The smallest indivisible particle. Chemistry

The founder of “atomism” - a philosophical doctrine according to which all elements of living and inanimate nature consist of atoms (chemically indivisible particles). Atoms exist forever and are so small that they cannot be measured; they are identical and differ only in appearance, but retain all the properties of the original substance.

In 1808 he revived atomism and proved that atoms are real. Atoms are chemical elements that cannot be created anew, divided into smaller components, or destroyed through any chemical transformations. Any chemical reaction simply changes the order of rearrangement of atoms.

In 1897, scientist J. Thompson proved the existence of electrons - negatively charged particles. In 1904, he proposed a model of the atom - “raisin pudding.” An atom is a positively charged body, inside which small particles with a negative charge are distributed, like raisins in a pudding.

1911 - Together with his students, he conducted an experiment that refuted the theory of J. Thompson and proposed a model of the atom like a planetary system. At the center of the atom there is a positively charged nucleus, around which negatively charged electrons rotate. In this case, the bulk of the atom's mass is concentrated in the nucleus, the mass of electrons is very small. The total charge of the nucleus and electrons must be zero, since the atom as a whole is electrically neutral.

Particle Mass Charge Absolute (kg) Relative Electric Relative Electron 9.109* .00051.602* Proton 1.673* .602* Neutron 1.675* Z – proton number (shows the number of protons in the nucleus and their total mass (relative)) N – neutron number (shows number of neutrons in the nucleus and their total mass (relative)) A - mass (nucleon) number - this is the sum of neutrons and protons in the nucleus and their total mass (relative))

Nucleon number (equal to relative atomic mass) - Proton number (equal to the atomic number of the element) A = 23 Z = 11 N = = 12 e = 11

OPTION 1 1) An atom is a particle consisting of ...... 2) The mass of an atom is determined by the sum of the masses of the particles: ... 3) The serial number of an element shows the number ..... and the number ..... in an atom 4) Atoms of one chemical element, differing in the value of the relative atomic mass are called ....... 5) The type of atoms with a certain nuclear charge is called.... 6) Write down the composition of the zinc atom using symbols (protons, neutrons, electrons, nucleon number) OPTION 2 1) Atomic nucleus comprises …. 2) Isotopes differ in the amount of ..... 3) The mass number of an atom is the sum of the masses of particles .... 4) Number…. = number.... = serial number of the element. 5) An electron is designated by the symbol..., has a charge...., and a relative mass.... 6) Write down the composition of the copper atom using symbols (protons, neutrons, electrons, nucleon number)

1. Basic concepts, definitions and laws of chemistry

1.2. Atom. Chemical element. Simple substance

The atom is a central concept in chemistry. All substances are made up of atoms. Atom - the limit of fragmentation of a substance by chemical means, i.e. an atom is the smallest chemically indivisible particle of matter. Atomic fission is possible only in physical processes - nuclear reactions and radioactive transformations.

Modern definition of an atom: an atom is the smallest chemically indivisible electrically neutral particle, consisting of a positively charged nucleus and negatively charged electrons.

In nature, atoms exist both in a free (individual, isolated) form (for example, noble gases are made up of individual atoms) and as part of various simple and complex substances. It is clear that in the composition of complex substances, atoms are not electrically neutral, but have an excess positive or negative charge (for example, Na + Cl −, Ca 2+ O 2−), i.e. In complex substances, atoms can be found in the form of monoatomic ions. Atoms and the monoatomic ions formed from them are called atomic particles.

The total number of atoms in nature cannot be counted, but they can be classified into narrower types, just as, for example, all the trees in a forest are classified according to characteristic features divided into birch, oak, spruce, pine, etc. The basis for classifying atoms into certain types is the charge of the nucleus, i.e. the number of protons in the nucleus of an atom, since it is this characteristic that is preserved, regardless of whether the atom is in a free or chemically bound form.

Chemical element- This is a type of atomic particles with the same nuclear charge.

For example, we mean the chemical element sodium, regardless of whether free sodium atoms or Na + ions in the composition of salts are considered.

The concepts of atom should not be confused, chemical element And simple substance. An atom is a concrete concept, atoms really exist, but a chemical element is an abstract, collective concept. For example, in nature there are specific copper atoms with rounded relative atomic masses of 63 and 65. But the chemical element copper is characterized by an averaged relative atomic mass, given in the periodic table of chemical elements by D.I. Mendeleev, which, taking into account the content of isotopes, is equal to 63.54 (in nature, there are no copper atoms with such an A r value). An atom in chemistry is traditionally understood as an electrically neutral particle, while a chemical element in nature can be represented by both electrically neutral and charged particles - monatomic ions: , , , .

A simple substance is one of the forms of existence of a chemical element in nature (another form is a chemical element in the composition of complex substances). For example, the chemical element oxygen in nature exists in the form of a simple substance O 2 and as part of a number of complex substances (H 2 O, Na 2 SO 4 ⋅ 10H 2 O, Fe 3 O 4). Often the same chemical element forms several simple substances. In this case, they talk about allotropy - the phenomenon of the existence of an element in nature in the form of several simple substances. The simple substances themselves are called allotropic modifications ( modifications) . A number of allotropic modifications are known for carbon (diamond, graphite, carbyne, fullerene, graphene, tubulenes), phosphorus (white, red and black phosphorus), oxygen (oxygen and ozone). Due to the phenomenon of allotropy, there are approximately 5 times more known simple substances than chemical elements.

Causes of allotropy:

• differences in the quantitative composition of molecules (O 2 and O 3);
• differences in the structure of the crystal lattice (diamond and graphite).

Allotropic modifications of a given element always differ in physical properties and chemical activity. For example, ozone is more active than oxygen, and the melting point of diamond is higher than fullerene. Allotropic modifications under certain conditions (changes in pressure, temperature) can transform into each other.

In most cases, the names of a chemical element and a simple substance are the same (copper, oxygen, iron, nitrogen, etc.), so it is necessary to distinguish between the properties (characteristics) of a simple substance as a collection of particles and the properties of a chemical element as a type of atom with the same nuclear charge.

A simple substance is characterized by structure (molecular or non-molecular), density, a certain state of aggregation under given conditions, color and odor, electrical and thermal conductivity, solubility, hardness, boiling and melting points (t boil and t pl), viscosity, optical and magnetic properties , molar (relative molecular) mass, chemical formula, chemical properties, methods of preparation and use. We can say that the properties of a substance are the properties of a collection of chemically related particles, i.e. physical body, since one atom or molecule has no taste, odor, solubility, melting and boiling points, color, electrical and thermal conductivity.

Properties (characteristics) chemical element: atomic number, chemical sign, relative atomic mass, atomic mass, isotopic composition, occurrence in nature, position in the periodic table, atomic structure, ionization energy, electron affinity, electronegativity, oxidation states, valence, allotropy phenomenon, mass and mole fraction in the composition of a complex substance, absorption and emission spectra. We can say that the properties of a chemical element are the properties of one particle or isolated particles.

The differences between the concepts of “chemical element” and “simple substance” are shown in table. 1.2 using nitrogen as an example.

Table 1.2

Differences between the concepts of “chemical element” and “simple substance” for nitrogen

Example 1.2. Indicate which of the following statements mention oxygen as a chemical element:

• a) the mass of the atom is 16u;
• b) forms two allotropic modifications;
• c) molar mass is 32 g/mol;
• d) poorly soluble in water.

Solution. Statements c), d) refer to a simple substance, and statements a), b) refer to the chemical element oxygen.

Answer: 3).

Each chemical element has its own symbol - chemical sign (symbol): K, Na, O, N, Cu, etc.

A chemical symbol can also express the composition of a simple substance. For example, the symbol of the chemical element Fe also reflects the composition of the simple substance iron. However, the chemical symbols O, H, N, Cl denote only chemical elements; simple substances have the formulas O 2, H 2, N 2, Cl 2.

As already noted, in most cases the names of chemical elements and simple substances are the same. Exceptions are the names of allotropic modifications of carbon (diamond, graphite, carbyne, fullerene) and one of the modifications of oxygen (oxygen and ozone). For example, when we use the word “graphite,” we mean only the simple substance (but not the chemical element) carbon.

The abundance of chemical elements in nature is expressed in mass and mole fractions. Mass fraction w - the ratio of the mass of atoms of a given element to total mass atoms of all elements. Mole fraction χ is the ratio of the number of atoms of a given element to the total number of atoms of all elements.

In the earth's crust (a layer about 16 km thick), oxygen atoms have the largest mass (49.13%) and molar (55%) shares, followed by silicon atoms (w (Si) = 26%, χ(Si) = 16 .35%). In the Galaxy, almost 92% of the total number of atoms are hydrogen atoms, and 7.9% are helium atoms. Mass fractions of atoms of the main elements in the human body: O - 65%, C - 18%, H - 10%, N - 3%, Ca - 1.5%, P - 1.2%.

The absolute values ​​of the atomic masses are extremely small (for example, the mass of an oxygen atom is about 2.7 ⋅ 10 −23 g) and are inconvenient for calculations. For this reason, a relative scale was developed atomic masses elements. Currently, the unit of measurement for relative atomic masses is 1/12 of the mass of an atom of the C-12 nuclide. This quantity is called constant atomic mass or atomic mass unit(a.u.m.) and has the international designation u:

m u = 1 a. e.m. = 1 u = 1 / 12 (m a 12 C) =

1.66 ⋅ 10 − 24 g = 1.66 ⋅ 10 − 27 kg.

It is easy to show that the numerical value of u is equal to 1/N A:

1 u = 1 12 m a (12 C) = 1 12 M (C) N A = 1 12 12 N A = 1 N A =

1 6.02 ⋅ 10 23 = 1.66 ⋅ 10 − 24 (g).

Relative atomic mass of an element A r (E) is a physical dimensionless quantity that shows how many times the mass of an atom or the average mass of an atom (respectively for isotopically pure and isotopically mixed elements) is greater than 1/12 of the mass of an atom of the C-12 nuclide:

A r (E) = m a (E) 1 a. e.m. = m a (E) 1 u. (1.1)

Knowing the relative atomic mass, you can easily calculate the mass of an atom:

m a (E) = A r (E)u = A r (E) ⋅ 1.66 ⋅ 10 −24 (g) =

A r (E) ⋅ 1.66 ⋅ 10 −27 (kg).

Molecule. And he. Substances of molecular and non-molecular structure. Chemical equation

When atoms interact, more complex particles are formed - molecules.

A molecule is the smallest electrically neutral isolated collection of atoms, capable of independent existence and being the bearer of the chemical properties of a substance.

Molecules have the same qualitative and quantitative composition as the substance they form. The chemical bond between atoms in a molecule is much stronger than the interaction forces between molecules (which is why a molecule can be considered as a separate, isolated particle). In chemical reactions, molecules, unlike atoms, are not preserved (destroyed). Like an atom, an individual molecule does not have such physical properties substances such as color and odor, melting and boiling points, solubility, thermal and electrical conductivity, etc.

Let us emphasize that the molecule is precisely the carrier of the chemical properties of a substance; one cannot say that the molecule retains (has exactly the same) Chemical properties substances, since the chemical properties of a substance are significantly influenced by intermolecular interactions, which are absent for an individual molecule. For example, the substance trinitroglycerin has the ability to explode, but not an individual molecule of trinitroglycerin.

An ion is an atom or group of atoms that has a positive or negative charge.

Positively charged ions are called cations, and negatively charged ones are called anions. Ions can be simple, i.e. monoatomic (K +, Cl −), and complex (NH 4 +, NO 3 −), single-charged (Na +, Cl −) and multi-charged (Fe 3+, PO 4 3 −).

1. For a given element, a simple ion and a neutral atom have the same number of protons and neutrons, but differ in the number of electrons: the cation has fewer and the anion has more than the electrically neutral atom.

2. The mass of a simple or complex ion is the same as the mass of the corresponding electrically neutral particle.

It should be borne in mind that not all substances are composed of molecules.

Substances made up of molecules are called substances of molecular structure. These can be either simple (argon, oxygen, fullerene) or complex (water, methane, ammonia, benzene) substances.

All gases and almost all liquids (with the exception of mercury) have a molecular structure; solids can have both a molecular (sucrose, fructose, iodine, white phosphorus, phosphoric acid) and non-molecular structure (diamond, black and red phosphorus, carborundum SiC, table salt NaCl). In substances with a molecular structure, the bonds between molecules (intermolecular interaction) are weak. When heated, they are easily destroyed. It is for this reason that substances of molecular structure have relatively low temperatures melting and boiling, volatile (as a result they often have an odor).

Substances of non-molecular structure consist of electrically neutral atoms or simple or complex ions. For example, diamond, graphite, black phosphorus, silicon, boron are made of electrically neutral atoms, and salts are made of simple and complex ions, for example KF and NH 4 NO 3. Metals are made up of positively charged atoms (cations). Carborundum SiC, silicon oxide (IV) SiO 2, alkalis (KOH, NaOH), most salts (KCl, CaCO 3), binary compounds of metals with non-metals (basic and amphoteric oxides, hydrides, carbides, silicides, nitrides, phosphides), intermetallic compounds (compounds of metals with each other). In substances of non-molecular structure, individual atoms or ions are connected to each other by strong chemical bonds, therefore, under normal conditions, these substances are solid, non-volatile, and have high melting points.

For example, sucrose (molecular structure) melts at 185 °C, and sodium chloride (non-molecular structure) melts at 801 °C.

In the gas phase, all substances consist of molecules, and even those that at ordinary temperatures have a non-molecular structure. For example, when high temperature molecules of NaCl, K 2 , and SiO 2 were found in the gas phase.

For substances that decompose when heated (CaCO 3, KNO 3, NaHCO 3), molecules cannot be obtained by heating the substance

Molecular substances form the basis of the organic world, and non-molecular substances form the basis of the inorganic (mineral) world.

Chemical formula. Formula unit. Chemical equation

The composition of any substance is expressed using a chemical formula. Chemical formula is an image of the qualitative and quantitative composition of a substance using symbols of chemical elements, as well as numerical, alphabetic and other signs.

For simple substances of non-molecular structure, the chemical formula coincides with the sign of the chemical element (for example, Cu, Al, B, P). In the formula of a simple substance of molecular structure, indicate (if necessary) the number of atoms in the molecule: O 3, P 4, S 8, C 60, C 70, C 80, etc. The formulas of noble gases are always written with one atom: He, Ne, Ar, Xe, Kr, Rn. When writing equations chemical reactions the chemical formulas of some polyatomic molecules of simple substances can (unless specifically stated) be written in the form of symbols of elements (single atoms): P 4 → P, S 8 → S, C 60 → C (this cannot be done for ozone O 3, oxygen O 2 , nitrogen N2, halogens, hydrogen).

For complex substances of molecular structure, empirical (simple) and molecular (true) formulas are distinguished. Empirical formula shows the smallest integer ratio of the numbers of atoms in a molecule, and molecular formula - true integer ratio of atoms. For example, the true formula of ethane is C 2 H 6, and the simplest is CH 3. The simplest formula is obtained by dividing (reducing) the numbers of atoms of the elements in the true formula by some suitable number. For example, the simplest formula for ethane was obtained by dividing the numbers of C and H atoms by 2.

The simplest and true formulas can either coincide (methane CH 4, ammonia NH 3, water H 2 O) or not coincide (phosphorus oxide (V) P 4 O 10, benzene C 6 H 6, hydrogen peroxide H 2 O 2, glucose C 6 H 12 O 6).

Chemical formulas allow you to calculate the mass fractions of atoms of elements in a substance.

The mass fraction w of atoms of the element E in a substance is determined by the formula

w (E) = A r (E) ⋅ N (E) M r (V) , (1.2)

where N (E) is the number of atoms of the element in the formula of the substance; M r (B) - relative molecular (formula) mass of a substance.

For example, for sulfuric acid M r (H 2 SO 4) = 98, then mass fraction oxygen atoms in this acid

w (O) = A r (O) ⋅ N (O) M r (H 2 SO 4) = 16 ⋅ 4 98 ≈ 0.653 (65.3%).

Using formula (1.2), the number of atoms of an element in a molecule or formula unit is found:

N (E) = M r (V) ⋅ w (E) A r (E) (1.3)

or molar (relative molecular or formula) mass of a substance:

M r (V) = A r (E) ⋅ N (E) w (E) . (1.4)

In formulas 1.2–1.4, the values ​​of w (E) are given in fractions of unity.

Example 1.3. In a certain substance, the mass fraction of sulfur atoms is 36.78%, and the number of sulfur atoms in one formula unit is two. Specify the molar mass (g/mol) of the substance:

Solution . Using formula 1.4, we find

M r = A r (S) ⋅ N (S) w (S) = 32 ⋅ 2 0.3678 = 174 ,

M = 174 g/mol.

Answer: 2).

The following example shows a method for finding the simplest formula of a substance based on the mass fractions of elements.

Example 1.4. In some chlorine oxide, the mass fraction of chlorine atoms is 38.8%. Find the formula of the oxide.

Solution . Since w (Cl) + w (O) = 100%, then

w(O) = 100% − 38.8% = 61.2%.

If the mass of the substance is 100 g, then m (Cl) = 38.8 g and m (O) = 61.2 g.

Let's imagine the oxide formula as Cl x O y. We have

x : y = n (Cl) : n (O) = m (Cl) M (Cl) : m (O) M (O) ;

x: y = 38.8 35.5: 61.2 16 = 1.093: 3.825.

Dividing the resulting numbers by the smallest of them (1.093), we find that x: y = 1: 3.5 or, multiplying by 2, we get x: y = 2: 7. Therefore, the formula of the oxide is Cl 2 O 7.

Answer: Cl 2 O 7.

For all complex substances of non-molecular structure, chemical formulas are empirical and reflect the composition not of molecules, but of the so-called formula units.

Formula unit(FE) - a group of atoms corresponding to the simplest formula of a substance of non-molecular structure.

Thus, the chemical formulas of substances of non-molecular structure are formula units. Examples of formula units: KOH, NaCl, CaCO 3, Fe 3 C, SiO 2, SiC, KNa 2, CuZn 3, Al 2 O 3, NaH, Ca 2 Si, Mg 3 N 2, Na 2 SO 4, K 3 PO 4, etc.

Formula units can be thought of as structural units substances of non-molecular structure. For substances with a molecular structure, these are obviously actually existing molecules.

Using chemical formulas, the equations of chemical reactions are written.

Chemical equation is a conventional notation of a chemical reaction using chemical formulas and other signs (equals, plus, minus, arrows, etc.).

A chemical equation is a consequence of the law of conservation of mass, so it is written so that the numbers of atoms of each element in both its sides are equal.

The numbers before the formulas are called stoichiometric coefficients, in this case the unit is not written down, but is implied (!) and taken into account when calculating the total sum of stoichiometric coefficients. Stoichiometric coefficients show in what molar ratios the starting substances react and the reaction products are formed. For example, for a reaction whose equation is

3Fe 3 O 4 + 8Al = 9Fe + 4Al 2 O 3

n (Fe 3 O 4) n (Al) = 3 8; n (Al) n (Fe) = 8 9 etc.

In reaction schemes, coefficients are not placed and an arrow is used instead of an equal sign:

FeS 2 + O 2 → Fe 2 O 3 + SO 2

The arrow is also used when writing equations of chemical reactions involving organic substances (so as not to confuse the equals sign with a double bond):

CH 2 =CH 2 + Br 2 → CH 2 Br–CH 2 Br,

as well as equations for the electrochemical dissociation of strong electrolytes:

NaCl → Na + + Cl − .

Law of Constancy of Composition

For substances of molecular structure this is true law of constancy of composition(J. Proust, 1808): every substance of molecular structure, regardless of the method and conditions of production, has a constant qualitative and quantitative composition.

From the law of constancy of composition it follows that in molecular compounds the elements must be in strictly defined mass proportions, i.e. have a constant mass fraction. This is true if the isotopic composition of the element does not change. For example, the mass fraction of hydrogen atoms in water, regardless of the method of its preparation from natural substances (synthesis from simple substances, heating of copper sulfate CuSO 4 · 5H 2 O, etc.) will always be equal to 11.1%. However, in water obtained by the interaction of deuterium molecules (hydrogen nuclide with A r ≈ 2) and natural oxygen (A r = 16), the mass fraction of hydrogen atoms

w (H) = 2 ⋅ 2 2 ⋅ 2 + 16 = 0.2 (20%).

Substances that obey the law of constancy of composition, i.e. substances of molecular structure are called stoichiometric.

Substances of non-molecular structure (especially carbides, hydrides, nitrides, oxides and sulfides of d-family metals) do not obey the law of constant composition, which is why they are called non-stoichiometric. For example, depending on the production conditions (temperature, pressure), the composition of titanium(II) oxide is variable and ranges from TiO 0.7 –TiO 1.3, i.e. in a crystal of this oxide, for every 10 titanium atoms there can be from 7 to 13 oxygen atoms. However, for many substances of non-molecular structure (KCl, NaOH, CuSO 4), deviations from a constant composition are very insignificant, so we can assume that their composition is practically independent of the method of preparation.

Relative molecular and formula weight

To characterize substances of molecular and non-molecular structure, respectively, the concepts of “relative molecular mass” and “relative formula mass” are introduced, which are denoted by the same symbol - M r

Relative molecular weight- dimensionless physical quantity, which shows how many times the mass of the molecule is greater than 1/12 of the mass of the atom of the C-12 nuclide:

M r (B) = m mol (B) u . (1.5)

Relative formula mass- a dimensionless physical quantity that shows how many times the mass of a formula unit is greater than 1/12 of the mass of an atom of the C-12 nuclide:

M r (B) = m PU (B) u . (1.6)

Formulas (1.5) and (1.6) allow you to find the mass of a molecule or physical unit:

m (mol, FU) = uM r . (1.7)

In practice, M r values ​​are found by summing the relative atomic masses of the elements forming a molecule or formula unit, taking into account the number of individual atoms. For example:

M r (H 3 PO 4) = 3A r (H) + A r (P) + 4A r (O) =

3 ⋅ 1 + 31 + 4 ⋅ 16 = 98.

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