3. Periodic law and periodic system chemical elements

3.4. Periodic change in the properties of substances

The following properties of prime and complex substances:

• structure simple substances(first non-molecular, for example from Li to C, and then molecular: N 2 - Ne);
• the melting and boiling points of simple substances: when moving from left to right along the period t melt and t boil, at first, in general, they increase (diamond is the most refractory substance), and then decrease, which is associated with a change in the structure of simple substances (see above);
• metal and non-metallic properties simple substances. Over the period, with increasing Z, the ability of atoms to donate an electron decreases (E and increases), respectively, the metallic properties of simple substances weaken (non-metallic ones increase, since E cf atoms increases). From top to bottom in groups A, on the contrary, the metallic properties of simple substances are enhanced, while non-metallic ones are weakened;
• composition and acid-base properties of oxides and hydroxides (Tables 3.1–3.2).

Table 3.1

The composition of higher oxides and the simplest hydrogen compounds of elements of A-groups

As can be seen from Table. 3.1, the composition of higher oxides changes smoothly in accordance with the gradual increase in the covalence (oxidation state) of the atom.

With an increase in the charge of the nucleus of an atom in a period, the basic properties of oxides and hydroxides weaken, and the acid properties increase. The transition from basic oxides and hydroxides to acid ones in each period occurs gradually, through amphoteric oxides and hydroxides. As an example, in Table. 3.2 shows the change in the properties of oxides and hydroxides of elements of the 3rd period.

Table 3.2

oxides and hydroxides, formed by elements 3rd period, and their classification

In groups A, with an increase in the charge of the atomic nucleus, the basic properties of oxides and hydroxides increase. For example, for the IIA-group we have:

1. BeO, Be (OH) 2 - amphoteric (weak basic and acid properties).

2. MgO, Mg(OH) 2 - weak, basic properties.

3. CaO, Ca (OH) 2 - pronounced basic properties (alkalis).

4. SrO, Sr(OH) 2 - pronounced basic properties (alkalis).

5. BaO, Ba (OH) 2 - pronounced basic properties (alkalis).

6. RaO, Ra (OH) 2 - pronounced basic properties (alkalis).

The same trends can be traced for elements of other groups (composition and acid-base properties of binary hydrogen compounds, see Table 3.1). In general, with an increase in the atomic number over the period, the basic properties of hydrogen compounds weaken, and the acidic properties of their solutions increase: sodium hydride dissolves in water with the formation of alkali:

NaH + H 2 O \u003d NaOH + H 2,

and aqueous solutions of H 2 S and HCl are acids, with hydrochloric acid being stronger.

1. In groups A, with an increase in the charge of the atomic nucleus, the strength of oxygen-free acids also increases.

2. In hydrogen compounds, the number of hydrogen atoms in a molecule (or formula unit) first increases from 1 to 4 (groups IA–IVA), and then decreases from 4 to 1 (groups IVA–VIIA).

3. Volatile (gaseous) at n.o. are only hydrogen compounds of elements of groups IVA–VIIA (except for H 2 O and HF)

The described trends in the change in the properties of atoms of chemical elements and their compounds are summarized in Table. 3.3

Table 3.3

Change in the properties of atoms of elements and their compounds with an increase in the charge of the atomic nucleus

Example 3.3. Specify the formula of the oxide with the most pronounced acidic properties:

Decision. The acidic properties of oxides increase from left to right along the period, and weaken from top to bottom in group A. With this in mind, we come to the conclusion that the acidic properties are most pronounced in the oxide Cl 2 O 7 .

Answer: 4).

Example 3.4. The anion of the element E 2− has the electronic configuration of the argon atom. Specify the formula of the highest oxide of the element atom:

Decision. The electronic configuration of the argon atom is 1s 2 2s 2 2p 6 3s 2 3p 6, therefore the electronic configuration of the atom E (atom E contains 2 electrons less than the ion E 2−) - 1s 2 2s 2 2p 6 3s 2 3p 4, which corresponds to the atom sulfur. The element sulfur is in the VIA group, the formula of the highest oxide of the elements of this group is EO 3.

Answer: 1).

Example 3.5. Indicate the symbol of the element whose atom has three electron layers and forms a volatile (n.o.) compound of the composition EN 2 (H 2 E):

Decision. Hydrogen compounds of the composition EN 2 (H 2 E) form atoms of elements of the IIA- and VIA-groups, however, they are volatile at n.o. are compounds of elements of the VIA-group, which include sulfur.

Answer: 3).

The characterized trends in the change in the acid-base properties of oxides and hydroxides can be understood on the basis of the analysis of the following simplified diagrams of the structure of oxides and hydroxides (Fig. 3.1).

From a simplified reaction scheme

it follows that the efficiency of the interaction of the oxide with water with the formation of a base increases (according to the Coulomb law) with an increase in the charge on the ion E n + . The value of this charge increases as the metallic properties of the elements increase, i.e. from right to left across the period and from top to bottom across the group. It is in this order that the main properties of the elements increase.

Rice. 3.1. Scheme of the structure of oxides (a) and hydroxides (b)

Let us consider the reasons underlying the described changes in the acid-base properties of hydroxides.

With an increase in the degree of oxidation of the element + n and a decrease in the radius of the ion E n + (this is exactly what is observed with an increase in the charge of the nucleus of the element's atom from left to right along the period), the E–O bond is strengthened, and the O–H bond weakens; the process of dissociation of the hydroxide according to the acid type becomes more probable.

From top to bottom in the group, the radius E n + increases, and the value of n + does not change, as a result, the strength of the E–O bond decreases, its rupture becomes easier, and the process of hydroxide dissociation according to the main type becomes more likely.

General properties of base classes inorganic compounds. Conditions for the occurrence of "reactions of exchange".

1. Acid-base properties of hydrogen compounds.

a) Comment on the ability of water to self-ionize (equation, K W). Based on the structure of molecules (their polarizability), explain the patterns of changes in solubility in water and the acid-base properties of the corresponding solutions of methane (CH 4), ammonia (NH 3), hydrogen fluoride (HF) and hydrogen chloride (HCl). Write the necessary equations.

b) Using the concept of the polarizing effect of cations on the H–O bond, and also, taking into account the number of hydroxo groups, explain the pattern of changes in the acid-base properties of hydroxides LiOH–Be(OH) 2 –H 3 BO 3 –H 2 CO 3 –HNO 3 –H 3 PO 4 -H 2 SO 4 - (H 2 SeO 4) - HClO 4. Make up the dissociation equations of the proposed substances.

2. Mandatory and optional(including special ones) reactions of acids and bases.

a) Which of the following substances (solutions) can interact with 20% solutions of nitric, sulfuric and acetic acids: solutions of KOH, NH 3, H 2 S; Zn(OH) 2 , H3PO2; BaCl 2 and crystalline Cu, Ca 3 (PO 4) 2 .

b) Which of the following substances (solutions) can interact with 20% solutions of potassium hydroxide and ammonia: solutions of H 2 SO 4 , CH 3 COOH; Zn(OH) 2, Al(OH) 3 ; MgCl 2 and crystalline Ag2O, AgCl.

In both variants of the experiment, formulas of substances are highlighted in bold type, interaction with which will require writing non-obvious equations.

The task involves only a theoretical discussion, but ... The reaction equations must be thought out, written in advance, including in ionic form.

3. Conditions for the occurrence of exchange reactions with salts.

What exchange reactions can be performed using the proposed reagents: dilute solutions MnSO 4, Ba(NO3)2, saturated solution SrSO4, crystalline CuS and FeS, as well as concentrated solutions of HCl, CO 2 and NH 3. Consider the possibility of performing reactions with the mandatory participation of salt. Justify your proposals by calculating the constants of the corresponding exchange equilibria. Consider possible signs of a reaction taking place.

At the same time, it should be borne in mind that if substances that are sparingly soluble in water (in this case, CuS and FeS) are used as a reagent, then reactions involving them must necessarily be accompanied by dissolution, i.e. the products of such reactions should not precipitate themselves. For example, it is illiterate to think over the reaction of FeS ↓ and H 2 CO 3 in the hope of obtaining a precipitate of FeCO 3.

Reactions with rich solution SrSO4 suggest the use of solution over sediment rather than the sediment itself.

4. Dependence of the pH of solutions on the composition of salts.

Determine the hydrolyzability of the ions of the proposed salts (NH 4 NO 3, KCl, CH 3 COONa, Na 2 CO 3, AlCl 3, CH 3 COONH 4),

Make the equations for the hydrolysis of an ion (ions, if both the cation and the anion of the salt are involved in the hydrolysis); calculate the hydrolysis constant ( To G (Al 3+) take equal to ~10 -5).

write an equation in molecular form

(molecular equation make up by predominant ionic reaction ).

Arrange the salts in order of increasing hydrolyzability.

Check hydrolyzability experimentally. To do this, pour ~1 ml of the corresponding solution into a clean test tube, moisten a glass rod in this solution, and apply the solution to indicator paper. Use the colored scale to estimate the approximate pH value of the solution. Why in two cases the pH corresponds neutral environment?

5. Medium in solutions of medium and acidic salts.

Make up the equations of the prevailing ionic reactions that affect the environment in solutions of potassium ortho-, hydro- and dihydrogen phosphate (K 3 PO 4, K 2 HPO 4, KH 2 RO 4). In this case, it must be borne in mind that in solutions of acidic salts, in addition to hydrolysis reactions, the dissociation of the anions H 2 PO 4 – , HPO 4 2 – also takes place. The medium will be determined by the predominant reaction. Compare the constants of competing reactions of hydrolysis and dissociation of anions and draw a conclusion about the pH (more or less than 7). Compare the results of the preliminary analysis with the actual pH value (determine with a universal indicator).

Reference data for preparation for experiments 3, 4, 5

Modern wording Periodic Law : the properties of simple substances, as well as the forms and properties of compounds of elements, are in a periodic dependence on the magnitude of the charge of the nuclei of their atoms (serial number).

Periodic properties are, for example, the radius of an atom, the ionization energy, the electron affinity, the electronegativity of the atom, and some physical properties elements and compounds (melting and boiling points, electrical conductivity, etc.).

The expression of the Periodic Law is

periodic table of elements .

The most common variant short form periodic system, in which the elements are divided into 7 periods and 8 groups.

At present, the nuclei of atoms of elements up to number 118 have been obtained. The name of the element with the serial number 104 is rutherfordium (Rf), 105 is dubnium (Db), 106 is seaborgium (Sg), 107 is bohrium (Bh), 108 is hassium (Hs ), 109 – meitnerium ( Mt), 110 - darmstadtium (Ds), 111 - roentgenium (Rg), 112 - copernicium (Cn).
On October 24, 2012, in Moscow, at the Central House of Scientists of the Russian Academy of Sciences, a solemn ceremony was held to name the 114th element "Flerovium" (Fl), and the 116th - "Livermorium" (Lv).

Periods 1, 2, 3, 4, 5, 6 contain 2, 8, 8, 18, 18, 32 elements, respectively. The seventh period is not completed. Periods 1, 2 and 3 are called small the rest - large.

In periods from left to right, metallic properties gradually weaken and non-metallic properties increase, since with an increase in the positive charge of the nuclei of atoms, the number of electrons in the outer electron layer increases and a decrease in the atomic radii is observed.

At the bottom of the table are placed 14 lanthanides and 14 actinides. AT recent times lanthanum and actinium were classified as lanthanides and actinides, respectively.

Groups are divided into subgroups - main, or subgroups A and side, or subgroup B. Subgroup VIII B - special, it contains triads elements that make up the families of iron (Fe, Co, Ni) and platinum metals (Ru, Rh, Pd, Os, Ir, Pt).

From top to bottom, in the main subgroups, metallic properties increase and non-metallic properties weaken.

The group number usually indicates the number of electrons that can participate in the formation chemical bonds. This is what physical meaning group numbers. For elements of secondary subgroups, the valence electrons are not only the outer, but also the penultimate layers. This is the main difference in the properties of the elements of the main and secondary subgroups.

Periodic system and electronic formulas of atoms

To predict and explain the properties of elements, it is necessary to be able to write down the electronic formula of an atom.

In an atom located in basic condition, each electron occupies a vacant orbital with the lowest energy. The energy state is determined primarily by temperature. The temperature on the surface of our planet is such that the atoms are in the ground state. At high temperatures, other states of atoms, which are called excited.

Location sequence energy levels in ascending order of energy is known from the results of solving the Schrödinger equation:

1s< 2s < 2p < 3s < Зр < 4s 3d < 4p < 5s 4d < 5p < 6s 5d 4f < 6p.

Consider the electronic configurations of atoms of some elements of the fourth period (Fig. 6.1).



Rice. 6.1. The distribution of electrons over the orbitals of some elements of the fourth period

It should be noted that there are some features in electronic structure atoms of elements of the fourth period: for atoms Cr and C u by 4 s-shell contains not two electrons, but one, i.e., there is "failure" external s -electron to the previous d-shell.

Electronic formulas of 24 Cr and 29 Cu atoms can be represented as follows:

24 Cr 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 ,

29 Cu 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 1 .

The physical reason for the “violation” of the filling order is related to the different penetrating power of electrons into the inner layers, as well as the special stability of the electronic configurations d 5 and d 10 , f 7 and f 14 .

All elements are divided into four types

:

1. At atoms s-elements filled with s - outer layer shells ns . These are the first two elements of each period.

2. At atoms p-elements electrons fill the p-shells of the outer level np . These include the last 6 elements of each period (except the first and seventh).

3. Do d-elements filled with electrons d - sublevel of the second outside level ( n-1)d . These are elements of intercalated decades of large periods located between s- and p-elements.

4. Do f-elements filled with electrons f - sublevel of the third outside level ( n-2)f . These are the lanthanides and actinides.

Changes in the acid-base properties of compounds of elements by groups and periods of the periodic system
(Kossel scheme)

To explain the nature of the change in the acid-base properties of the compounds of the elements, Kossel (Germany, 1923) proposed using a simple scheme based on the assumption that there are purely ionic bond and there is a Coulomb interaction between the ions. The Kossel scheme describes the acid-base properties of compounds containing E–H and E–O–H bonds, depending on the charge of the nucleus and the radius of the element forming them.

Kossel scheme for two metal hydroxides (for LiOH and KOH molecules ) is shown in Fig. 6.2. As can be seen from the presented scheme, the ion radius Li + less than the ion radius K+ and OH - group is more strongly bonded to the lithium ion than to the potassium ion. As a result, KOH will be easier to dissociate in solution and the basic properties of potassium hydroxide will be more pronounced.



Rice. 6.2. Kossel scheme for LiOH and KOH molecules

Similarly, one can analyze the Kossel scheme for two bases CuOH and Cu(OH) 2 . Since the radius of the Cu ion 2+ less, and the charge is greater than that of an ion Cu + OH - - the group will be stronger to hold the Cu 2+ ion .
As a result, the base Cu(OH)2 will be weaker than CuOH.

Thus, base strength increases as the cation radius increases and its positive charge decreases .

Kossel's scheme for the two anoxic acids HCl and HI shown in fig. 6.3.



Rice. 6.3. Kossel scheme for HCl and HI molecules

Since the radius of the chloride ion is smaller than that of the iodide ion, the H + ion more strongly bound to the anion in the hydrochloric acid molecule, which will be weaker than hydroiodic acid. Thus, the strength of anoxic acids increases with increasing negative ion radius.

The strength of oxygen-containing acids changes in the opposite way. It increases with a decrease in the ion radius and with an increase in its positive charge. On fig. 6.4 shows the Kossel scheme for two acids HClO and HClO 4 .



Rice. 6.4. Kossel scheme for HClO and HClO 4

Ion С1 7+ is strongly bound to the oxygen ion, so the proton will be more easily split off in the HClO molecule 4 . At the same time, the bond of the C1 ion+ with O ion 2- less strong, and in the HClO molecule the proton will be more strongly retained by the O anion 2-. As a result, HClO 4 is a stronger acid than HClO.

Thus, an increase in the oxidation state of an element and a decrease in the radius of the element's ion enhance the acidic nature of the substance. On the contrary, a decrease in the degree of oxidation and an increase in the radius of the ion enhance the basic properties of substances.

Examples of problem solving

Compose electronic formulas of the zirconium atom and ions O 2–, Al 3+, Zn 2+ . Determine what type of elements Zr, O, Zn, Al atoms belong to.

40 Zr 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 2 5s 2 ,

O 2– 1s 2 2s 2 2p 6 ,

Zn 2+ 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 ,

Al 3+ 1s 2 2s 2 2p 6 ,

Zr - d-element, O - p-element, Zn - d-element, Al - p-element.

Arrange the atoms of the elements in order of increasing ionization energy: K, Mg, Be, Ca. Justify the answer.

Decision. Ionization energy is the energy required to detach an electron from an atom in the ground state. In the period from left to right, the ionization energy increases with increasing nuclear charge, in the main subgroups from top to bottom it decreases, as the distance from the electron to the nucleus increases.

Thus, the value of the ionization energy of the atoms of these elements increases in the series K, Ca, Mg, Be.

Arrange atoms and ions in ascending order of their radii: Ca 2+ , Ar, Cl – , K + , S 2– . Justify the answer.

Decision. For ions containing the same number of electrons (isoelectronic ions), the radius of the ion will increase with a decrease in positive and an increase in its negative charge. Therefore, the radius increases in the series Ca 2+ , K + , Ar, Cl – , S 2– .

Determine how the radii of ions and atoms change in the series Li + , Na + , K + , Rb + , Cs + and Na, Mg, Al, Si, P, S.

Decision. In the series Li + , Na + , K + , Rb + , Cs + the radius of the ions increases, since the number of electron layers increases in ions of the same sign with a similar electronic structure.

In the series Na, Mg, Al, Si, P, S, the radius of atoms decreases, since with the same number of electron layers in atoms, the charge of the nucleus increases, and, hence, the attraction of electrons by the nucleus.

Compare the strength of the acids H 2 SO 3 and H 2 SeO 3 and the bases Fe (OH) 2 and Fe (OH) 3.

Decision. According to the Kossel scheme H 2 SO 3 stronger acid than H 2 SeO 3 , since the ion radius Se4+ greater than the ion radius S 4+, therefore, the bond S 4+ - O 2– is stronger than bond Se 4+ - O 2-.

According to the Kossel scheme, Fe(OH)

2 stronger base, because the radius of the Fe ion 2+ more than Fe ion 3+ . In addition, the charge of the Fe ion 3+ more than the Fe ion 2+ . As a result, the bond Fe 3+ – O 2– is stronger than Fe 2+ - O 2- and OH ion - easily split off in the molecule Fe(OH)2.

Tasks for independent solution

6.1.Compose electronic formulas of elements with a nuclear charge of +19, +47, +33 and in the ground state. Specify what type of elements they belong to. What oxidation states are typical for an element with a nuclear charge of +33?




6.2.Compose the electronic formula of the ion Cl – .

Acid properties are those that are most pronounced in a given environment. There are a number of them. It is necessary to be able to determine the acidic properties of alcohols and other compounds, not only to reveal the content of the corresponding medium in them. It is also important for the recognition of the studied substance.

There are many tests for the presence of acidic properties. The most elementary - immersion in the indicator substance - litmus paper, which reacts to the content of hydrogen, turning pink or blushing. Moreover, a more saturated color demonstrates a stronger acid. And vice versa.

The acidic properties increase along with the increase in the radii of the negative ions and hence the atom. This provides easier detachment of hydrogen particles. This quality is hallmark strong acids.

There are the most characteristic acid properties. These include:

Dissociation (cleavage of the hydrogen cation);

Decomposition (formation and water under the influence of temperature and oxygen);

Interaction with hydroxides (as a result of which water and salt are formed);

Interaction with oxides (as a result, salt and water are also formed);

Interaction with metals preceding hydrogen in the activity series (salt and water are formed, sometimes with gas evolution);

Interaction with salts (only if the acid is stronger than the one that formed the salt).

Often chemists have to make acids themselves. There are two ways to get them out. One of them is mixing acid oxide with water. This method is used most often. And the second is the interaction of a strong acid with a weaker salt. It is used less frequently.

It is known that acidic properties are manifested and in many they can be more or less pronounced, depending on K, the properties of alcohols are manifested in the ability to split off a hydrogen cation when interacting with alkalis and metals.

Alcoholates - salts of alcohols - are able to hydrolyze under the action of water and release alcohol with metal hydroxide. This proves that the acidic properties of these substances are weaker than those of water. Consequently, the environment is expressed in them more strongly.

The acidic properties of phenol are much stronger due to the increased polarity of the OH compound. Therefore, this substance can also react with hydroxides of alkaline earth and alkali metals. As a result, salts - phenolates are formed. To detect phenol, it is most effective to use with (III), in which the substance acquires a blue-violet color.

So, the acidic properties in various compounds manifest themselves in the same way, but with different intensity, which depends on the structure of the nuclei and the polarity of the hydrogen bonds. They help determine the environment of a substance and its composition. Along with these properties, there are also basic ones, which increase with the weakening of the first.

All these characteristics appear in most complex substances and form an important part of the world around us. After all, it is at their expense that many processes take place not only in nature, but also in living organisms. Therefore, acidic properties are extremely important, without them life on earth would be impossible.