The arrangement of elements in the periodic table in accordance with their atomic number and external electronic configuration determines the manifestation of two important patterns in the chemical properties of non-transition elements and their compounds:
1. Elements with similar chemical properties are divided into groups.
For example, all alkali metals are in group I, and all halogens are in group VII.
2. The most electropositive elements, and therefore the most reactive metals, are located in the lower left corner of the periodic table. The electropositivity of the elements gradually decreases as one moves from bottom to top along each group and as one moves from left to right along each period.
The most electronegative elements, and therefore the most reactive nonmetals, are located in the upper right corner of the periodic table. The electronegativity of elements increases as you move along each period in the direction from group I to group VII, but decreases as you move from top to bottom along each group.
Table 11.11. Regularities in the formation of compounds by elements of the 2nd and 3rd periods
Table 11.12. Examples of p-element ligands in d-element complex ions
The electronegativity or electropositivity of elements is directly related to the types of chemical reactions that the elements are capable of engaging in, and therefore to the types of compounds formed by the elements. s-Metals are characterized by their ability to readily form cations and thus ionic compounds (see Table 11.11). -Elements located closer to the center of the periodic table are characterized by the ability to form only covalent compounds. More electronegative p-elements located closer to the right edge of the periodic period
Rice. 11.11. Periodic changes in oxidation states of non-transition elements
Rice. 11.12. Periodic changes in the oxidation states of d-elements of the first, second and third transition series (i.e., 4th, 5th and 6th periods, respectively).
Table 11.13. Characteristic valences of elements of the 3rd period
tables are capable of forming both covalent and ionic compounds. Noble gases, which have a stable electronic configuration, form relatively few compounds.
As can be seen from Fig. 11.3, d-elements are located in the periodic table between groups II and III. They are all metals, but less electropositive and therefore more electronegative than the s-metals (alkali and alkaline earth metals). As a result, their compounds, such as oxides and chlorides, tend to be either ionic with a high degree of covalent character or covalent. Together with p-elements located closer to the central part of the periodic table, they often form compounds of a high molecular weight type or compounds with a layered or chain structure.
d-elements have the ability to form both cationic and anionic complex ions, which is not typical for s-metals. p-Elements are often included in ligands in both cationic and anionic complexes (Table 11.12).
The valences (see Chapter 4) of intransitive elements also show periodic changes. From the table 11.13 it is clear that all elements of the 3rd period exhibit valences that numerically coincide with the group number of the element. In addition, all elements of groups IV-VII exhibit valences equal to the difference between the number 8 and their group number.
The maximum oxidation states of elements also show periodic changes (Figs. 11.11 and 11.12). As a rule, they increase when moving from left to right along the period and reach maximum values in groups V-VII. Noteworthy is the fact that elements with higher oxidation states exhibit, in addition to them, many other oxidation states. For example, chlorine can exist in states with all oxidation states from -1 to
In all three series of transition metals (elements), the maximum oxidation state is achieved in the middle part of the series (Fig. 11.12). -Elements with higher oxidation states exhibit, in addition to them, a maximum number of other oxidation states. For example, in the first row of transition metals, manganese exhibits five positive oxidation states from to
Periodicity of redox properties
The redox properties of elements also show periodic changes. The pattern of these changes is as follows: the elements occupying the left side of the periodic table, i.e. alkali and alkaline earth metals (metals), are strong reducing agents. Then, as you move to the right along each period, the elements become increasingly weaker reducing agents and increasingly stronger oxidizing agents. Finally, when moving to group VII, the elements become strong oxidizing agents. Let us now consider this pattern in somewhat more detail.
The reducing properties of metals are characterized by:
low ionization energy,
low electron affinity,
low electronegativity,
high “electropositivity” (qualitative term - see previous footnote),
negative standard redox potential.
1. Reaction with air or oxygen
2. Reaction with chlorine
Reaction with dilute acids
All of these are examples of the reducing power of s-metals, since in each case the metal easily gives up electrons:
A detailed discussion of the chemistry of alkali and alkaline earth metals is provided in Chap. 13.
The oxidative properties of group VII elements are characterized by: high ionization energy, high electron affinity, high electronegativity, low “electropositivity”,
positive standard redox potential.
Chlorine has the properties of a strong oxidizing agent. It reacts violently with hydrogen in sunlight to form hydrogen chloride. In contrast, it does not react with other oxidizing agents such as oxygen or dilute acids. A detailed discussion of the chemistry of chlorine and other halogens is provided in Chap. 16.
Properties of elements from the middle part of periods. Group VII elements are p-elements, which are located on the right side of the periodic table. p-elements located closer to the middle part of the periods exhibit weak reducing and (or) weak oxidizing properties. For example, silicon belonging to group IV reacts slowly with oxygen to form an oxide
Nitrogen belonging to group V can act both as a weak reducing agent and as a weak oxidizing agent. For example, it behaves as a weak reducing agent when reacting with oxygen:
In contrast, when reacting with hydrogen, nitrogen behaves as a weak oxidizing agent:
Transition elements have the properties of weak reducing agents. For example, red-hot iron reacts with water vapor to form hydrogen:
Periodicity of properties of compounds
Periodic patterns of change are also found in the formation, structure, as well as physical and chemical properties of compounds. We will trace these patterns using the example of oxides, hydrides, hydroxides and halides.
Oxides. The reactivity of elements in interaction with oxygen, generally speaking, decreases as one moves to the right along each period. For example, in the 3rd period, two s-metals, sodium and magnesium, and two p-elements, aluminum and phosphorus, react violently with oxygen to form oxides. In the same period, the elements silicon and sulfur are only able to react slowly with oxygen. Chlorine and argon, located at the right end of the period, do not react with oxygen at all.
Electropositive s-metals form ionic oxides, such as sodium oxide and magnesium oxide. Oxides of elements located in the middle and right parts of the period are predominantly covalent compounds, such as oxides of nitrogen and sulfur.
The acid-base character of the oxides also changes from basic in oxides of elements on the left side of the period to amphoteric in oxides of elements in the middle part of the period and then to acidic in oxides of elements on the right side of the period. For example, s-metals usually form oxides that dissolve in water to form alkaline solutions:
Molecular oxides of p-elements, such as carbon dioxide and sulfur trioxide, usually have acidic properties. The natural change in basic properties with the transition to acidic properties is clearly manifested in oxides of elements of the 3rd period.
Oxides of d-elements are usually insoluble in water and have basic properties, although one or two of them. for example, zinc oxide, exhibit amphoteric properties (see Chapter 14).
A detailed discussion of the chemistry of oxides is given in Sect. 15.4.
Hydrides. In the formation, structure and properties of hydrides, patterns can be traced that are similar to those described above for oxides, although not completely identical to them.
s-Metals, such as sodium and magnesium, typically react violently with dry hydrogen when heated, forming ionic hydrides. These ionic hydrides have basic properties. The most electronegative elements on the right side of the periods, such as sulfur and chlorine, react with hydrogen to form covalent hydrides, which have acidic properties. The exceptions are methane, which is a neutral compound, and ammonia, which has basic properties.
More electronegative elements, such as aluminum, silicon and phosphorus, do not react with hydrogen when heated.
Transition d-metals react with hydrogen when heated, forming non-stoichiometric hydrides.
The preparation, structure and properties of hydrides are described in detail in Chap. 12.
Hydroxides. The hydroxides of the most electropositive elements, such as sodium and calcium, are ionic compounds with strongly basic properties. In contrast, the highly electronegative element chlorine forms an acidic hydroxide, hypochlorous acid. In this compound, the bond between the chlorine and oxygen atoms is covalent. Hydroxides of some less electronegative elements have amphoteric properties. They are often unstable and form oxides.
Table 11.14. Properties of chlorides of elements of the 3rd period
Halides. Halides exhibit periodic changes in properties similar to those described above for oxides, hydrides and hydroxides. When moving to the right along the period from the most electropositive to the most electronegative elements, a decrease in the boiling point and melting point is observed (Table 11.14). Thus, the chlorides of the first three elements in the 3rd period under normal conditions are solids, the chlorides of the next three elements are liquids, and chlorine is a gaseous substance.
The ionic character of chlorides decreases as one moves to the right along the period, while the covalent character, on the contrary, increases.
Halides of s-elements are generally salts of strong acids and strong bases. They dissolve in water to form neutral solutions. Chlorides of p- and d-elements are characterized by the ability to react with water, forming acidic solutions. For example,
The reactions of d-element chlorides in water are described in Chapter. 14, and halide chemistry is discussed in more detail in Chap. 16.
Diagonal relationships between elements
It was previously noted that the electropositivity of elements generally decreases as one moves to the right along a period, but increases as one moves down the group. This gives rise to so-called diagonal relationships in the periodic table. Each diagonal relationship links a pair of elements with similar chemical properties. The most important pairs of elements related to each other by diagonal relationships are lithium and magnesium, beryllium and aluminum, boron and silicon.
The presence of diagonal relationships is explained by the fact that the decrease in electropositivity when moving to each next element to the right along the period is compensated by an increase in electropositivity when moving to the next element down the group. A more detailed discussion of diagonal relations is carried out in Chap. 13.
Anomalies
Head elements in main subgroups. Period 2 elements that “head” groups I-VII (the main subgroups in the short-period form of the periodic table - Transl.) are sometimes called head elements. They are of interest due to the fact that some properties of these elements and their compounds differ significantly from similar properties characteristic of other elements of the corresponding groups. These anomalous properties can be attributed to the smaller size of the parent element atoms and their higher electronegativity and ionization energies. For example, lithium and beryllium halides exhibit a more covalent character than the halides of other metals from the corresponding groups. Lithium, unlike other alkali metals, does not form solid hydrogen carbonate. While other alkali metal nitrates decompose when heated to form the corresponding nitrites and oxygen, lithium nitrate decomposes to form lithium oxide, oxygen and nitrogen dioxide. Finally, unlike hydroxides of other alkali metals, lithium hydroxide is thermally unstable. The anomalous properties of lithium and other lead elements are discussed in detail in Chap. 13, 14 and 16.
So let's say it again!
1. The elements in the modern periodic table are arranged in ascending order of their atomic number.
2. Elements of the same period have the same electron core, with the same configuration as the noble gas that completes the previous period.
3. Elements of the same group have the same external electronic configuration.
4. All -elements (with the exception of hydrogen and helium), as well as d- and -elements belong to metals.
5. Hydrogen and helium are non-metals. All other non-metals belong to p-elements.
6. When moving from left to right along a period, the metallic properties of the elements weaken, and when moving from top to bottom along the group, the metallic properties of the elements increase.
7. The physical properties of elements (melting and boiling points, enthalpies of melting and evaporation, density) when moving from left to right along a period, first increase, and then, after reaching a maximum in the middle part of the period, decrease.
8. Atomic and ionic radii of elements decrease when moving from left to right along a period, and increase when moving from top to bottom along a group.
9. The first ionization energy of elements increases when moving from left to right along a period, and decreases when moving from top to bottom along a group.
10. The electronegativity of elements within each period increases, reaching a maximum for the halogens, and decreases when moving from top to bottom in the group.
11. The most electropositive, and therefore the most reactive
capable (active) metals are located in the lower left corner of the periodic table.
12. The most electronegative elements are located in the upper right corner of the periodic table.
13. s-Elements, as a rule, exhibit valences that coincide with their group number.
14. p-elements have major valences equal to their group number, as well as equal differences between the number 8 and their group number.
15. d-Elements exhibit many different valences and oxidation states.
16. The restorative properties of elements weaken when moving from top to bottom along the group.
17. The reactivity of elements towards oxygen decreases when moving from top to bottom along the group.
18. When moving from left to right along a period, the ionic character of the oxides decreases, and the covalent character increases.
19. Oxides, hydrides, hydroxides and halides of elements exhibit the same periodicity in changes in properties.
20. Lithium and magnesium have similar chemical properties and thus demonstrate a diagonal relationship between them.
21. The head elements that lead the main groups have anomalous properties in relation to the remaining elements of their groups.
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Tests A2.
2-1. The number of the period in which the chemical element is located characterizes
3) higher oxide formula 4) higher valence
2-2. Group number element in the periodic table corresponds to
charge of the nucleus of an atom of this element
number of electrons in the valence shell of an atom
number of electronic levels of an atom of this element
the average value of the mass numbers of isotopes of this element.
1) basic properties of higher hydroxides
2) reducing properties of hydrogen compounds;
3) acidic properties of higher hydroxides
4) basic properties of higher oxides.
2-4. Among all the elements of the 3rd period, the element sodium has...
1) the highest electronegativity 3) the smallest atomic radius
2) the most pronounced metallic properties 4) the highest density
2-5. For elements of main subgroups group number characterizes...
1) number of filled energy levels 2) number of valence electrons
3) state of aggregation of a simple substance 4) lower valency
2-6. Among all the elements of the main subgroup of group VII (excluding hydrogen), the element fluorine has
1) the weakest non-metallic properties 2) the largest atomic mass
3) the smallest electronegativity 4) the smallest atomic radius
2-7. Among all the elements of the main subgroup of group IV, the element lead has...
1) the highest valence
2) the least pronounced metallic properties
3) the largest atomic radius
4) the highest electronegativity
2-8. The two elements have the following valence electron configurations:
Ps 2 etc 3 and ms 2 (m-1)d 3 . It is known that T not equal P. What do these elements have in common?
1) highest oxidation state 2) formula of hydrogen compound
3) period number 4) number of filled energy levels
2-9. The hundredth element of the periodic table D.I. Mendeleev is
2-10. The chemical element with atomic number 81 is
1)s – element 2)p – element 3)d – element 4)f – element
2-11. All d-elements are:
1) typical non-metals 2) metals
3) efficient semiconductors 4) ideal insulators
2-12. The s-elements include:
1) sodium and zinc; 2) potassium and barium; 3) silver and gold
2-13. An element of period IV, which forms the higher oxide EO 3 with oxygen, but does not form volatile compounds with hydrogen - this is...
1) selenium 2) lead 3) germanium 4) chromium
2-14. Are the following judgments true about the properties of compounds of an element whose atomic electronic configuration is 1s 2 2s 2 2р 6 3з 2 3р 4
A. This element forms a hydroxide with pronounced acidic properties.
B. The oxidation state of this element in the higher hydroxide is +4. .
1) only A is true 2) only B is true
3) both judgments are correct 4) both judgments are incorrect
2-15. An element of the fourth period, the highest oxide of which has the formula EO and which forms a salt-like compound with the composition EN 2 with hydrogen - this is...
1) calcium 2) zinc 3) beryllium 4) selenium
2-16. The formula of the highest oxide of an element is E 2 O. What configuration of valence electrons is possible for this atom? 1) 3s 1 2) 3d 1 4s 2 3) 2s 2 2p 1 4) 2s 2 2p 5
2-17.Determine the (hypothetical) formula of the highest oxide of element 115 of the periodic table. 1)EO 2 2)E 2 O 5 3)EO 4 4)E 2 O 3
2-18.The formula of the highest oxide of some element is EO 3. What configuration of valence electrons can this element have in its ground state?
1) 4d 6 3) 3s 2 Зр 4 2) 2s 2 2р 4 4) 3s 1 3d 5
2-19. Formula of the highest oxide of an element with the electronic configuration of the atom 1s 2 2s 2 2p 6 3s 2 3p 6 3d 3 4s 2 1) EO 2) E 2 O 3) E 2 O 3 4) E 2 O 5
2-20. The formula for a hydrogen compound of an element with the electronic configuration of the atom is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 5s 2
1) EN 2) EN 2 3) EN 3 4) EN 6
2-21. What volatile hydrogen compound is characteristic of an element whose higher oxide has the formula E 2 O 7: 1) NE 2) H 2 E 3) EN 3 4) EN 7
2-22. An element with a core charge of +32 corresponds to a higher oxide:
1)EO 2 2)E 2 O 5 3)EO 4)E 2 O 3
2-23. In the row: Al Si P S
1) the metallic properties of the elements are enhanced
2) the metallic properties of the elements weaken
3) non-metallic properties of elements weaken
4) the highest oxidation state of elements decreases
2-24. Among the listed elements, metals include:
1) barium 2) silicon 3) helium 4) boron 5) fluorine
2-25. The most pronounced metallic properties are
1) Al 2) Na 3) Mg 4) Be 5) Fe
2-26. Metallic properties are enhanced in the following order:
1) K – Na – Li 2) Mg – Ca – K 3) Rb – Sr – Y 4) In – Ga – Ge
2-27. Which element has the most pronounced metallic properties?
1)K 2)Na 3)Ca 4)Mg 5)Be
2-38.In which series are simple substances arranged in order of increasing metallic properties?
1)Mg, Ca, Ba 2)Na, Mg, Al 3)K, Ca, Fe 4)Sc, Ca, Mg
2-29.In which series are simple substances arranged in order of increasing their metallic properties?
1) Na, Mg, Al 2) K, Na, Be 3) Li, Na, K 4) Ba, Sr, Ca
2-30.Which group of elements contains only metals?
1) Li, Be, B 2) K, Ca, Sr 3) Li, Si, Na 4) Se, Te, Po
2-31. Has the most pronounced metallic properties
1) Na 2) K 3) Mg 4) A1
2-32. Has the least pronounced metallic properties
1) Rb 2) Sr 3) Ca 4) K
2-33. In which series are the chemical elements arranged in order of increasing metallic properties?
1) Na, Mg, Al 2) Al, Mg, Na 3) Ca, Mg, Be 4) Mg, Be, Ca
2-34. Which element has the most pronounced non-metallic properties?
1)S 2)Se 3)Te 4)Po 5)As
2-35. In order of increasing non-metallic properties, they are located
1) S-Se 2) Se-Br 3) Br-I 4) I-Te
2-36. Non-metallic properties of group A elements are enhanced
1) from left to right and in groups from bottom to top
2) from right to left and in groups from top to bottom
3) from right to left and in groups from bottom to top
4) from left to right and in groups from top to bottom
2-37. Are the following statements true? nonmetals?
A. In the periodic table, nonmetals are located in the right, mainly upper part.
B. Among non-metals there is not a single d-element.
1) only A is true 2) only B is true 3) both judgments are correct 4) both judgments are incorrect
2-38. Which metal atoms contain five electrons in the ground state at the energy d-sublevel:
1) Iron 4) Vanadium
2) Manganese 5) Chromium
2-39. Which metal atoms contain five electrons in the ground state at the energy d-sublevel:
1) Chrome 5) Germany
2) Silver 6) Rubidium
3) Zinc 7) Cadmium
4) Potassium 8) Manganese
2-40. In which series are the chemical elements arranged in order of increasing atomic radius? 1) Li, Na, K, Rb 2) Sr, Ca, Mg, Be 3) In, Ga, Al, B 4) Sn, Ge, Si, C
2-41. Which of the following series of chemical elements is characterized increasing atomic radii?
1)Te, Se, S, O 2)Na, Mg, Al, Si 3)C, B, Be, Li 4)Ba, Al, Ga, Ge
2-42. Among the elements of group VIA, the maximum radius atom has
1) oxygen 2) sulfur 3) tellurium 4) polonium
2-43.Chemical elements arranged in order increasing their atomic radius in the series:
1) Be, B, C, N 2) O, S, Se, Te 3) Rb, K, Na, Li 4) Mg, Al, Si, P
2-44. Which of the following series of ions are characterized decrease ionic radii?
1)S 2-, Cl -, K +, Ca 2+ 2)As 5+, V 5+, Cl 5+, I 5+
3)Rb + , K + , Ag + , Cu + 4)F - , Si 4+ , Mg 2+ , Na +
2-45. Which of the following series of ions is characterized increasing ionic radii?
1)O 2-, F -, Al 3+, Mg 2+ 2)S 6+, P 5+, Al 3+, Na +
3)Ca 2+, Mg 2+, Be 2+, Ba 2+ 4)Ra 2+, Ba 2+, Hg 2+, Zn 2+
2-46. Among the listed elements greatest atomic radius has:
1) beryllium 2) fluorine 3) lithium 4) sodium 5) magnesium
2-47. The atom of which chemical element, from among those given below, has least radius? 1)K 2)Al 3)Sn 4)C 5)O 6)S
2-48. K and Rb are the same...
2-49. Rb and Sr are the same...
1) atomic radii 2) relative electronegativity values
3) oxidation state 4) number of electronic layers
2-50. Sr and Va are the same...
1) atomic radii 2) relative electronegativity values
3) nuclear charges 4) higher oxidation states
2-51. In the series Be – B – C – N occurs
increase in atomic radius
increase in electronegativity
increasing the force of attraction of valence electrons to the nucleus
reduction in the number of unpaired electrons in the ground state of the atom
1) the number of valence electrons in atoms increases
2) the number of electronic layers in atoms decreases
3) the number of protons in the nuclei of atoms decreases
4) the radii of atoms increase
2-53. In the series Be-Mg-Ca-Sr occurs
1) weakening of metallic properties
2) increase in electronegativity
3) reduction in the number of valence electrons
4) decreasing the force of attraction of valence electrons to the nucleus
2-54. Attraction of electrons from the outer layer to the nucleus increases in a row:
1)Si – P - N 2) S - P –As 3) Na - K – Rb 4) Sr – Ca – K
2-55. Attraction of electrons from the outer layer to the nucleus weakens in a row:
1)Al - Mg - Ca 2) Al - Si - C 3) Na - Mg - Be 4) Se - S - Cl
2-56. Atom ability attract valence electrons of other atoms increases in the series: 1) Mg Ca Ba 2) Si S Cl 3) P O S 4) F Cl Br
2-57. Ability give away electrons atom of element increases in a row
l)Ca, Mg, Be 2)B, C, F 3) Al, Mg, Na 4) S, Cl, F
2-58. The ability to accept electrons by an atom of an element increases by
next to serial numbers: 1) 16.20 2) 6, 11 3) 12.17 4) 9.10
2-59. The greatest energy must be expended to remove electrons from an atom
1) Ca 2) A1 3) Si 4) C
2-60. The least energy required to remove an electron from an atom
1) As 2) Se 3) S 4) P
2-61. The greatest energy must be expended to remove an electron from
1) Ga 2) Al 3) Si 4) C
2-62.The easiest way to add electrons is to the atom.
1) sulfur 2) chlorine 3) selenium 4) bromine
1. Which simple substance - formed from elements No. 11, No. 12 or No. 13 - has the most pronounced metallic properties? Why? Write the formulas of the hydroxides of these elements and indicate their nature.
2. Identify the most typical metal and non-metal of the 4th period. Justify your answer.
A typical metal of the fourth period is K, a non-metal is Kr (metallicity increases from right to left across the period). Typical metals contain 1-2 electrons in their outer energy level (potassium has 1), and non-metals have more than 4 (krypton has 8).
3. Which simple substance - formed by element No. 14, No. 15 or No. 16 - has the most pronounced non-metallic properties? Why? Write the formulas of acids corresponding to the higher oxides of these elements. 
4. An element with a constant valence of two is located in the 4th period. Its oxide and hydroxide are basic in nature. What element is this? What is the structure of its atom? Write the formulas for the oxide and hydroxide of this chemical element.
Ca. Atomic structure: 20 electrons, 20 protons, 20 neutrons. Core charge +20. There are two electrons in the outer energy level. CaO – calcium oxide, Ca(OH)₂ – calcium hydroxide.
5. Indicate the serial number, the charge of the nucleus and calculate the number of protons, neutrons and electrons in volumes of: a) potassium (41K); b) beryllium (9Be); c) magnesium (24Mg); d) calcium (42Ca); e) aluminum (27Al); e) titanium (48Ti); g) vanadium (51V); h) iron (56Fe). 
6. How does the composition of the isotope nuclei differ: a) 63Cu and 65Cu; b) 107Ag and 109Ag? Give a reasoned answer based on calculations. 
7. Calculate the molar masses of substances whose formulas are ₂SO, NO₃, O. Write the formulas of similar compounds containing the H isotope and calculate their molar masses. 
8. How many molecules of sulfur oxide (IV) of different masses can be formed by the interaction of isotopes 16O, 17O, 18O and 32S? Write down formulas for all molecules and calculate their molar masses. 
9. Write the electronic formulas of the atoms: a) sulfur and sodium; b) argon and potassium; c) calcium and chlorine; d) neon and aluminum; e) silicon and bromine; e) arsenic and carbon. 
10. Based on the theory of atomic structure, explain the reasons for the periodicity of changes in the properties of elements and their compounds. Support your answer with specific examples.
The properties of simple substances and compounds of elements repeat periodically because the electronic configurations of atoms repeat periodically. Thus, the atoms Be, Mg, Ca, Sr, Ba have the same structure of the outer electron shell; it contains 2 electrons. These elements easily give them up, exhibiting reducing properties and an oxidation state of +2.
11. How to explain, based on the theory of atomic structure, the sharp transition from inert gases to alkali metals?
In noble gases, the outer energy level is complete; after adding another electron to it, another energy level is filled, which becomes external. One electron in the outer energy level is a characteristic of alkali metals.
12. Determine the place of elements in the periodic table using the electronic formulas of their atoms: 1s2 2s2 2p6 3s2; 1s2 2s2 2p6 3s2 3p6; 1s2 2s2 2p6 3s2 3p6 4s2. Make up formulas of oxides and hydroxides and acids corresponding to these elements.
13. Based on their position on the periodic table, characterize the elements potassium, bromine, and aluminum. 
14. The oxide of an element (3rd period metal) has a molecular weight of 102. Identify the element and draw a diagram of the structure of its atom. Write the formulas of the oxide and hydroxide of this element and indicate their nature. 
15. Calculate the mass (n.s.): a) 8.96 liters of a gaseous compound of fluorine with hydrogen; b) 13.44 liters of a gaseous compound of phosphorus with hydrogen; c) 17.92 liters of a gaseous compound of sulfur and hydrogen. 
16. Elements A and B belong to the same period. A simple substance formed by elements A reacts violently with water. A simple substance formed by element B is a heavy, toxic, red-brown liquid with an unpleasant odor. What elements are we talking about?
Br – element B (Br₂ is a heavy toxic red-brown liquid). K – element A, reacts violently with water. Ca - also interacts with water, but not as violently as K.
17. The substance used as a mineral fertilizer consists of two elements - A and B. The simple substance corresponding to element A is an alkali metal. Element A belongs to the same period as the element that forms the only non-metal that is liquid under ordinary conditions. Element B is in group VIIA, its atoms are part of table salt. Identify elements A and B and write: a) electronic formulas of their atoms; b) the formula of the substance formed during their interaction.
18. Elements X and Y are in the 2nd period. A simple substance formed by element X reacts with water to release hydrogen. The molecular mass of the hydrogen compound of element Y is 20. Identify these elements and write: a) electronic formulas of their atoms; b) the equation of interaction of simple substances formed by these elements; c) the equation for the reaction of the hydroxide of element X with the volatile hydrogen compound of element Y.
19. What chemical properties does the element with atomic number 34 have? Which element is its properties most similar to? give a reasoned answer based on the positions of the elements in the periodic table. 
20. Find in the periodic table an element located in the 4th period, 5th row and exhibiting a valence of six when combined with oxygen. What is its valence when combined with hydrogen? Write the formulas of these compounds.
Se. SeO₃ – selenium oxide (VI), H₂Se (valency of selenium in combination with hydrogen – II).
