About the book: 1971 / There is a persistent opinion that in science there are facts that it is too early for those starting to get acquainted with it to know, but it is a shame to inform those experienced in it. Most often this is remembered when trying to explain the structure of the atom. Perhaps that is why a book on quantum mechanics has not yet been written that is strict enough not to offend an expert, simple enough not to scare off a beginner, and at the same time interesting to both of them. This book is not for experts, although they will find a few unexpected things here facts. It is for those who are finishing school, and for those who are trying to look at the world a little more broadly than their specialty allows them - it is necessary to be narrow in order to be productive. In this book, the history of the atom is told quite strictly. But rigor in it is not an end in itself: as a rule, we are interested not only in the facts themselves, but also in their interpretation and the circumstances under which they are discovered. Therefore, the main thing in the book is the evolution of ideas and concepts of atomic physics, forming a single system - simple and harmonious. It was this inner beauty that motivated the book. I will consider my work not in vain if those who read it feel the power of the logical constructions of quantum mechanics and the beauty of their unexpectedly simple consequences.