Human memory is built on associative-logical connections. Therefore, it is so important for us to streamline reality, distribute its elements in cells and find a system. Otherwise, the memory refuses to obey, and the mind - to analyze. It is no coincidence that in the history of chemistry such an important breakthrough is considered to be the creation of the periodic table - order, system and logic have triumphed. The influence of this discovery was so great that it inspired many. For example, Lyubishchev, the genius of Russian time management, developed his system in order to create a periodic table of living organisms. This goal was not achieved, but the fact itself proves the importance of logic for human understanding. But reality is richer than beautiful schemes. For example, in the main chemistry table, some elements are out of order by weight. Why? You can answer in one word - "isotopes". This word literally means "occupying one place."
Many worked, not everyone remembers
The periodic table is the fruit of the painstaking work of many scientists, and not just its creator. He brilliantly created the principle itself and found the basic laws. But the figure of the mass that you see under each element is the result of the work of many chemists, and in addition, it is inaccurate. How can this be? Perhaps you have ever noticed that an atomic number is a beautiful integer. But the mass is fractional with a huge number of decimal places. Why? And the same isotopes are to blame. This is explained quite simply. If you look at the numbers for the nitrogen element, you’ll see 14.0067 near the atomic mass number. However, if you “catch” a free nitrogen atom, then it can weigh both 10 and 25 atomic units. They are different. What is the difference? The mass of the isotope consists of the mass of protons and the mass of neutrons. The mass of protons is a constant value, it is it that makes the element an element (nitrogen - nitrogen). But an atom can be either rich or poor with neutrons. In the example with nitrogen, there can be 18, and only 3 pieces. Isotopes are types of atoms of an element depending on the number of neutrons in the nucleus. Nitrogen has 16 isotopes. Some other elements have more.
What should a scientist do?
If you were scientists, what would you do? How would mass be recorded in a table for calculations? It was possible, of course, to take the arithmetic mean. But many isotopes are very unstable objects, some of them are artificially created. Therefore, it would be wrong to carry out calculations with an orientation to inaccurate data. Scientists acted differently - they calculated the atomic mass of an element in proportion to the natural abundance of a particular isotope. As a result, the probability that during an arbitrary search you came across exactly an isotope of 14 mass units is very high. The nucleus of an isotope found by chance is likely to contain 7 protons and 7 neutrons.
Reasons for Inaccuracy
Why is atomic mass not entirely accurate? Because the conclusion about it is made inductively - from the particular to the general. We do not know the exact composition of the Earth under the crust; we have not geologically investigated even the entire surface of the Earth. Therefore, the atomic mass figure is probabilistic. It is based on the knowledge that people have today. Why does this satisfy scientists? Because greater accuracy is needed only for molecular physics, for the banal calculation of quantities, the error can be neglected. But the violation of order by mass is due to the fact that the quantity of charged particles - protons - is important for chemical properties. And it is precisely by the number of protons that the elements are arranged in series.