Atom modern definition. What is an atom? What parts does it consist of and how is its mass measured?

In 1913 the Danish physicist Niels Bohr proposed his theory of atomic structure. He took as a basis the planetary model of the atom developed by the physicist Rutherford. In it, the atom was likened to objects of the macrocosm - a planetary system, where planets move in orbits around a large star. Similarly, in the planetary model of the atom, electrons move in orbits around a heavy nucleus located at the center.

Bohr introduced the idea of ​​quantization into atomic theory. According to it, electrons can only move in fixed orbits corresponding to certain energy levels. It was Bohr's model that became the basis for the creation of the modern quantum mechanical model of the atom. In this model, the atomic nucleus, consisting of positively charged protons and uncharged neutrons, is also surrounded by negatively charged electrons. However, according to quantum mechanics, it is impossible to determine any exact trajectory or orbit of motion for an electron - there is only a region in which electrons with a similar energy level are located.

What's inside an atom?

Atoms are made up of electrons, protons and neutrons. Neutrons were discovered after physicists developed a planetary model of the atom. Only in 1932, while conducting a series of experiments, James Chadwick discovered particles that had no charge. The absence of charge was confirmed by the fact that these particles did not react in any way to the electromagnetic field.

The nucleus of an atom itself is formed by heavy particles - protons and neutrons: each of these particles is almost two thousand times heavier than an electron. Protons and neutrons are also similar in size, but protons have a positive charge and neutrons have no charge at all.

In turn, protons and neutrons consist of elementary particles called quarks. In modern physics, quarks are the smallest, fundamental particle of matter.

The dimensions of the atom itself are many times greater than the dimensions of the nucleus. If you enlarge an atom to the size of a football field, then the size of its nucleus can be comparable to the size of a tennis ball in the center of such a field.

In nature, there are many atoms that differ in size, mass and other characteristics. A collection of atoms of the same type is called a chemical element. Today, more than one hundred chemical elements are known. Their atoms differ in size, mass, and structure.

Electrons inside an atom

Negatively charged electrons move around the nucleus of an atom, forming a kind of cloud. The massive nucleus attracts electrons, but the energy of the electrons themselves allows them to “run away” further from the nucleus. Thus, the higher the energy of the electron, the farther it is from the nucleus.

The electron energy value cannot be arbitrary; it corresponds to a clearly defined set of energy levels in the atom. That is, the electron energy changes abruptly from one level to another. Accordingly, an electron can move only within a limited electron shell corresponding to one or another energy level - this is the meaning of Bohr’s postulates.

Having received more energy, the electron “jumps” to a layer higher from the nucleus, having lost energy - on the contrary, to a lower layer. Thus, the cloud of electrons around the nucleus is ordered in the form of several “sliced” layers.

History of ideas about the atom

The word “atom” itself comes from the Greek “indivisible” and goes back to the ideas of ancient Greek philosophers about the smallest indivisible part of matter. In the Middle Ages, chemists became convinced that some substances could not be further broken down into their constituent elements. These smallest particles of matter are called atoms. In 1860, at an international congress of chemists in Germany, this definition was officially enshrined in world science.

At the end of the 19th and beginning of the 20th centuries, physicists discovered subatomic particles and it became clear that the atom is not in fact indivisible. Theories about the internal structure of the atom were immediately put forward, one of the first of which was the Thomson model or the “raisin pudding” model. According to this model, small electrons were located inside a massive, positively charged body, like raisins inside a pudding. However, the practical experiments of the chemist Rutherford refuted this model and led him to the creation of a planetary model of the atom.

Bohr's development of the planetary model, along with the discovery of neutrons in 1932, formed the basis for modern theory about the structure of the atom. The next stages in the development of knowledge about the atom are already associated with the physics of elementary particles: quarks, leptons, neutrinos, photons, bosons and others.

Take any object, well, at least a spoon. Put it down - it lies calmly, does not move. Touch it - cold, motionless metal.

But in reality, a spoon, like everything around us, consists of tiny particles - atoms, with large gaps between them. The particles are constantly swaying and oscillating.

Why is the spoon hard if the atoms in it are arranged freely and move all the time? The fact is that they are, as it were, firmly tied to each other by special forces. And the gaps between them, although much larger than the atoms themselves, are still negligible, and we cannot notice them.

Atoms are different - there are 92 types of atoms in nature. Everything in the world is built from them, just as from 32 letters - all the words of the Russian language. Scientists created another 12 types of atoms artificially in their own.

People have known about the existence of atoms for a long time. More than two thousand years ago in ancient Greece there lived the great scientist Democritus, who believed that the whole world consists of tiny particles. He called them “atomos,” which in Greek means “indivisible.”

It took a long time for scientists to prove that atoms really exist. This happened at the end of the last century. And then it turned out that their very name was a mistake. They are not indivisible: an atom consists of even smaller particles. Scientists call them elementary particles.

Here is an artist drawing an atom. In the middle is the core, around which, like planets around the Sun, tiny balls move - . The core is also not solid. It consists of nuclear particles - protons and neutrons.

That's what we thought just recently. But then it became clear that atomic particles are not like balls. It turned out that the atom is structured in a special way. If you try to imagine what particles look like, you can say that an electron is like a cloud. Such clouds surround the core in layers. And nuclear particles are also kind of clouds.

Different types of atoms have different numbers of electrons, protons, and neutrons. The properties of atoms depend on this.

It's easy to split an atom. Electrons easily break away from nuclei and lead an independent life. For example, electric current in a wire is the movement of such independent electrons.

But the core is extremely strong. Protons and neutrons in it are tightly bound together by special forces. Therefore, it is very difficult to break the core. But people learned to do it and got it. We learned to change the number of particles in the nucleus and thus transform some atoms into others and even create new atoms.

Studying the atom is difficult: scientists require extraordinary ingenuity and resourcefulness. After all, even its size is difficult to imagine: in a microbe invisible to the eye there are billions of atoms, more than there are people on Earth. And yet scientists are achieving their goal, they were able to measure and compare the weights of all atoms and the particles that make up an atom, they found out that a proton or neutron is almost two thousand times more massive than an electron, they discovered and continue to discover many other atomic secrets.

Modern people constantly hear phrases that contain derivatives of the word “atom”. This is energy, a power plant, a bomb. Some take it for granted, and some ask the question: “What is an atom?”

What does this word mean?

It has ancient Greek roots. Comes from “atomos”, which literally means “uncut”.

Someone already somewhat familiar with the physics of the atom will be indignant: “How is it “uncut”? It consists of some kind of particles!” The thing is that the name appeared when scientists did not yet know that atoms are not the smallest particles.

After experimental proof of this fact, it was decided not to change the usual name. And in 1860, an “atom” began to be called the smallest particle that has all the properties of the chemical element to which it belongs.

What is larger than an atom and smaller than it?

The molecule is always larger. It is formed from several atoms and is the smallest particle of matter.

But smaller ones are elementary particles. For example, electrons and protons, neutrons and quarks. There are a lot of them.

A lot has already been said about him. But it is still not very clear what an atom is.

What he really is?

The question of how to represent a model of an atom has long occupied scientists. Today, the one proposed by E. Rutherford and finalized by N. Bohr has been accepted. According to it, the atom is divided into two parts: the nucleus and the electron cloud.

Most of the mass of an atom is concentrated at its center. The nucleus consists of neutrons and protons. And the electrons in an atom are located at a fairly large distance from the center. It turns out something similar to the solar system. In the center, like the Sun, is a core, and electrons revolve around it in their orbits, like planets. That is why the model is often called planetary.

Interestingly, the nucleus and electrons occupy a very small space compared to the overall dimensions of the atom. It turns out that there is a small core in the center. Then emptiness. A very big void. And then a narrow strip of small electrons.

Scientists did not immediately arrive at this model of atoms. Before this, many assumptions were made that were refuted by experiments.

One of these ideas was to represent the atom as a solid body that has a positive charge. And it was proposed to place electrons in an atom throughout this body. This idea was put forward by J. Thomson. His model of the atom was also called "Raisin Pudding". The model very much resembled this dish.

But it was untenable because it could not explain some of the properties of the atom. That's why she was rejected.

Japanese scientist H. Nagaoka, when asked what an atom is, proposed such a model. In his opinion, this particle has a vague resemblance to the planet Saturn. There is a nucleus in the center, and electrons rotate around it in orbits connected in a ring. Although the model was not accepted, some of its provisions were used in the planetary diagram.

About the numbers associated with the atom

First, about physical quantities. The total charge of an atom is always zero. This is due to the fact that the number of electrons and protons in it is the same. And their charge is the same in magnitude and has opposite signs.

Situations often arise when an atom loses electrons or, conversely, attracts extra ones. In such situations they say that it has become an ion. And its charge depends on what happened to the electrons. If their number decreases, the ion's charge is positive. When there are more electrons than required, the ion becomes negative.

Now about chemistry. This science, like no other, gives the greatest understanding of what an atom is. After all, even the main table that is studied in it is based on the fact that the atoms are located in it in a certain order. We are talking about the periodic table.

In it, each element is assigned a specific number, which is associated with the number of protons in the nucleus. It is usually denoted by the letter z.

The next value is the mass number. It is equal to the sum of protons and neutrons found in the nucleus of an atom. It is usually designated by the letter A.

The two numbers indicated are related to each other by the following equation:

A = z + N.

Here N is the number of neutrons in the atomic nucleus.

Another important quantity is the mass of the atom. To measure it, a special value has been introduced. It is abbreviated: a.e.m. And it is read as an atomic mass unit. Based on this unit, the three particles that make up all the atoms of the Universe have masses:

These values ​​are often needed when solving chemical problems.

An atom is made up of atomic nucleus and electrons. If the number of protons in the nucleus coincides with the number of electrons, then the atom as a whole turns out to be electrically neutral. Otherwise, it has some positive or negative charge and is called an ion. In some cases, atoms are understood only as electrically neutral systems in which the charge of the nucleus is equal to the total charge of the electrons, thereby contrasting them with electrically charged ions.

Core, which carries almost the entire (more than 99.9%) mass of an atom, consists of positively charged protons and uncharged neutrons bound together through strong interaction. Atoms are classified according to the number of protons and neutrons in the nucleus: the number of protons Z corresponds to the serial number of the atom in the periodic table and determines its belonging to a certain chemical element, and the number of neutrons N - a specific isotope of this element. The Z number also determines the net positive electric charge (Ze) of the atomic nucleus and the number of electrons in a neutral atom, which determines its size.

Atoms of different types in different quantities, connected by interatomic bonds, form molecules.

Properties of the atom

By definition, any two atoms with the same number of protons in their nuclei belong to the same chemical element. Atoms with the same number of protons but different numbers of neutrons are called isotopes of a given element. For example, hydrogen atoms always contain one proton, but there are isotopes without neutrons (hydrogen-1, sometimes also called protium - the most common form), with one neutron (deuterium) and two neutrons (tritium). The known elements form a continuous natural series according to the number of protons in the nucleus, starting with the hydrogen atom with one proton and ending with the ununoctium atom, which has 118 protons in the nucleus. All isotopes of the elements of the periodic table, starting with number 83 (bismuth), are radioactive.

Weight

Since protons and neutrons make the largest contribution to the mass of an atom, the total number of these particles is called the mass number. The rest mass of an atom is often expressed in atomic mass units (a.m.u.), which is also called a dalton (Da). This unit is defined as 1⁄12th of the rest mass of a neutral carbon-12 atom, which is approximately equal to 1.66 × 10−24 g. Hydrogen-1 is the lightest isotope of hydrogen and the atom with the smallest mass, having an atomic weight of about 1.007825 a. e.m. The mass of an atom is approximately equal to the product of the mass number per atomic mass unit. The heaviest stable isotope is lead-208 with a mass of 207.9766521 a. eat.

Since the masses of even the heaviest atoms in ordinary units (for example, grams) are very small, moles are used in chemistry to measure these masses. One mole of any substance, by definition, contains the same number of atoms (approximately 6.022·1023). This number (Avogadro's number) is chosen in such a way that if the mass of an element is 1 a. e.m., then a mole of atoms of this element will have a mass of 1 g. For example, carbon has a mass of 12 a. e.m., so 1 mole of carbon weighs 12 g.

Size

Atoms do not have a clearly defined external boundary, so their sizes are determined by the distance between the nuclei of neighboring atoms that have formed a chemical bond (Covalent radius) or by the distance to the farthest stable electron orbit in the electron shell of this atom (Atomic radius). The radius depends on the position of the atom in the periodic table, the type of chemical bond, the number of nearby atoms (coordination number) and a quantum mechanical property known as spin. In the periodic table of elements, the size of an atom increases as you move down a column and decreases as you move down a row from left to right. Accordingly, the smallest atom is a helium atom with a radius of 32 pm, and the largest is a cesium atom (225 pm). These sizes are thousands of times smaller than the wavelength of visible light (400-700 nm), so atoms cannot be seen with an optical microscope. However, individual atoms can be observed using a scanning tunneling microscope.

The smallness of atoms is demonstrated by the following examples. A human hair is a million times thicker than a carbon atom. One drop of water contains 2 sextillion (2 1021) oxygen atoms, and twice as many hydrogen atoms. One carat of diamond weighing 0.2 g consists of 10 sextillion carbon atoms. If an apple could be enlarged to the size of the Earth, then the atoms would reach the original size of the apple.

Scientists from the Kharkov Institute of Physics and Technology presented the first photographs of an atom in the history of science. To obtain images, scientists used an electron microscope that records radiation and fields (field-emission electron microscope, FEEM). Physicists sequentially placed dozens of carbon atoms in a vacuum chamber and passed an electrical discharge of 425 volts through them. The radiation of the last atom in the chain onto a phosphorus screen made it possible to obtain an image of a cloud of electrons around the nucleus.

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