What is called an atom. Molecule and atom: definition

Chemistry is the science of substances and their transformations into each other.

Substances are chemically pure substances

A chemically pure substance is a collection of molecules that have the same qualitative and quantitative composition and the same structure.

CH 3 -O-CH 3 -

CH 3 -CH 2 -OH

Molecule - the smallest particles of a substance that have all its chemical properties; a molecule is made up of atoms.

An atom is a chemically indivisible particle from which molecules are formed. (for noble gases the molecule and the atom are the same, He, Ar)

An atom is an electrically neutral particle consisting of a positively charged nucleus around which negatively charged electrons are distributed according to their strictly defined laws. Moreover, the total charge of electrons is equal to the charge of the nucleus.

The nucleus of an atom consists of positively charged protons (p) and neutrons (n) that do not carry any charge. The common name for neutrons and protons is nucleons. The mass of protons and neutrons is almost the same.

Electrons (e -) carry a negative charge equal to the charge of a proton. The mass of e is approximately 0.05% of the mass of the proton and neutron. Thus, the entire mass of an atom is concentrated in its nucleus.

The number p in an atom, equal to the charge of the nucleus, is called the serial number (Z), since the atom is electrically neutral; the number e is equal to the number p.

The mass number (A) of an atom is the sum of protons and neutrons in the nucleus. Accordingly, the number of neutrons in an atom is equal to the difference between A and Z (mass number of the atom and atomic number). (N=A-Z).

17 35 Cl р=17, N=18, Z=17. 17р + , 18n 0 , 17е - .

Nucleons

The chemical properties of atoms are determined by their electronic structure (number of electrons), which is equal to the atomic number (nuclear charge). Therefore, all atoms with the same nuclear charge behave chemically in the same way and are calculated as atoms of the same chemical element.

A chemical element is a collection of atoms with the same nuclear charge. (110 chemical elements).

Atoms, having the same nuclear charge, can differ in mass number, which is associated with a different number of neutrons in their nuclei.

Atoms having the same Z but different mass numbers are called isotopes.

17 35 Cl 17 37 Cl

Isotopes of hydrogen H:

Designation: 1 1 N 1 2 D 1 3 T

Name: protium deuterium tritium

Core composition: 1р 1р+1n 1р+2n

Protium and deuterium are stable

Tritium decays (radioactive) Used in hydrogen bombs.

Atomic mass unit. Avogadro's number. Mol.

The masses of atoms and molecules are very small (approximately 10 -28 to 10 -24 g); in order to practically display these masses, it is advisable to introduce your own unit of measurement, which would lead to a convenient and familiar scale.

Since the mass of an atom is concentrated in its nucleus, consisting of protons and neutrons of almost equal mass, it is logical to take the mass of one nucleon as a unit of atomic mass.

We agreed to take one twelfth of the carbon isotope, which has a symmetrical structure of the nucleus (6p+6n), as the unit of mass of atoms and molecules. This unit is called the atomic mass unit (amu), it is numerically equal to the mass of one nucleon. In this scale, the masses of atoms are close to integer values: He-4; Al-27; Ra-226 a.u.m……

Let's calculate the mass of 1 amu in grams.

1/12 (12 C) = =1.66*10 -24 g/a.u.m

Let's calculate how many amu are contained in 1g.

N A = 6.02 *-Avogadro number

The resulting ratio is called Avogadro's number and shows how many amu are contained in 1g.

Atomic masses given in the Periodic Table are expressed in amu

Molecular mass is the mass of a molecule, expressed in amu, and is found as the sum of the masses of all atoms that form a given molecule.

m(1 molecule H 2 SO 4)= 1*2+32*1+16*4= 98 a.u.

To move from amu to 1 g, which is practically used in chemistry, a portion calculation of the amount of a substance was introduced, with each portion containing the number N A of structural units (atoms, molecules, ions, electrons). In this case, the mass of such a portion, called 1 mole, expressed in grams, is numerically equal to the atomic or molecular mass expressed in amu.

Let's find the mass of 1 mol H 2 SO 4:

M(1 mol H 2 SO 4)=

98a.u.m*1.66**6.02*=

As you can see, the molecular and molar masses are numerically equal.

1 mole– the amount of a substance containing the Avogadro number of structural units (atoms, molecules, ions).

Molecular weight(M)- mass of 1 mole of a substance, expressed in grams.

Amount of substance - V (mol); mass of substance m(g); molar mass M(g/mol) - related by the relationship: V=;

2H 2 O+ O 2 2H 2 O

2 mole 1 mole

2.Basic laws of chemistry

The law of constancy of the composition of a substance - a chemically pure substance, regardless of the method of preparation, always has a constant qualitative and quantitative composition.

CH3+2O2=CO2+2H2O

NaOH+HCl=NaCl+H2O

Substances with a constant composition are called daltonites. As an exception, substances of unchanged composition are known - bertholites (oxides, carbides, nitrides)

Law of conservation of mass (Lomonosov) - the mass of substances that enter into a reaction is always equal to the mass of the reaction products. It follows from this that atoms do not disappear during the reaction and are not formed; they pass from one substance to another. This is the basis for the selection of coefficients in the equation of a chemical reaction; the number of atoms of each element in the left and right sides of the equation must be equal.

Law of equivalents - in chemical reactions, substances react and are formed in quantities equal to the equivalent (How many equivalents of one substance are consumed, exactly the same number of equivalents are consumed or formed of another substance).

Equivalent is the amount of a substance that, during a reaction, adds, replaces, or releases one mole of H atoms (ions). The equivalent mass expressed in grams is called the equivalent mass (E).

Gas laws

Dalton's law - the total pressure of a gas mixture is equal to the sum of the partial pressures of all components of the gas mixture.

Avogadro's Law: Equal volumes of different gases under the same conditions contain an equal number of molecules.

Consequence: one mole of any gas under normal conditions (t=0 degrees or 273K and P=1 atmosphere or 101255 Pascal or 760 mm Hg. Col.) occupies V=22.4 liters.

V which occupies one mole of gas is called the molar volume Vm.

Knowing the volume of gas (gas mixture) and Vm under given conditions, it is easy to calculate the amount of gas (gas mixture) =V/Vm.

The Mendeleev-Clapeyron equation relates the amount of gas to the conditions under which it is found. pV=(m/M)*RT= *RT

When using this equation, all physical quantities must be expressed in SI: p-gas pressure (pascal), V-gas volume (liters), m-gas mass (kg), M-molar mass (kg/mol), T- temperature on an absolute scale (K), Nu-amount of gas (mol), R-gas constant = 8.31 J/(mol*K).

D - the relative density of one gas compared to another - the ratio of M gas to M gas, chosen as a standard, shows how many times one gas is heavier than another D = M1 / ​​M2.

Methods of expressing the composition of a mixture of substances.

Mass fraction W - the ratio of the mass of the substance to the mass of the entire mixture W=((m mixture)/(m solution))*100%

Mole fraction æ is the ratio of the number of substances to the total number of all substances. in the mixture.

Most chemical elements in nature are present as a mixture of different isotopes; Knowing the isotopic composition of a chemical element, expressed in mole fractions, the weighted average value of the atomic mass of this element is calculated, which is converted into ISHE. А= Σ (æi*Аi)= æ1*А1+ æ2*А2+…+ æn*Аn, where æi is the mole fraction of the i-th isotope, Аi is the atomic mass of the i-th isotope.

Volume fraction (φ) is the ratio of Vi to the volume of the entire mixture. φi=Vi/VΣ

Knowing the volumetric composition of the gas mixture, the Mav of the gas mixture is calculated. Мср= Σ (φi*Mi)= φ1*М1+ φ2*М2+…+ φn*Мn

Every day we use some objects: we take them in our hands, perform any manipulations on them - turn them over, examine them, and ultimately break them. Have you ever thought about what these objects are made of? "What can we think about here? Made of metal/wood/plastic/fabric!" - many of us will answer in bewilderment. This is partly the correct answer. What are these materials made of - metal, wood, plastic, fabric and many other substances? Today we will discuss this issue.

Molecule and atom: definition

For a knowledgeable person, the answer is simple and banal: from atoms and molecules. But some people get puzzled and start asking questions: “What are an atom and a molecule? What do they look like?” etc. Let's answer these questions in order. Well, first of all, what are an atom and a molecule? Let us tell you right away that these definitions are not the same thing. And even more than that, these are completely different terms. So, an atom is the smallest part of a chemical element, which is the bearer of its properties, a particle of substance of scanty mass and size. A molecule is an electrically neutral particle that is formed by several connected atoms.

What is an atom: structure

An atom consists of an electron shell and (photo). In turn, the core consists of protons and neutrons, and the shell consists of electrons. In an atom, protons are positively charged, electrons are negatively charged, and neutrons are not charged at all. If the number of protons corresponds, then the atom is electrically neutral, i.e. If we touch a substance formed from molecules with such atoms, we will not feel the slightest electrical impulse. And even super-powerful computers will not catch it due to the absence of the latter. But it happens that there are more protons than electrons, and vice versa. Then it would be more correct to call such atoms ions. If there are more protons in it, then it is electrically positive, but if electrons predominate, it is electrically negative. Each specific atom has a strict number of protons, neutrons and electrons. And it can be calculated. A template for solving problems of finding the number of these particles looks like this:

Chem. element - R (insert element name)
Protons (p) - ?
Electrons (e) - ?
Neutrons (n) - ?
Solution:
p = serial number of chemical. element R in the periodic table named after D.I. Mendeleev
e = p
n = A r (R) - No. R

What is a molecule: structure

A molecule is the smallest particle of a chemical substance, that is, it is already directly included in its composition. A molecule of a certain substance consists of several identical or different atoms. The structural features of molecules depend on the physical properties of the substance in which they are present. Molecules are made up of electrons and atoms. The location of the latter can be found using the structural formula. allows you to determine the progress of a chemical reaction. They are usually neutral (have no electrical charge) and have no unpaired electrons (all valences are saturated). However, they can also be charged, in which case their correct name is ions. Molecules can also have unpaired electrons and unsaturated valencies - in this case they are called radicals.

Conclusion

Now you know what an atom is and all substances, without exception, are composed of molecules, and the latter, in turn, are built of atoms. The physical properties of a substance determine the arrangement and connection of atoms and molecules in it.

from gr. a - negative particle and temnein - to divide): the smallest element of the body, which, as the word itself shows, is indivisible (at least this was believed until the beginning of our century).

Excellent definition

Incomplete definition ↓

ATOM

(Greek ?????? - indivisible) - the smallest particle of a chemical. element, preserving its properties. Concept "A." as the smallest indivisible particle of substance (matter) was introduced in the 5th century. BC. Democritus. Philosophers and natural scientists of the 16th-18th centuries. used this concept along with the terms “corpuscula” (Latin corpuscula - small body) and “individual” (Latin individuum - lit. indivisible) in approximately the same sense. Until the end of the 19th century. in physics and chemistry, the idea of ​​the indivisibility of aluminum dominated, but after the discovery of the electron by J. J. Thomson (1897), it became clear that aluminum has a complex structure. As a result of the experiments of E. Rutherford (1909-11), the nuclear model of atoms was established. The first quantum theory of atoms was developed by N. Bohr (1911-13). According to modern According to ideas, electrons consist of a nucleus and electron shells. The nucleus consists of protons and neutrons; almost the entire mass of the element and the entire positive charge are concentrated in it: qя = Ze, where Z is the serial number of the element in the table. Mendeleev, e = 1.6 10-19 C - elementary charge. The number of protons in the nucleus is Np = Z. Electrons move around the nucleus, forming electron shells. The number of electrons in A. is also equal to Z. Their negative charge -Ze neutralizes the positive charge of the nucleus, which leads to the neutrality of A. The number of neutrons in the nucleus Nн = A–Z, where A is the mass number (an integer number closest to the mass of A. in the periodic table). Electrons are distributed among energy states and shells in accordance with the Pauli principle. The average size of an asteris is ~10-10 m, its core is ~10-15 m. F.M. Dyagilev

ATOM(from the Greek atomos - indivisible), the smallest particle of a chemical. element, its holy. Each chem. An element corresponds to a collection of specific atoms. By bonding with each other, atoms of the same or different elements form more complex particles, for example. . All the variety of chemicals. in-in (solid, liquid and gaseous) due to decomposition. combinations of atoms with each other. Atoms can also exist freely. state (in , ). The properties of the atom, including the most important ability of the atom to form chemicals. conn., are determined by the features of its structure.

General characteristics of the structure of the atom. An atom consists of a positively charged nucleus surrounded by a cloud of negatively charged ones. The dimensions of an atom as a whole are determined by the dimensions of its electron cloud and are large compared to the dimensions of the atomic nucleus (the linear dimensions of an atom are ~ 10~8 cm, its nucleus ~ 10" -10" 13 cm). The electron cloud of an atom does not have strictly defined boundaries, so the size of the atom means. degrees are conditional and depend on the methods of their determination (see). The nucleus of an atom consists of Z and N, held together by nuclear forces (see). Positive charge and negative. the charge is the same abs. magnitude and are equal to e = 1.60*10 -19 C; does not have electric power. charge. Nuclear charge +Ze - basic. characteristic of an atom that determines its belonging to a particular chemical. element. element in periodic periodic system () is equal to the number in the nucleus.

In an electrically neutral atom, the number in the cloud is equal to the number in the nucleus. However, under certain conditions, it can lose or add, turning respectively. in positive or deny. , eg. Li + , Li 2+ or O - , O 2- . When talking about atoms of a certain element, we mean both neutral atoms and that element.

The mass of an atom is determined by the mass of its nucleus; the mass (9.109*10 -28 g) is approximately 1840 times less than the mass or (1.67*10 -24 g), so the contribution to the mass of the atom is insignificant. Total number and A = Z + N called. . and nuclear charge are indicated respectively. superscript and subscript to the left of the element symbol, e.g. 23 11 Na. The type of atoms of one element with a certain value N is called. . Atoms of the same element with the same Z and different N are called. this element. The difference in mass has little effect on their chemistry. and physical St. Vah. Most importantly, the differences () are observed due to the large relative. differences in the masses of an ordinary atom (), D and T. The exact values ​​of the masses of atoms are determined by methods.

The stationary state of a one-electron atom is uniquely characterized by four quantum numbers: n, l, m l and m s. The energy of an atom depends only on n, and a level with a given n corresponds to a number of states differing in the values ​​of l, m l, m s. States with given n and l are usually denoted as 1s, 2s, 2p, 3s, etc., where the numbers indicate the values ​​of l, and the letters s, p, d, f and further in Latin correspond to the values ​​d = 0, 1, 2 , 3, ... Number of dec. states with given p and d is equal to 2(2l+ 1) the number of combinations of values ​​m l and m s. Total number of divers. states with given n equals , i.e., levels with values ​​n = 1, 2, 3, ... correspond to 2, 8, 18, ..., 2n 2 decomp. . A level to which only one (one wave function) corresponds is called. non-degenerate. If a level corresponds to two or more , it is called. degenerate (see). In an atom, the energy levels are degenerate in the values ​​of l and m l; degeneracy in m s occurs only approximately if the interaction is not taken into account. spin magnet moment with magnetic field caused by orbital motion in electric. nuclear field (see). This is a relativistic effect, small in comparison with the Coulomb interaction, but it is fundamentally significant, because leads to additional splitting of energy levels, which manifests itself in the form of the so-called. fine structure.

For given n, l and m l, the square of the modulus of the wave function determines the average distribution for the electron cloud in the atom. Diff. atoms differ significantly from each other in distribution (Fig. 2). Thus, at l = 0 (s-states) it is different from zero at the center of the atom and does not depend on the direction (i.e., spherically symmetric), for other states it is equal to zero at the center of the atom and depends on the direction.

Rice. 2. Shape of electron clouds for different states of the atom.

In multielectron atoms due to mutual electrostatic. repulsion significantly reduces their connection with the nucleus. For example, the energy of separation from He + is 54.4 eV; in a neutral He atom it is much less - 24.6 eV. For heavier atoms, the bond is ext. with a core even weaker. Specificity plays an important role in multielectron atoms. , associated with indistinguishability, and the fact that they obey, according to Krom, each one characterized by four quantum numbers cannot contain more than one. For a multi-electron atom, it makes sense to talk only about the entire atom as a whole. However, approximately, in the so-called. In the one-electron approximation, each one-electron state (a certain orbital described by the corresponding function) can be considered individually and characterized by a set of four quantum numbers n, l, m l and m s. The collection 2(2l+ 1) in a state with given n and l forms an electron shell (also called a sublevel, subshell); if all these states are occupied, the shell is called. filled (closed). A set of 2n 2 states with the same n, but different l forms an electronic layer (also called a level, shell). For n = 1, 2, 3, 4, ... layers are designated by the symbols K, L, M, N, ... The numbers in shells and layers when completely filled are given in the table:

Between stationary states in an atom are possible. When transitioning from a higher energy level E i to a lower energy level E k, the atom gives up energy (E i - E k), and during the reverse transition it receives it. During radiative transitions, an atom emits or absorbs an electromagnetic quantum. radiation (photon). They are also possible when an atom gives or receives energy during interaction. with other particles with which it collides (for example, in) or is bound for a long time (in. Chemical properties are determined by the structure of the outer electron shells of atoms, in which they are bonded relatively weakly (binding energies from several eV up to several tens of eV). The structure of the outer shells of atoms of chemical elements of one group (or subgroup) of the periodic system is similar, which determines the similarity of the chemical properties of these elements. With an increase in the number in the filling shell, their binding energy, as a rule, , increases; the highest binding energy is in a closed shell. Therefore, atoms with one or more in a partially filled outer shell give them up in chemical reactions. Atoms that lack one or more to form a closed outer shell. shells, usually accept them. Atoms with closed outer shells, under ordinary conditions, do not enter into chemical reactions.

Internal structure shells of atoms, which are bound much more tightly (binding energy 10 2 -10 4 eV), manifests itself only during interaction. atoms with fast particles and high-energy photons. Such interactions determine the nature of the X-ray spectra and the scattering of particles (,) on atoms (see). The mass of an atom determines its physical properties. holy, like an impulse, kinetic. energy. From mechanical and related mag. and electric moments of the atomic nucleus depend on certain subtle physical factors. effects (depends on the frequency of radiation, which determines the dependence of the refractive index of the atom associated with it on it. The close connection between the optical properties of an atom and its electrical properties is especially clearly manifested in the optical spectra.

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Spanish literature for the article "ATOM": Karapetyants M. X., Drakin S. I., Structure, 3rd ed., M., 1978; Shloliekiy E.V., Atomic physics, 7th ed., vol. 1-2, M., 1984. M.A. Elyashevich.

Page "ATOM" prepared based on materials.

Our world is fraught with many secrets and unsolved things, because physical and chemical processes are truly amazing. But scientists have constantly sought to understand the essence of matter from which life in the universe is woven. This question has often arisen among humanity for a long time. This article will tell you what a simple atom is, what elementary particles it consists of, and how scientists discovered the existence of the smallest part of a chemical element.

What is an atom and how was it discovered?

An atom is the smallest part of a chemical element. Atoms of different elements differ in the number of protons and neutrons.

The first who began to seriously think about what all objects consist of were the ancient Greeks. By the way, the word “atom” comes from the Greek language and translated means “indivisible”. The Greeks believed that sooner or later there would be a particle that could not be divided. But their reasoning was more speculative than scientific, so it cannot be said that this ancient people were the first to make great discoveries about the existence of small particles.

Let's consider the earliest ideas about what an atom is.

Ancient Greek philosopher Democritus assumed that the main parameters of any substance are shape and mass, and that any substance consists of small particles. Democritus gave an example with fire: if it burns, then the particles of which it consists are sharp. Water, on the contrary, is smooth because it is capable of flowing. And the state of the particles of solid objects, in his opinion, is rough, since they are capable of completely bonding to each other. Democritus was also confident that the human soul consists of atoms.

Interesting fact: if until the 19th century only philosophers dealt with the question of the atom, then John Dalton became the first experimenter to study small particles. In the process of experiments, he found out that atoms have different masses, as well as different properties. By the way, studying the arrangement of atoms in the molecules of specific substances is much more interesting if you observe the chemical reactions that occur during experiments. Dalton’s works, although they did not explain what an atom is as a whole, did provide guidance for some other scientists.

In 1904 John Thomson put forward an assumption about the model of the atom: the scientist believed that the atom consists of a positively charged substance, inside of which negatively charged corpuscles are located. The problem with the assumption is that Thompson tried to use his own model to consider the spectral lines of elements, but his experiments began to not work out very well.

At the same time, the Japanese physicist Hataro Nagaoka admitted that the atom is similar to the planet Saturn: it supposedly consists of a nucleus with a positive charge and electrons that revolve around it. But his model of the atom was not entirely correct.

In 1911, the scientist Rutherford put forward another hypothesis about the structure of the atom. The result of his hypotheses was stunning: now modern science largely relies on the discovery of this physicist.

In 1913 Niels Bohr put forward a semi-classical theory of the structure of the atom, based on the works of Rutherford.

Creation of Rutherford's model of the atom

Let's look at this model because it describes in detail some of the properties of the atom. As stated earlier, Ernest Rutherford, the "father" of nuclear physics, began working on a model of the atom in 1911. The physicist began to get the desired result when he began to refute Thomson’s model of the atom. The Geiger and Marsden alpha particle scattering experiment came to the scientist's aid. The scientist suggested that the atom has a very small positively charged nucleus. These arguments helped in creating a model of the atom, which is similar to the solar system, which is why it was given the name "Planetary model of the atom".

At the center of the atom is the nucleus, which contains almost the entire mass of the atom and has a positive charge. The nucleus consists of protons and neutrons. Protons are elementary particles with a positive charge, and neutrons are elementary particles that have no charge. Electrons revolve around the core, like the planets of the solar system.