Simple and complex substances. Chemical element
About atoms and chemical elements
In chemistry, in addition to the terms “atom” and “molecule,” the concept “element” is often used. What do these concepts have in common and how do they differ?
Chemical element are atoms of the same type. So, for example, all hydrogen atoms are the element hydrogen; all oxygen and mercury atoms are the elements oxygen and mercury, respectively.
Currently, more than 107 types of atoms are known, that is, more than 107 chemical elements. It is necessary to distinguish between the concepts of “chemical element”, “atom” and “simple substance”
Simple and complex substances
According to their elemental composition, they distinguish between simple substances consisting of atoms of one element (H2, O2,Cl2, P4, Na, Cu, Au), and complex substances consisting of atoms of different elements (H2O, NH3, OF2, H2SO4, MgCl2, K2SO4) .
Currently, 115 chemical elements are known, which form about 500 simple substances.
Native gold is a simple substance.
The ability of one element to exist in the form of various simple substances that differ in properties is called allotropy. For example, the element oxygen O has two allotropic forms - dioxygen O2 and ozone O3 with different numbers of atoms in the molecules.
Allotropic forms of the element carbon C - diamond and graphite - differ in the structure of their crystals. There are other reasons for allotropy.
Allotropic forms of carbon:
graphite:
diamond:
Complex substances are often called chemical compounds, for example, mercury(II) oxide HgO (obtained by combining atoms of simple substances - mercury Hg and oxygen O2), sodium bromide (obtained by combining atoms of simple substances - sodium Na and bromine Br2).
So, let's summarize the above. There are two types of molecules of matter:
1. Simple– the molecules of such substances consist of atoms of the same type. In chemical reactions they cannot decompose to form several simpler substances.
2.Complex– the molecules of such substances consist of atoms of different types. In chemical reactions they can decompose to form simpler substances.
The difference between the concepts of “chemical element” and “simple substance”
The concepts of “chemical element” and “simple substance” can be distinguished by comparing the properties of simple and complex substances. For example, a simple substance – oxygen – is a colorless gas necessary for breathing and supporting combustion. The smallest particle of the simple substance oxygen is a molecule that consists of two atoms. Oxygen is also included in carbon monoxide (carbon monoxide) and water. However, water and carbon monoxide contain chemically bound oxygen, which does not have the properties of a simple substance; in particular, it cannot be used for respiration. Fish, for example, do not breathe chemically bound oxygen, which is part of the water molecule, but free oxygen dissolved in it. Therefore, when we talk about the composition of any chemical compounds, it should be understood that these compounds do not contain simple substances, but atoms of a certain type, that is, the corresponding elements.
When complex substances decompose, atoms can be released in a free state and combine to form simple substances. Simple substances consist of atoms of one element. The difference between the concepts of “chemical element” and “simple substance” is also confirmed by the fact that the same element can form several simple substances. For example, atoms of the element oxygen can form diatomic oxygen molecules and triatomic ozone molecules. Oxygen and ozone are completely different simple substances. This explains the fact that much more simple substances are known than chemical elements.
Using the concept of “chemical element”, we can give the following definition to simple and complex substances:
Simple are called substances that consist of atoms of one chemical element.
Complex are called substances that consist of atoms of different chemical elements.
The difference between the concepts of “mixture” and “chemical compound”
Complex substances are often called chemical compounds.
Follow the link and view the experience of the interaction of simple substances iron and sulfur.
Try to answer the questions:
1. How do mixtures differ in composition from chemical compounds?
2. Compare the properties of mixtures and chemical compounds?
3. In what ways can you separate the components of a mixture and a chemical compound?
4. Is it possible to judge by external signs the formation of a mixture or chemical compound?
Comparative characteristics of mixtures and chemicals
connections
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Questions to match mixtures to chemical compounds | Comparison |
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Mixtures | Chemical compounds |
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How do mixtures differ in composition from chemical compounds? | Substances can be mixed in any ratio, i.e. variable composition of mixtures | The composition of chemical compounds is constant. |
Compare the properties of mixtures and chemical compounds? | Substances in mixtures retain their properties | Substances that form compounds do not retain their properties, since chemical compounds with other properties are formed |
In what ways can a mixture and a chemical compound be separated into its constituent components? | Substances can be separated by physical means | Chemical compounds can only be broken down through chemical reactions |
Is it possible to judge by external signs the formation of a mixture and a chemical compound? | Mechanical mixing is not accompanied by the release of heat or other signs of chemical reactions | The formation of a chemical compound can be judged by the signs of chemical reactions |
Tasks for consolidation
I. Work with simulators
Simulator No. 1
Simulator No. 2
Simulator No. 3
II. Solve the problem
From the proposed list of substances, write out simple and complex substances separately:
NaCl, H2SO4, K, S8, CO2, O3, H3PO4, N2, Fe.
Explain your choice in each case.
III. Answer the questions
№1
How many simple substances are written in a series of formulas:
H2O, N2, O3, HNO3, P2O5, S, Fe, CO2, KOH.
№2
Both substances are complex:
A) C (coal) and S (sulfur);
B) CO2 (carbon dioxide) and H2O (water);
B) Fe (iron) and CH4 (methane);
D) H2SO4 (sulfuric acid) and H2 (hydrogen).
№3
Choose the correct statement:
Simple substances consist of atoms of the same type.
A) Correct
B) Incorrect
№4
What is typical for mixtures is that
A) They have a constant composition;
B) Substances in the “mixture” do not retain their individual properties;
C) Substances in “mixtures” can be separated by physical properties;
D) Substances in “mixtures” can be separated using a chemical reaction.
№5
The following are typical for “chemical compounds”:
A) Variable composition;
B) Substances contained in a “chemical compound” can be separated by physical means;
C) The formation of a chemical compound can be judged by the signs of chemical reactions;
D) Permanent composition.
№6
In what case are we talking about iron as a chemical element?
A) Iron is a metal that is attracted by a magnet;
B) Iron is part of rust;
C) Iron is characterized by a metallic luster;
D) Iron sulfide contains one iron atom.
№7
In what case are we talking about oxygen as a simple substance?
A) Oxygen is a gas that supports respiration and combustion;
B) Fish breathe oxygen dissolved in water;
C) The oxygen atom is part of the water molecule;
D) Oxygen is part of air.
§ 9. Simple and complex substances
Having mastered this topic, you will be able to:
Distinguish between the concepts of “simple substance” and “complex substance”, formulas of simple and complex substances;
Understand the concept of “chemical compound”;
Give examples of simple and complex substances;
Describe simple and complex substances known to you from everyday use;
Make judgments about a variety of substances.
Most atoms of chemical elements have the ability to combine with each other or with atoms of other chemical elements. As a result, chemical compounds are formed. Regardless of the composition of their structural particles, both simple and complex substances are chemical compounds, since chemical bonds arise between them.
You have already become familiar with the structure of atoms of chemical elements. Substances whose components are atoms are called atomic.
However, among all the variety of chemical compounds, there are also molecular substances. Their constituent parts are molecules.
Molecules are the smallest particles of a substance that retain its chemical properties.
A molecule is considered to be the limit of divisibility of a substance. If it is destroyed, then the substance is destroyed. A characteristic feature of molecules is continuous movement.
Remember from your natural history course what phenomenon is called diffusion.
Each molecule consists of a certain number of atoms of one or different chemical elements.
Remember from your natural history course how substances are divided according to composition and origin.
What substances are called: a) simple; b) difficult? Give some examples of simple and complex substances that you most often use in everyday life.
Simple substances are substances formed by one chemical element.
For example, simple substances hydrogen, oxygen, nitrogen formed in accordance with the chemical elements Hydrogen, Oxygen, Nitrogen. Their molecules contain two atoms of these elements connected to each other (Fig. 41 a, 6, c).
The element Oxygen, under certain conditions, forms another simple substance - ozone, the molecule of which contains three atoms (Fig. 41 d).
Rice. 41. Models of molecules of simple substances: a - hydrogen; b - oxygen; c - ozone; g - nitrogen
Complex substances are substances formed by two or more chemical elements.
Complex substances include; water, sugar, soap, table salt, chalk, methane (a component of natural gas), carbon dioxide. The substances that make up the cells of living organisms (proteins, fats and carbohydrates) are complex and contain mainly atoms of Carbon, Oxygen, Hydrogen, Nitrogen, Sulfur, Phosphorus and have a molecular structure.
Remember how to prove that water is a complex substance. What research methods did scientists use to determine the composition of water?
Figure 42 shows models of methane, carbon dioxide and water molecules. A methane molecule consists of one Carbon atom and four Hydrogen atoms, a carbon dioxide molecule - from one Carbon atom and two Oxygen atoms, a water molecule - from one Oxygen atom and two Hydrogen atoms.
Rice. 42. Models of molecules of complex substances: a - methane; b - carbon dioxide; c - water
So, depending on their composition, substances are classified into simple and complex. The classification scheme for substances is shown in Figure 43.
Rice. 43. Classification of substances
Simple substances: metals and non-metals. Simple substances are divided into two groups. Metallic elements form metals, non-metallic elements form nonmetals. They are distinguished by physical properties.
Remember what physical properties of substances you have already become familiar with. Name them.
Let's turn to the demonstrations and look at samples of simple substances of metals and non-metals. Of the metals, the most common in technology, various industries, and everyday life are iron, zinc, aluminum, copper, silver, gold; Non-metals in the laboratory include sulfur, carbon, red phosphorus, bromine, and iodine.
Pay attention to the state of aggregation of metals and non-metals. Why do you think bromine is stored in sealed ampoules?
The division of simple substances into metals and non-metals is based on their physical properties (Table 2).
table 2
Physical properties of simple substances
Nonmetals are substances that are mostly made up of molecules. The molecules of many of them are diatomic. However, there are also polyatomic molecules: the already mentioned ozone, crystalline sulfur - contains eight atoms of Sulfur, white phosphorus - four atoms of this element. In simple substances formed by the element Carbon, atoms combine in a certain order without forming molecules.
Metals are composed of atoms of the corresponding elements. The names of metals often coincide with the names of the metallic elements that form them. For example, substances aluminum, zinc, nickel, chromium, magnesium formed by the corresponding chemical elements. However, the substance copper consists of atoms of the element Cuprum, silver - Argentum, gold - Aurum, mercury - Mercury, iron - Iron. The names of nonmetals, elements and simple substances coincide for a small number of substances (Table 3).
Table C
Names of chemical elements and simple substances
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Metal |
Non-metallic |
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Chemical element |
Simple substance |
Chemical element |
Simple substance |
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Aluminum |
aluminum |
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Argentum |
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Mercury |
oxygen |
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Laboratory experience 2
Familiarization with samples of simple and complex substances
Task 1. Look carefully at the substances given to you in banks. Read the labels: H 2 O (water), S (sulfur), P (phosphorus), Mg (magnesium), NaOH (sodium hydroxide), C (carbon), Fe 3 O 4 (ferum (II, III) oxide ), Fe (iron), ZnO (zinc oxide), CaCO 3 (calcium carbonate), Al (aluminum), Zn (zinc), CaO (calcium oxide), Na 2 CO 3 (sodium carbonate).
Distribute these substances into two groups: simple and complex. Simple classify substances into metals and non-metals.
Task 2. Describe: a) how simple and complex substances differ in composition; 6) what criteria did you use to make the classification?
Task 3. Describe the physical properties of substances based on your observations.
After completing the task, write down the data in your workbook in the form of a table. At the end of the work, formulate conclusions.
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substances |
substances |
Description of properties based on observations |
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Nonmetals |
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Variety of substances. The diversity of substances is explained by the ability of atoms of elements to combine with each other. Depending on which atoms, in what quantity and how they combine, many simple and complex substances are formed (Fig. 44).
Rice. 44. The simple substance sulfur (a) and the complex substance amethyst (b)
There are slightly more simple substances than chemical elements - 400, because, as you already know, the same element (Oxygen, Carbon, Phosphorus, Sulfur) can form two or more substances.
Much more complex substances are known (almost 20 million). This is water, the molecule of which includes Hydrogen and Oxygen, carbon dioxide - Carbon and Oxygen, table salt - Sodium and Chlorine. The composition of these substances includes only two elements - these are binary compounds. However, a significant number of substances consist of three or more elements. Thus, glucose contains three elements: Carbon, Hydrogen and Oxygen, and baking soda contains four elements: Sodium, Hydrogen, Carbon and Oxygen.
All organic substances are classified as complex. In addition, there is a whole industry for the extraction of synthetic and artificial compounds, which have huge industrial and household purposes.
Remember from your natural history course what substances are called inorganic and organic. Give examples of inorganic and organic compounds.
Under normal conditions (temperature 0 °C, pressure 101.3 kPa), substances are in three states of aggregation: liquid (water, oil, alcohol), solid (zinc, iron, sulfur, phosphorus, carbon, copper) and gaseous (hydrogen, oxygen, ozone, nitrogen, carbon dioxide, inert gases).
LET'S SUMMARY WHAT WE'VE LEARNED
Substances are divided into simple and complex.
Complex substances are formed from two or more chemical elements. there are much more of them than simple ones.
Each simple and complex substance is characterized by certain properties, that is, signs by which their similarities and differences can be identified.
Complex substances are of organic and inorganic origin.
The diversity of substances is explained by the ability of atoms of elements to combine with each other.
TASKS TO CONTROL KNOWLEDGE
1. Explain what the concepts of “molecule”, “simple substance”, “complex substance”, “chemical compound” mean.
2. Give examples: a) simple and complex substances; b) organic and inorganic substances.
3. Justify whether the concepts of “chemical compound” and “mixture of substances” are identical.
4. Describe the physical properties of: a) sugar; b) water; c) oils.
5. Justify why there are more complex substances than simple ones.
6. Express your own opinion about the importance of substances for human life and health.
INTERESTING TO KNOW
The English chemist G. Davy was the first to isolate the metals sodium, potassium, calcium, strontium, barium, and magnesium in a free state by electrolysis. These works marked the beginning of the production of powerful lamps for searchlights, lighthouses, etc. Subsequently, the scientist created a safe miner's lamp, which was used all over the world until it was replaced by a battery-powered lamp.
Maria Sklodowska-Curie (1867-1934) - French physicist and chemist, teacher, public figure. Science owes him the discovery and study of two radioactive elements - Polonium and Radium. The discovery of the element Radium began a method for treating skin cancer. For her work she was awarded two Nobel Prizes, which she donated for the construction of a sanatorium in Zakopane and the Radiological Institute in Warsaw (Poland).
Chemistry belongs to the natural sciences. She studies the composition, structure, properties and transformations of substances, as well as the phenomena accompanying these transformations.
Substance is one of the main forms of existence of matter. Substance as a form of matter consists of individual particles of varying degrees of complexity and has its own mass, the so-called
rest mass.
Simple and complex substances. Allotropy.
All substances can be divided into simple And complex .
Simple substances consist of atoms of one chemical element, complex - from atoms of several chemical elements.
Chemical element - this is a certain type of atom with the same nuclear charge. Hence, atom is the smallest particle of a chemical element.
Concept simple substance cannot be identified with the concept
chemical element . A chemical element is characterized by a certain positive charge of the atomic nucleus, isotopic composition, and chemical properties. The properties of an element refer to its individual atoms. A simple substance is characterized by a certain density, solubility, melting and boiling points, etc. These properties relate to a collection of atoms and are different for different simple substances.
Simple substance - this is the form of existence of a chemical element in a free state. Many chemical elements form several simple substances that differ in structure and properties. This phenomenon is called allotropy , and the forming substances are allotropic modifications . Thus, the oxygen element forms two allotropic modifications - oxygen and ozone, the carbon element - diamond, graphite, carbyne, fullerene.
The phenomenon of allotropy is caused by two reasons: a different number of atoms in the molecule (for example, oxygen ABOUT 2 and azon ABOUT 3 ) or the formation of various crystalline forms (for example, carbon forms the following allotropic modifications: diamond, graphite, carbine, fullerene), carbine was discovered in 1968 (A. Sladkov, Russia), and fullerene was discovered theoretically in 1973 (D. Bochvar, Russia) , and in 1985 - experimentally (G. Kroto and R. Smalley, USA).
Complex substances They consist not of simple substances, but of chemical elements. Thus, hydrogen and oxygen, which are part of water, are contained in water not in the form of gaseous hydrogen and oxygen with their characteristic properties, but in the form elements - hydrogen and oxygen.
The smallest particle of substances with a molecular structure is a molecule that retains the chemical properties of a given substance. According to modern concepts, molecules consist mainly of substances in liquid and gaseous states. Most solids (mostly inorganic) do not consist of molecules, but of other particles (ions, atoms). Salts, metal oxides, diamond, metals, etc. do not have a molecular structure.
Relative atomic mass
Modern research methods make it possible to determine extremely small atomic masses with greater accuracy. For example, the mass of a hydrogen atom is 1,674 10 -27 kg, carbon – 1,993 10 -26 kg.
In chemistry, not absolute values of atomic masses are traditionally used, but relative ones. In 1961, the unit of atomic mass was adopted atomic mass unit (abbreviated a.u.m.), which is 1/12 part of the mass of a carbon isotope atom 12 WITH.
Most chemical elements have atoms with different masses (isotopes). That's why relative atomic mass (or just atomic mass) A r of a chemical element is a value equal to the ratio of the average mass of an atom of the element to 1/12 carbon atom mass 12 WITH.
The atomic masses of the elements are A r, where index r– the initial letter of an English word relative – relative. Posts A r (H), A r (O) A r (C) mean: relative atomic mass of hydrogen, relative atomic mass of oxygen, relative atomic mass of carbon.
Relative atomic mass is one of the main characteristics of a chemical element.
Simple substances and their classification When studying the material in the previous paragraphs, you have already become acquainted with some substances. For example, a molecule of hydrogen gas consists of two atoms of the chemical element hydrogen - H + H = H2. Simple substances are substances that contain atoms of the same type Simple substances known to you include: oxygen, graphite, sulfur, nitrogen, all metals: iron, copper, aluminum, gold, etc. Sulfur consists only of atoms of the chemical element sulfur, while graphite consists of atoms of the chemical element carbon. It is necessary to clearly distinguish between concepts "chemical element" And "simple matter". For example, diamond and carbon are not the same thing. Carbon is a chemical element, and diamond is a simple substance formed by the chemical element carbon. In this case, the chemical element (carbon) and the simple substance (diamond) are called differently. Often a chemical element and its corresponding simple substance are named the same. For example, the element oxygen corresponds to a simple substance - oxygen. It is necessary to learn how to distinguish between where we are talking about an element and where about a substance! For example, when they say that oxygen is part of water, we are talking about the element oxygen. When they say that oxygen is a gas necessary for breathing, we are talking about the simple substance oxygen. Simple substances of chemical elements are divided into two groups - metals and non-metals. Metals and non-metals radically different in their physical properties. All metals are solid substances under normal conditions, with the exception of mercury - the only liquid metal. Metals are opaque and have a characteristic metallic luster. Metals are ductile and conduct heat and electricity well. Nonmetals are not similar to each other in physical properties. So, hydrogen, oxygen, nitrogen are gases, silicon, sulfur, phosphorus are solids. The only liquid non-metal is bromine, a brownish-red liquid. If you draw a conventional line from the chemical element boron to the chemical element astatine, then in the long version of the Periodic System there are non-metallic elements above the line, and below it - metal. In the short version of the Periodic Table, there are non-metallic elements below this line, and both metallic and non-metallic elements above it. This means that it is more convenient to determine whether an element is metallic or non-metallic using the long version of the Periodic Table. This division is arbitrary, since all elements in one way or another exhibit both metallic and non-metallic properties, but in most cases this distribution corresponds to reality.
Complex substances and their classification
If the composition of simple substances includes atoms of only one type, it is easy to guess that the composition of complex substances will include several types of different atoms, at least two. An example of a complex substance is water; you know its chemical formula - H2O. Water molecules are made up of two types of atoms: hydrogen and oxygen. Complex substances- substances containing atoms of various types Let's conduct the following experiment. Mix sulfur and zinc powders. Place the mixture on a metal sheet and set it on fire using a wooden torch. The mixture ignites and quickly burns with a bright flame. After the completion of the chemical reaction, a new substance was formed, which included sulfur and zinc atoms. The properties of this substance are completely different from the properties of the starting substances - sulfur and zinc. Complex substances are usually divided into two groups: inorganic substances and their derivatives and organic substances and their derivatives. For example, rock salt is an inorganic substance, and the starch contained in potatoes is an organic substance.Types of structure of substances
Based on the type of particles that make up the substances, substances are divided into substances molecular and non-molecular structure. The substance may contain various structural particles, such as atoms, molecules, ions. Consequently, there are three types of substances: substances of atomic, ionic and molecular structure. Substances of different types of structure will have different properties.Substances of atomic structure
An example of substances of atomic structure are substances formed by the element carbon: graphite and diamond. These substances contain only carbon atoms, but the properties of these substances are very different. Graphite– a fragile, easily exfoliating substance of gray-black color. Diamond– transparent, one of the hardest minerals on the planet. Why do substances consisting of the same type of atom have different properties? It's all about the structure of these substances. The carbon atoms in graphite and diamond join together in different ways. Substances of atomic structure have high boiling and melting points, are usually insoluble in water, and nonvolatile. Crystal lattice – an auxiliary geometric image introduced to analyze the structure of a crystalSubstances of molecular structure
Substances of molecular structure– These are almost all liquids and most gaseous substances. There are also crystalline substances whose crystal lattice includes molecules. Water is a substance of molecular structure. Ice also has a molecular structure, but unlike liquid water, it has a crystal lattice where all molecules are strictly ordered. Substances of molecular structure have low boiling and melting points, are usually fragile, and do not conduct electricity.Substances of ionic structure
Substances of ionic structure are solid crystalline substances. An example of an ionic compound substance is table salt. Its chemical formula is NaCl. As we can see, NaCl consists of ions Na+ and Cl⎺, alternating in certain places (nodes) of the crystal lattice. Substances with an ionic structure have high melting and boiling points, are fragile, are usually highly soluble in water, and do not conduct electric current. The concepts of “atom”, “chemical element” and “simple substance” should not be confused.- "Atom"– a specific concept, since atoms really exist.
- "Chemical element"– this is a collective, abstract concept; In nature, a chemical element exists in the form of free or chemically bonded atoms, that is, simple and complex substances.
Complex substances must be distinguished from mixtures, which also consist of different elements. The quantitative ratio of the components of the mixture can be variable, but the chemical compounds have a constant composition. For example, in a glass of tea you can add one spoon of sugar, or several, and sucrose molecules С12Н22О11 contains exactly 12 carbon atoms, 22 hydrogen atoms and 11 oxygen atoms. Thus, the composition of compounds can be described by one chemical formula, and the composition no mixture. The components of the mixture retain their physical and chemical properties. For example, if you mix iron powder with sulfur, a mixture of two substances is formed. Both sulfur and iron in this mixture retain their properties: iron is attracted by a magnet, and sulfur is not wetted by water and floats on its surface. If sulfur and iron react with each other, a new compound is formed with the formula FeS, which does not have the properties of either iron or sulfur, but has a set of its own properties. In connection FeS iron and sulfur are bound to each other, and it is impossible to separate them using the methods used to separate mixtures. Thus, substances can be classified according to several parameters: Conclusions from an article on the topic Simple and complex substances
- Simple substances- substances that contain atoms of the same type
- Simple substances are divided into metals and non-metals
- Complex substances- substances containing atoms of various types
- Complex substances are divided into organic and inorganic
- There are substances of atomic, molecular and ionic structure, their properties are different
- Crystal cell– an auxiliary geometric image introduced to analyze the crystal structure
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 are called complex substances. 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 of various types with each other, 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, at least due to the fact that the molecular weight of ozone is 1.5 times greater than that of 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 that differ 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 a 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 gas lighter than air that forms explosive mixtures with air, sulfur is a yellow solid that can burn, 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.
Obviously, in addition to individual chemicals, there are also mixtures of them. It is primarily mixtures of various substances that form the world around us: metal alloys, food, drinks, various materials that make up the objects around us.
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.