What is an atom, a molecule. Chemical elements. Isotopes.

What is an atom, a molecule. Chemical elements. Isotopes.


Trees, birds, clouds, houses and cars are made up of atoms. Everything is made up of atoms.

Figure 1.1. Everything consists of atoms.

Atoms are the smallest constituent particles of matter.

Figure 1.1 shows samples of inanimate and living matter. Clouds, houses and cars are inanimate matter Trees and birds represent living matter. What inanimate and living matter have in common is that they are made up of atoms.

“Atom” is by origin a Greek word meaning indivisible. It was introduced into the lexicon by the Greek philosopher Democritus, who lived about 2500 years ago. At that time, it was believed that the atom was the smallest particle of matter. Today we know more about this.

The size of the atom.

Figure 1.2. From the whole to the atom.

Imagine a piece of paper divided into two parts. You can divide a sheet as long as each part of it consists of a single atom. In this case, the sheet will be divided into approximately 1022 parts, that is, 1 +22 zeros (see Figure 1.2). For example: the volume of Land is 1021 m3.

A small stick made with a pencil consists of a huge number of atoms, more than the number of inhabitants on Earth. The diameter of the nucleus of an atom is approximately 1/1,000, 000, 000 or 10-12 cm, that is, one hundred millionth of a centimeter. The diameter of an entire atom, including electrons, is approximately equal to 1/100, 000,000 or 10-8 cm, that is, one hundred millionth of a centimeter.

This means that the trajectories of the movement of electrons are located relatively far from the nucleus — like the planets of our Solar system — far from the Sun.

Thus, the largest part of the volume of an atom is empty space. The core density is approximately 1014 g/cm3, which means that 1 cubic centimeter of nuclear mass would weigh 100,000,000 tons. This can give you an idea of how tiny the atoms are.

The structure of the atom.

The atoms are so small that you won’t be able to distinguish them with your eyes, you won’t even be able to see them under the most powerful electron microscope. In other words, we don’t know what an atom really looks like. But there are models that describe our idea of the atom.

The simplest model was created by the Danish physicist Niels Bohr at the beginning of the last century. Over the past decades, the Bohr model has been improved by physicists of the world.

In the Bohr model, an atom consists of a nucleus represented by united spheres, as shown in Figure 1.3, negatively charged electrons rotate around the nucleus in their orbits. The core consists of two types of particles. The light particles shown in the figure are called neutrons – they have no electric charge. Dark particles — protons – are positively charged (+).

The positive charge of protons neutralizes the negative charge of electrons.

Therefore, an atom is electrically neutral, since it has an equal number of protons and electrons.

Figure 1.3. Model of the Boron atom. Protons and neutrons are about the same size, electrons are about 1800 times smaller.

Elementary particles

The table below shows some comparative data on elementary particles – neutrons, protons and electrons.

Table. Elementary particles.

The “Charge” column shows that the electric charge of the neutron is zero, i.e. the neutron is electrically neutral.

The proton is positively charged, the electron is negatively charged. Their charges are numerically equal. Thus, they balance each other’s charges.

A neutron and a proton have almost the same weight. An electron is much lighter – approximately 1/1800 of the mass of a proton or neutron (column 4). Therefore, the mass of an atom is actually the mass of the nucleus: the weight of electrons can be ignored.


There are various types of chemical elements in nature. Some have a relatively simple structure, others are more complex. We use the word element to describe a substance consisting only of atoms of one kind.

An element is determined by the number of protons in its nucleus. Consequently, the number of protons in all atoms of some element is the same. The number of neutrons, however, can vary.

Figure 1.4. illustrates examples of some chemical elements. The number before the name of the element indicates the ordinal number of the element in the periodic system of D.I. Mendeleev. The symbol after the name of the element is its chemical designation: H – for hydrogen; He – for helium, C — for carbon, Sr- for strontium, Cs – for caesium, etc.

Figure 1.4. Some Elements, their chemical designations and atomic numbers.

There are 92 elements in nature.

Isotopes or nuclides

The atoms that make up the chemical elements can have a variety of shapes. The simplest of all atoms is the hydrogen atom, its nucleus consists of a single proton. In addition, there are two more types of hydrogen atoms. The first, with one neutron, is called deuterium, the second, with two neutrons, is called tritium. They are isotopes of hydrogen. Most other elements also have such isotopes.

Chemically, the isotopes of the elements behave almost the same. From the point of view of nuclear physics, isotopes have very different properties.

Another name for isotopes of all chemical elements is nuclide. In other words: all elements are nuclides.

The designation of hydrogen isotopes in Figure 1.5 is “H”.

The sum of the number of protons and neutrons is called the mass number and is indicated by the upper-left index before the symbol of the chemical element (1H, 2H and 3H).

Hydrogen is the lightest of all chemical elements: its nucleus consists of a single proton.

The nucleus of heavy hydrogen, deuterium, consists of one proton and one neutron. This hydrogen isotope is approximately twice as heavy as a normal hydrogen atom.

An isotope of hydrogen whose nucleus consists of a proton and two neutrons is called the third.

                                             1Н                                                 2H(D)                                         3Н(Т)

Figure 1.5. Types of hydrogen atoms (isotopes). Of these 3 Nuclides 1H is the most common.


All matter consists of atoms. Atoms of various elements can combine to form a molecule. The properties of this molecule can be very different from the properties of individual atoms. In Figure 1.6, an oxygen atom is surrounded by two hydrogen atoms. The combination of one oxygen atom and two hydrogen atoms forms a water molecule. The chemical designation of water is H2O.

A water molecule is a relatively idle molecule consisting of only three atoms.

There are, however, much more complex molecules that can consist of hundreds of thousands of atoms, for example, a DNA molecule in the nucleus of a cell. This molecule will be discussed in Chapter 2, “The biological effect of ionizing radiation.”

Figure 1.6. A water molecule consists of one oxygen atom and two hydrogen atoms.

Ions and ionization

We mentioned that the atom is electrically neutral. But if an atom emits or absorbs one or more electrons, it ceases to be electrically neutral. It turns into a positively or negatively charged ion.

Figure 1.7. Example of atom ionization.

In Figure 1.7 you can see the neutral atom. It contains six protons and is therefore surrounded by six electrons. The arrow indicates that the electron is being removed. In this case, the atom takes the form illustrated in the right part of the figure. It has six protons, but the number of electrons around the nucleus is five, not six. The atom was transformed into a positively charged ion. This process is called ionization, and requires energy costs.

After ionization, the atom more readily interacts with other substances.

Matter can be ionized when exposed to radiation from radioactive sources, the so-called ionizing radiation. This radiation can remove electrons from atoms in a substance and form free electrons and positively charged ions.


FSUE “St. Petersburg Research Institute of Radiation Hygiene named after Professor P.V. Ramzaev”, 2011

Shutov V.N., Kaduka M.V., Kravtsova O.S., Parkhomenko V.I., Samoylenko V.M. Protection from radiation. Popular science manual. – St. Petersburg, 2011.-88s.

Artist Melnikova T.Yu.

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