All the variety of existing substances is explained by combinations of different types of atoms. It so happened that the species of these atoms - a little more than a hundred today. But they are rather capricious guys, and unite with each other not according to the rules of combinatorics, but in accordance with the laws of chemistry. And still, the amount of substances is huge, it is growing. But the number of known chemical elements is almost not increasing. Each of them is unique and has its own "portrait". And the main feature of every element is atomic mass.
The unit of this mass is a very small number. None of the available types of atoms was ideally suited as a candidate for being a unit of mass (but the closest was light hydrogen). As a result, scientists decided to take a number convenient for calculations - one twelfth of the absolute mass of such an element as carbon. It turned out that this quantity expresses very well the relationships in which the atoms of the elements are located to each other. So, the atomic unit of mass was recognized as a number in a very small degree, this is a small figure "ten to the power minus twenty-seventh."
It is clear that using just such a number is inconvenient. You yourself understand that when calculating everywhere you drag this minus twenty-seventh degree out of hand, and as a result the numbers can turn out to be just as uncomfortable, bulky. What to do? Use such a unit as the relative atomic mass of the element. What is it? Everything is done very simply - the absolute atomic mass is taken (the number is extremely uncomfortable, with almost the same minus degree), divided by our one-twelfth mass of carbon. So what? Correctly, the degrees are reduced and a quite decent number is obtained. For example, sixteen for an oxygen atom, fourteen for nitrogen. Carbon, logically, will have a mass of twelve. And the atomic mass of hydrogen is one, although not exactly one, which proves that nevertheless not hydrogen was taken for calculations, although the number is very close to its mass.
Then why are the relative atomic masses of each element - the numbers are not quite beautiful, non-integer? The thing is that the elements, although they are types of atoms, within the framework of the species allow themselves some “diversity”. Some of them are unstable, in other words, they are very easily destroyed spontaneously. But they exist for some time, therefore they cannot be ignored. It happens, and a generally stable kind of elements includes subspecies with different atomic masses. They are called isotopes. This means that they occupy one cell of a table known to every student - yes, you guessed it right, the periodic table.
But does atomic mass make an element an element? Not at all, the element is characterized by a much more fundamental number of protons in the nucleus. But it cannot be fractional and means a positive nuclear charge. A “calm” atom has as many electrons as there are protons in the nucleus, and therefore an self-respecting atom is electrically neutral. By the charge of the nucleus, the atoms line up in sequence in the periodic table, but their masses sometimes do not obey this law. Therefore, there are cases, exceptions, when the heavier atom in the sequence of the table is before. Well, it’s worth blaming exclusively isotopes. Nature “wanted” to have a lot of heavy isotopes for this element. But the relative atomic mass is set in proportion to the amounts of different isotopes. Simply put, if in nature there are more heavy isotopes - the atomic mass will be larger, if more light - then less. So the paradoxes of the Mendeleev system are obtained .
In fact, what has been said about atomic mass is somewhat simplified. There are deeper and more serious patterns regarding the periodic table. But they require a separate article, perhaps we will return to their consideration later, dear reader.