In the previous chapter it was said that not only atoms of the same chemical element can form bonds with each other, but also atoms of different elements. Substances formed by atoms of one chemical element are called simple substances, and substances formed by atoms of different chemical elements, - complex. Some simple substances have a molecular structure, i.e. consist of molecules. For example, substances such as oxygen, nitrogen, hydrogen, fluorine, chlorine, bromine, iodine have a molecular structure. Each of these substances is formed by diatomic molecules, so their formulas can be written as O 2, N 2, H 2, F 2, Cl 2, Br 2 and I 2, respectively. As you can see, simple substances can have the same name as the elements that form them. Therefore, one should clearly distinguish between situations when we are talking about a chemical element and when about a simple substance.
Often simple substances have not a molecular, but an atomic structure. In such substances, atoms can form bonds with each other various types, which will be discussed in detail a little later. Substances of a similar structure are all metals, for example, iron, copper, nickel, as well as some non-metals - diamond, silicon, graphite, etc. These substances are usually characterized not only by the coincidence of the name of the chemical element with the name of the substance formed by it, but also by the identical recording of the formula of the substance and the designation of the chemical element. For example, the chemical elements iron, copper and silicon, designated Fe, Cu and Si, form simple substances whose formulas are Fe, Cu and Si, respectively. There is also a small group of simple substances consisting of isolated atoms that are not connected in any way. Such substances are gases, which are called noble gases due to their extremely low chemical activity. These include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn).
Since there are only about 500 known simple substances, the logical conclusion follows that many chemical elements are characterized by a phenomenon called allotropy.
Allotropy is a phenomenon when one chemical element can form several simple substances. Different chemical substances formed by one chemical element are called allotropic modifications or allotropes.
So, for example, the chemical element oxygen can form two simple substances, one of which has the name of the chemical element - oxygen. Oxygen as a substance consists of diatomic molecules, i.e. its formula is O 2. It is this compound that is part of the air we need for life. Another allotropic modification of oxygen is the triatomic gas ozone, whose formula is O 3 . Despite the fact that both ozone and oxygen are formed by the same chemical element, their chemical behavior is very different: ozone is much more active than oxygen in reactions with the same substances. In addition, these substances differ from each other in physical properties already at least due to the fact that molecular weight ozone is 1.5 times greater than oxygen. This leads to the fact that its density in the gaseous state is also 1.5 times greater.
Many chemical elements tend to form allotropic modifications, differing from each other in the structural features of the crystal lattice. So, for example, in Figure 5, you can see schematic images of fragments of the crystal lattices of diamond and graphite, which are allotropic modifications of carbon.
Figure 5. Fragments of crystal lattices of diamond (a) and graphite (b)
In addition, carbon can also have a molecular structure: such a structure is observed in a type of substance such as fullerenes. Substances of this type are formed by spherical carbon molecules. Figure 6 shows 3D models of the c60 fullerene molecule and a soccer ball for comparison. Notice their interesting similarities.
Figure 6. C60 fullerene molecule (a) and soccer ball (b)
Complex substances are substances that consist of atoms of different elements. They, just like simple substances, can have a molecular and non-molecular structure. The non-molecular type of structure of complex substances can be more diverse than that of simple ones. Any complex chemical substances can be obtained either by direct interaction of simple substances or by a sequence of their interactions with each other. It is important to realize one fact, which is that the properties of complex substances, both physical and chemical, are very different from the properties of the simple substances from which they are obtained. For example, table salt, which has the NaCl forum and is colorless transparent crystals, can be obtained by reacting sodium, which is a metal with properties characteristic of metals (brilliance and electrical conductivity), with chlorine Cl2, a yellow-green gas.
Sulfuric acid H 2 SO 4 can be formed by a series of successive transformations from simple substances - hydrogen H 2, sulfur S and oxygen O 2. Hydrogen is a lighter-than-air gas that forms explosive mixtures with air; sulfur is a solid. yellow, capable of burning, and oxygen is a gas slightly heavier than air in which many substances can burn. Sulfuric acid, which can be obtained from these simple substances, is a heavy oily liquid with strong water-removing properties, due to which it chars many substances of organic origin.
It is obvious that in addition to individual chemicals, there are also mixtures of them. Mainly mixtures various substances the world around us is formed: metal alloys, food, drinks, various materials, of which the objects around us are made.
For example, the air we breathe consists mainly of nitrogen N2 (78%), oxygen (21%), which is vital for us, and the remaining 1% consists of impurities of other gases (carbon dioxide, noble gases, etc.).
Mixtures of substances are divided into homogeneous and heterogeneous. Homogeneous mixtures are those mixtures that do not have phase boundaries. Homogeneous mixtures are a mixture of alcohol and water, metal alloys, a solution of salt and sugar in water, mixtures of gases, etc. Heterogeneous mixtures are those mixtures that have a phase boundary. Mixtures of this type include a mixture of sand and water, sugar and salt, a mixture of oil and water, etc.
The substances that make up mixtures are called components.
Mixtures of simple substances, unlike chemical compounds that can be obtained from these simple substances, retain the properties of each component.
All substances are divided into simple and complex.
Simple substances- These are substances that consist of atoms of one element.
In some simple substances, atoms of the same element combine with each other to form molecules. Such simple substances have molecular structure. These include: , . All these substances consist of diatomic molecules. (Note that the names of the simple substances are the same as the names of the elements!)
Other simple substances have atomic structure, i.e. they consist of atoms between which there are certain bonds. Examples of such simple substances are all (, etc.) and some (, etc.). Not only the names, but also the formulas of these simple substances coincide with the symbols of the elements.
There is also a group of simple substances called. These include: helium He, neon Ne, argon Ar, krypton Kr, xenon Xe, radon Rn. These simple substances are made up of atoms that are not bonded to each other.
Each element forms at least one simple substance. Some elements can form not one, but two or more simple substances. This phenomenon is called allotropy.
Allotropy is the phenomenon of the formation of several simple substances by one element.
Different simple substances that are formed by the same chemical element are called allotropic modifications.
Allotropic modifications may differ from each other in molecular composition. For example, the element oxygen forms two simple substances. One of them consists of diatomic molecules O 2 and has the same name as the element-. Another simple substance consists of triatomic O 3 molecules and has proper name- ozone.
Oxygen O 2 and ozone O 3 have different physical and chemical properties.
Allotropic modifications can be solids that have different crystal structures. An example is the allotropic modifications of carbon C - diamond and graphite.
The number of known simple substances (approximately 400) is significantly greater than the number of chemical elements, since many elements can form two or more allotropic modifications.
Complex substances- These are substances that consist of atoms of different elements.
Examples of complex substances: HCl, H 2 O, NaCl, CO 2, H 2 SO 4, etc.
Complex substances are often called chemical compounds. In chemical compounds, the properties of the simple substances from which these compounds are formed are not preserved. The properties of a complex substance differ from the properties of the simple substances from which it is formed.
For example, sodium chloride NaCl can be formed from simple substances - metallic sodium Na and gaseous chlorine Cl. The physical and chemical properties of NaCl differ from the properties of Na and Cl 2.
In nature, as a rule, there are not pure substances, but mixtures of substances. In practical activities, we also usually use mixtures of substances. Any mixture consists of two or more substances called mixture components.
For example, air is a mixture of several gaseous substances: oxygen O 2 (21% by volume), (78%), etc. Mixtures are solutions of many substances, alloys of some metals, etc.
Mixtures of substances are homogeneous (homogeneous) and heterogeneous (heterogeneous).
Homogeneous mixtures- these are mixtures in which there is no interface between the components.
Mixtures of gases (in particular, air) and liquid solutions (for example, a solution of sugar in water) are homogeneous.
Heterogeneous mixtures- These are mixtures in which the components are separated by an interface.
Heterogeneous include mixtures of solids (sand + chalk powder), mixtures of liquids insoluble in each other (water + oil), mixtures of liquids and solids insoluble in it (water + chalk).
The most important differences between mixtures and chemical compounds:
- In mixtures, the properties of individual substances (components) are preserved.
- The composition of mixtures is not constant.
Under chemical element understand a collection of atoms with the same positive nuclear charge and with a certain set of properties. Atoms of the same chemical element combine to form simple substance. When atoms of different chemical elements combine, complex substances (chemical compounds) or mixtures. The difference between chemical compounds and mixtures is that:
They have new properties that the simple substances from which they were obtained did not have;
They cannot be mechanically divided into their component parts;
Chemical elements in their composition can only be in strictly defined quantitative ratios.
Some chemical elements (carbon, oxygen, phosphorus, sulfur) can exist in the form of several simple substances. This phenomenon is called allotropy, and varieties of simple substances of the same chemical element are called its allotropic modifications(modifications).
Tasks
1.1. What more exists in nature: chemical elements or simple substances? Why?
1.2. Is it true that sulfur and iron are included in the composition of iron sulfide as substances? If not, what is the correct answer?
1.3. Name allotropic modifications of oxygen. Do they differ in their properties? If so, how?
1.4. Which of the allotropic modifications of oxygen is chemically more active and why?
1.5. The simple substances or chemical elements are zinc, sulfur and oxygen in the following reactions:
1) CuSO 4 + Zn = ZnSO 4 + Cu;
2) S + O 2 = SO 2;
3) Zn + 2HC1 = ZnCl 2 + H 2 ;
4) Zn + S = ZnS;
5) 2H 2 0 = 2H 2 + O 2 .
1.6. Is it possible to get another simple substance from one simple substance? Give a reasoned answer.
1.7. When a substance is burned in oxygen, sulfur (IV) oxide, nitrogen and water are produced. What chemical elements form the starting substance?
1.8. Indicate whether simple or complex substances include: H 2 O, C1 2, NaOH, O 2, HNO 3, Fe, S, ZnSO 4, N 2, AgCl, I 2, A1 2 O 3, O 3?
1.9. For which chemical elements are allotropic modifications known? Name these modifications.
1.10. Is it possible for a chemical element to transition from one allotropic modification to another? Give examples.
1.11. What chemical elements do they mean when they talk about diamond and ozone?
1.12. Which of the substances are chemical compounds and which are mixtures:
2) air;
4) sulfuric acid;
1.13. How to prove that sodium chloride is a complex substance?
1.14. Name three allotropic modifications of carbon.
1.15. What are allotropic modifications of phosphorus called and how do they differ from each other?
1.16. What are allotropic modifications of sulfur called and how do they differ from each other?
1.17. Indicate which of the statements is true and why - the composition of barium sulfate includes:
1) simple substances barium, sulfur, oxygen;
2) chemical elements barium, sulfur, oxygen.
1.18. How many liters of ammonia can be produced from a mixture of 10 liters of nitrogen and 30 liters of hydrogen?
1.19. How many liters of water vapor are produced from a mixture of 10 liters of hydrogen and 4 liters of oxygen? What gas and in what volume will remain in excess?
1.20. How many grams of zinc sulfide (ZnS) can be formed from a mixture of 130 g of zinc and 48 g of sulfur?
1.22. What is a solution of alcohol in water - a mixture or a chemical compound?
1.23. Can a complex substance consist of atoms of the same type?
1.24. Which of the following substances are mixtures and which are chemical compounds:
1) bronze;
2) nichrome;
3) kerosene;
4) potassium nitrate:
5) rosin;
6) superphosphate.
1.25. Given a mixture of Cl 2 + HCl + CaCl 2 + H 2 O.
1) How many different substances are in the mixture;
2) How many chlorine molecules are in the mixture;
3) How many chlorine atoms are in the mixture;
4) How many molecules of different substances are contained in the mixture.
The main difference between them is their composition. Thus, simple substances include atoms of one element. Their (simple substances) crystals can be synthesized in the laboratory, and sometimes at home. However, it is often necessary to create certain conditions for storing the resulting crystals.
There are five classes into which simple substances are divided: metals, semimetals, nonmetals, intermetallic compounds, and halogens (not found in nature). They can be represented by atomic (Ar, He) or molecular (O2, H2, O3) gases.
As an example, we can take the simple substance oxygen. It includes molecules consisting of two atoms of the element Oxygen. Or, for example, the substance iron consists of crystals containing only atoms of the element Iron. Historically, it was customary to name a simple substance by the name of the element whose atoms are included in its composition. The structure of these compounds can be molecular or non-molecular.
Complex substances include atoms various types and upon decomposition can form two (or more) compounds. For example, when water splits, it forms oxygen and hydrogen. However, not every compound can be broken down into simple substances. For example, iron sulfide, formed by sulfur and iron atoms, cannot be broken down. In this case, in order to prove that the compound is complex and includes dissimilar atoms, the reverse reaction principle is used. In other words, iron sulfide is obtained using the starting components.
Elements are forms of chemical elements that exist in free form. Today science knows more than four hundred types of these elements.
Unlike complex substances, simple substances cannot be obtained from other simple substances. They also cannot be decomposed into other compounds.
All allotropic modifications have the property of transforming into each other. Different types simple substances formed by one chemical element can have different and different levels of chemical activity. For example, oxygen exhibits less activity than ozone, and the melting point of fullerene, for example, is lower than that of diamond.
IN normal conditions for eleven elements the simple substances will be gases (Ar, Xe, Rn, N, H, Ne, O, F, Kr, Cl, He,), for two liquids (Br, Hg), and for other elements - solids.
At temperatures close to room temperature, the five metals will take on a liquid or semi-liquid state. This is due to the fact that their melting point is almost equal. Thus, mercury and rubidium melt at 39 degrees, francium at 27, cesium at 28, and gallium at 30 degrees.
It should be noted that the concepts of “chemical element”, “atom”, “simple substance” should not be confused. So, for example, an atom has a definite, specific meaning and really exists. The definition of “chemical element” is generally abstract and collective. In nature, elements are present in the form of free or chemically bonded atoms. At the same time, the characteristics of simple substances (collections of particles) and chemical elements (isolated atoms) specific type) have their own characteristics.
