People have long guessed that substances consist of individual tiny particles. They are called atoms. In the overwhelming majority of cases, atoms do not exist alone but are grouped into molecules. Atoms and molecules are extremely small: any tiny piece of substance that we can see (e.g. dust) contains more atoms than stars in our entire galaxy.
Compared to the variety of substances, the number of atoms is very limited. Atoms can combine with each other in different ways. Just as the letters of the alphabet can make up hundreds of thousands of words, so the same atoms form molecules or crystals of a huge number of different substances that make up the world around them.
A molecule is the smallest particle of a substance that defines its properties and is capable of independent existence. Molecules are built from atoms.
A substance (e.g. sugar) can be ground in the finest mill and still each grain will consist of a huge number of identical sugar molecules and will retain all the known properties of this substance. Even if we split the substance into separate molecules, as it happens when dissolving sugar in water, the substance continues to exist and exhibit its properties (it is easy to see this by trying the solution to taste). So, a sugar molecule that exists independently is also a substance called "sugar" (even a very small amount of this substance). But if you continue crushing further, you will have to destroy the molecules. And by destroying the molecules or even taking away a couple of atoms (out of three dozen that make up the sugar molecule!) from them, we are already destroying the substance itself. Of course, atoms do not disappear anywhere - they start to form part of some other molecules. But sugar as a substance ceases to exist - it turns into some other substances.
Substances are not eternal, because their molecules are not eternal. But atoms are practically eternal. In each of us, there will be atoms that existed in the days of dinosaurs. Or participating in Alexander the Great's campaigns, or in swimming Columbus, or visited at the court of Ivan Terrible.
Despite the fact that molecules are very small, their devices can be found out by various physical and chemical methods. A pure substance consists of molecules of the same type. If the physical body contains several types of molecules, we are dealing with a mixture of substances. The concepts of "pure" in chemistry and in everyday life are not the same. For example, when we say, "What clean air! - we actually inhale a complex mixture of several gaseous substances. The chemist will say about the forest air: "It takes a lot of work to isolate pure substances from this mixture. Interestingly, in the atmosphere of any one of them separately, a person would not be able to exist.
Nitrogen consists of nitrogen molecules, water is a well-known substance, water molecules, and terpineol consists of terpineol molecules. Molecules of substances can be very different - from the simplest, consisting of two or three atoms (nitrogen, oxygen, ozone, carbon dioxide) to molecules consisting of many atoms (such molecules are found in living organisms). For example, terpineol, which is formed in coniferous trees and gives the air the smell of freshness.
This means that there can be an infinite number of substances as well as molecular species. No one can name the exact number of substances known to people today. We can only tentatively say that there are more than seven million such substances.
Atoms in the molecules of different substances are connected in a strictly defined order, the establishment of which is one of the most interesting activities in the work of a chemist.
The sizes of balloons have a physical meaning and roughly correspond to the relative sizes of atoms. The same substances can be depicted differently with the help of chemical symbols. For a long time, each type of atom in chemistry has had a symbol of Latin letters.
There is a conditional division of substances into simple and complex ones. The molecules of simple substances consist of atoms of the same kind. Examples: nitrogen, oxygen, argon, ozone. Molecules of complex substances consist of two or more types of atoms: carbon dioxide, water, and terpineol.
Often the physical body consists of molecules of several different substances. Such a physical body is called a mixture. For example, the air is a mixture of several simple and complex substances. You should not confuse a complex substance with a mixture. A complex substance, if it consists of only one type of molecule, is not a mixture.
A simple substance consists of atoms of only one species or molecules built of atoms of the same species.
A complex substance consists of molecules constructed of atoms of different species.
A mixture is a substance consisting of molecules (or atoms) of two or more substances. The substances that make up the mixture can be simple and complex.