and e 2), so that a chemical bond is formed,

or “out of phase” (different signs of the wave functions), leading to the repulsion of atoms from each other. In the first case, there is a gain in energy (the green energy level V is lower, and the magnitude of this gain is exactly equal to the energy of the bond being formed). In the second case, there is a loss in energy (red level X).

Imagine that you are rolling a ball. If it rolls downhill, you don’t make any effort and the ball rolls into the hole. On the contrary, you are pushing the ball up the hill with the sweat of your brow, but as soon as you let it go

– and the ball rolls down to its foot.

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

What happens when a connection is formed with an electron cloud?

For simplicity of the picture, we take spherically symmetric s-AOs (l = 0).

1. If the clouds (gray balls) add up, the picture below appears - there is an overlap region in which the electron density has “doubled”, and in the rest of the region it coincides with either the density of the electron cloud of atom A or the density of the electron cloud of atom B.

In this case, the increased electron density, like a hamburger patty, binds

positively charged nuclei of atoms A and B.

2. If the clouds (gray balls) are subtracted, then a picture appears from above - in the middle there is complete mutual destruction, and at the edges - the density of the electron cloud of the atom before interaction.

In this case, there is no electron density between the nuclei - and Coulomb’s merciless law orders the atoms to fly apart in different directions.

So, covalent chemical bond arises when unpaired electrons with opposite spins, which originally belonged to different atoms, are shared.

In this case, the elements entering into a covalent chemical bond seem to exchange electrons, therefore such a mechanism (method) of formation

covalent bond is called exchange bond.

A· + ·B = A: B

(sharing of electrons, formation of a common electron pair)

A· + ·B = A – B

(formation of a chemical bond,

the dash between A and B indicates a chemical bond and is called a valence prime)

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

Thus, for the formation of a covalent chemical bond by exchange

For example, when a hydrogen molecule is formed, each atom provides 1e - a common (bonding) pair of electrons is obtained.

When a water molecule is formed, for 1 oxygen atom, which has

2 unpaired electrons, requires 2 hydrogen atoms, each with 1e -

2 O – H bonds are formed. In this case, the oxygen atom also has two pairs of electrons (on the 2s and on the 2p sublevel), which do not participate in the reaction. Such pairs are called lone electron pairs.

The image of atoms with electrons in the valence level is called Lewis structures. In this case, it is recommended to represent electrons of different atoms with different symbols, for example, · , *, etc.

The image of the order in which atoms bond together is called

structural formulas. In this case, each pair of electrons on the letter is replaced by a valence stroke.

Structural formulas of substances: H – H, H – O – H, O = O.

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

The number of covalent bonds that a given element forms is called

covalence, or valency of this element.

Valence is indicated by Roman numerals.

Thus, at this stage, the valence of an element is determined by the number of unpaired electrons that can take part in the formation of covalent bonds.

Valence possibilities of elements.

1. Carbon.

In the ground state, the electronic configuration of the carbon atom is 1s2 2s2 2p2, of which the valence electrons are 2s and 2p electrons.

In this state, the carbon atom is able to form 2 covalent bonds according to the exchange

mechanism.

However, in practice, stable compounds of divalent carbon do not exist.

Due to the small difference between 2s and 2p-

sublevel, a carbon atom with little energy expenditure is able to move into the first

excited state (denoted C*).

In this state, the carbon atom is capable of

form 4 covalent bonds via the exchange mechanism.

Examples of stable molecules in which the valency of carbon is IV are

compounds with hydrogen, oxygen, ...

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

The valency of carbon in all compounds is IV, hydrogen – I, oxygen – II.

Acetylene H–C ≡C–H is a flammable gas that is used to produce high-temperature flames, for example, in welding.

Conclusion: given this opportunity (vacant orbitals), atoms are able to pair their valence electrons in order to increase their covalency.

Donor-acceptor mechanism of covalent bond formation.

Mathematics is a great power. As follows from the above, 2 electrons (shared electron pair) are required to form a chemical bond.

Obviously, two electrons can be obtained:

However, there is another solution!

Donor-acceptor mechanism of covalent bond formation – a method of forming a covalent bond, in which one atom (donor) provides a pair of electrons for the formation of the bond, and the other atom (acceptor) provides a vacant (unoccupied) one.

orbital.

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

Example. The structure of the carbon monoxide molecule (carbon monoxide (II), carbon monoxide)

In a carbon monoxide molecule, the carbon and oxygen atoms are linked by two covalent bonds formed by metabolic mechanism.

However, since the carbon atom has an unfilled orbital at the 2p sublevel, and the oxygen atom has a lone pair of electrons, a third covalent bond is formed according to donor-acceptor mechanism

In writing, the donor-acceptor mechanism is represented by an arrow pointing away from

donor atom to the acceptor atom of a pair of electrons.

Correct structural formula of the carbon monoxide molecule.

Oxygen valence is III, carbon valence is III.

The triple bond between oxygen and carbon atoms is confirmed by the value

carbon-oxygen bond energy (the value is closer to the triple bond energy than to

double bond energy), data from spectral analysis methods.

2. Valence capabilities of atoms. Nitrogen.

The atoms of nitrogen, oxygen and fluorine differ significantly from their electronic

analogues due to the absence of the energy d-sublevel.

The electronic configuration of the nitrogen atom is 7 N 1s2 2s2 2p3.

Valence electrons 2s2 2p3 – 3 unpaired electrons and 1 electron pair.

It is obvious that in addition to three bonding pairs, the nitrogen atom has

1 lone pair of electrons (2s2).

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

Consequently, the nitrogen atom is capable of acting as a donor of a pair of electrons.

In the simplest case, PROTON acts as an acceptor: we are familiar with this example from the reaction of ammonia with acids to form ammonium salts.

Note:

1. The acceptor must have a vacant orbital (in this case, the hydrogen atom has lost an electron and has a vacant 1s-AO)

2. During a chemical reaction, charge is conserved (law of conservation of charge!).

The biggest mistake is the lack of charge, since the nitrogen atom is not able to form 4 bonds through the exchange mechanism.

3. The structure of the ammonium cation is depicted in the form of three covalent bonds N – H,

formed according to the exchange mechanism, indicated by valence primes, and

one covalent bond formed by a donor-acceptor mechanism,

indicated by an arrow from the nitrogen atom to the hydrogen atom. The positive charge must be shown either on the nitrogen atom (usually above the atom) or on the NH4 particle

is enclosed in square brackets and a “+” sign is drawn behind the brackets.

4. The maximum valency of nitrogen is FOUR - an atom has only 4 AOs, three of which contain unpaired electrons, and one contains an electron pair. The next energy level (3s) is too far away to be used to form a bond, so the nitrogen atom is unable to form the V valence.

You will learn about more complex cases of the formation of covalent bonds by a nitrogen atom a little later.

Lecture 3. Valence capabilities of atoms. Covalent chemical bond

3. Valence capabilities of atoms. Sulfur.

Electrons valence level sulfur atoms in the ground state have the configuration

16 S ... 3s 2 3p 4 – 2 electron pairs and 2 unpaired electrons.

Conclusion (octet rule) 1: when forming chemical compounds, atoms of elements tend to supplement their electronic configuration to the most stable one,

For example, in a hydrogen sulfide molecule, the sulfur atom forms an octet of electrons due to two bonding pairs with hydrogen atoms and two lone electron pairs

The octet rule is NOT MANDATORY, immutable - there are countless compounds in which the octet rule is not observed for one element or another, but it correctly predicts the general tendency to form compounds of similar stoichiometry.

For connections of d-elements there is a corresponding rule eighteen electrons, since this is the number of electrons that corresponds to a completely completed ns2 (n-1)d10 np6 – electron shell.

1 Doublet – 2, triplet – 3, quartet – 4, quintet – 5, sextet – 6, septet – 7, octet – 8. Thus, the octet rule is a rule eight electrons.

>> Chemistry: Valence capabilities of atoms of chemical elements

The structure of the outer energy levels of atoms of chemical elements mainly determines the properties of their atoms. Therefore, these levels are called valence levels. Electrons from these levels, and sometimes from pre-external levels, can take part in the formation of chemical bonds. Such electrons are also called valence electrons.

The valency of an atom of a chemical element is determined primarily by the number of unpaired electrons participating in the formation of a chemical bond.

Goals.

• Develop ideas about valency as the main property of an atom, identify patterns of changes in the radii of atoms of chemical elements in periods and groups of the periodic system.
• Using an integrated approach, develop students’ skills to compare, contrast, find analogies, and predict practical results based on theoretical reasoning.
• By creating situations of success, overcome the psychological inertia of students.
• Develop imaginative thinking and reflection abilities.

Equipment: Table “Valency and electronic configurations of elements”, multimedia.

Epigraph.Logic, if reflected in truth and common sense, always leads to the goal, to the correct result.

The lesson is combined, with elements of integration. Teaching methods used: explanatory-illustrated, heuristic and problem-based.

Stage I. Indicative and motivational

The lesson begins with “setting up” (music sounds - Symphony No. 3 by J. Brahms).

Teacher: The word “valency” (from Latin valentia) arose in the middle of the 19th century, during the period of completion of the second chemical-analytical stage in the development of chemistry. By that time, more than 60 elements had been discovered.

The origins of the concept of “valence” are contained in the works of various scientists. J. Dalton established that substances consist of atoms connected in certain proportions. E. Frankland, in fact, introduced the concept of valency as a connecting force. F. Kekule identified valency with a chemical bond. A.M. Butlerov drew attention to the fact that valency is related to the reactivity of atoms. DI. Mendeleev created the periodic system of chemical elements, in which the highest valency of the atoms coincided with the group number of the element in the system. He also introduced the concept of “variable valency”.

Question. What is valency?

Read through the definitions taken from various sources (the teacher shows slides through multimedia):

“Valency of a chemical element- the ability of its atoms to combine with other atoms in certain proportions.”

"Valence- the ability of atoms of one element to attach a certain number of atoms of another element.”

"Valence– property of atoms entering in chemical compounds, give or take a certain number of electrons (electrovalency) or combine electrons to form electron pairs common to two atoms (covalency).”

Which definition of valence do you think is more perfect and where do you see the others lacking? (Discussion in groups.)

Valence and valence possibilities are important characteristics of a chemical element. They are determined by the structure of atoms and change periodically with increasing nuclear charges.

Teacher. Thus, we conclude that:

What do you think the concept of “valence possibility” means?

Students express their opinions. They recall the meaning of the words “opportunity”, “possible”, clarify the meaning of these words in S.I. Ozhegov’s explanatory dictionary:

"Opportunity- a means, a condition necessary for the implementation of something”;

"Possible“one that can happen, feasible, permissible, permissible, conceivable.”

(teacher shows next slide)

Then the teacher summarizes.

Teacher. Valence possibilities of atoms are the permissible valences of an element, the entire range of their values ​​in various compounds.

Stage II. Operations and executive

Working with the table “Valence and electronic configurations of elements.”

Teacher. Since the valence of an atom depends on the number of unpaired electrons, it is useful to consider the structures of atoms in excited states, taking into account the valence possibilities. Let us write down the electron diffraction formulas for the distribution of electrons among orbitals in a carbon atom. With their help, we will determine what valency carbon C exhibits in compounds. An asterisk (*) denotes an atom in an excited state:

Thus, carbon exhibits valence IV due to vaporization
2s 2 – electrons and the transition of one of them to a vacant orbital. (Vacant - unoccupied, empty (S. I. Ozhegov))

Why is the valence C-II and IV, and H-I, He-O, Be – II, B – III, P-V?

Compare the electron diffraction formulas of the elements (Scheme No. 1) and establish the reason for the different valency.

Work in groups:

Teacher. So, what do the valence and valence capabilities of atoms depend on? Let's look at these two concepts in conjunction (diagram No. 2).

The energy consumption (E) to transfer the atom to an excited state is compensated by the energy released during the formation of a chemical bond.

What is the difference between an atom in the ground (stationary) state and an atom in an excited state (scheme No. 3)?

Teacher . Can the elements have the following valences: Li -III, O - IV, Ne - II?

Explain your answer using the electronic and electron diffraction formulas of these elements (diagram No. 4).

Work in groups.

Answer. No, because in this case the energy required to move the electron is

(1s -> 2p or 2p -> 3s) are so large that they cannot be compensated by the energy released during the formation of a chemical bond.

Teacher. There is another type of valence possibility of atoms - the presence of lone electron pairs (formation of a covalent bond according to the donor-acceptor mechanism):

Stage III. Evaluative-reflective

The results are summed up and the work of students in the lesson is characterized (return to the epigraph of the lesson). Then a summary is given - the attitude of the children to the lesson, subject, teacher.

1. What didn’t you like about the lesson?

2. What did you like?

3. What questions remain unclear for you?

4. Evaluation of the teacher’s work and your own work? (reasonable).

Homework(according to the textbook by O.S. Gabrielyan, Chemistry-10; profile level, paragraph No. 4, exercise 4)

The valence capabilities of an atom are determined by the number of unpaired electrons. In the process of formation of chemical compounds, these possibilities can be fully used or not realized, but they can also be surpassed. An increase in the number of unpaired electrons is possible when there are vacant orbitals in the atom, and the energy consumption for the transition of electrons from a normal to an excited state is compensated by the energy of formation of a chemical compound.

In the valence bond method, the formation of normal bonds requires the interaction of two half-occupied valence orbitals. Here it is assumed that atom A has one of the electrons and shares it with atom B, which in turn has another electron and allows atom A to also use this electron.

The valence capabilities of atoms are determined by the number of unpaired electrons, as well as the number of unshared electron pairs capable of moving to the free orbitals of an atom of another element (participate in the formation of a covalent bond according to the donor-acceptor mechanism).

The structure of the outer energy levels of atoms of chemical elements mainly determines the properties of their atoms. Therefore, these levels are called valence levels. Electrons of these levels, and sometimes of pre-external levels, can take part in the formation of chemical bonds. Such electrons are also called valence electrons.

The valence of an atom of a chemical element is determined primarily by the number of unpaired electrons participating in the formation of a chemical bond.

The valence electrons of the atoms of the elements of the main subgroups are located in the s- and p-orbitals of the outer electron layer. For elements of side subgroups, except for lanthanides and actinides, valence electrons are located in the s-orbital of the outer and d-orbitals of the pre-outer layer.

In order to correctly assess the valence capabilities of atoms of chemical elements, it is necessary to consider the distribution of electrons in them across energy levels and sublevels and determine the number of unpaired electrons in accordance with the Pauli principle and Hund’s rule for the unexcited (ground, or stationary) state of the atom and for the excited ( that is, having received additional energy, as a result of which the electrons of the outer layer are paired and transferred to free orbitals). An atom in an excited state is designated by the corresponding element symbol with an asterisk.

The valence capabilities of atoms of chemical elements are far from being limited to the number of unpaired electrons in the stationary and excited states of atoms. If you remember the donor-acceptor mechanism for the formation of covalent bonds, then two other valence possibilities of atoms of chemical elements will become clear to you, which are determined by the presence of free orbitals and the presence of unshared electron pairs that can give a covalent chemical bond according to the donor-acceptor mechanism.

Conclusion

The valence capabilities of atoms of chemical elements are determined:

1) the number of unpaired electrons (one-electron orbitals);

2) the presence of free orbitals;

3) the presence of unshared pairs of electrons.