ZIKA99 said:
I meant that if you have a more complete explanation of this topic, write it down.
If you have a book on quantum theory, introduce it.
sincerely
You have to forget all what you learned about the atom in terms of the Bohr-Sommerfeld old quantum theory. It's outdated since 1925 and shouldn't be taught except in lectures on the history of quantum theory (I recommend the huge work by Mehra and Rechenberg).
It is an observational fact that a (non-radioactive atom) is very stable, i.e., once put somewhere not interacting too energetically with anything else around it, will stay in its ground state (state of lowest energy) forever. That means it's in a time-independent state. In the lingo of formal QT we say it's in an energy eigenstate. The energy eigenstates are precisely the stationary states, describing a situation where nothing changes with time.
What one has get used to when entering the quantum realm is that observables like position or momentum don't take determined values except if the system were prepared in a state such that this is certain (and the quantum formalism tells you that this is impossible due to the Heisenberg-Robertson-Schrödinger uncertainty relation for position and momentum). All quantum theory describes are the probabilities for getting a value of an observable when measuring it given the system's state it is prepared in.
For an electron indide an atom you get a probability distribution for finding it at some position when measured. The same holds for momentum. To get an idea about the momentum of an electron when prepared in an energy eigenstate of an atom, you can ask for the expectation value. The quantum formalism tells you that the expectation value is 0. So on average the electron is indeed not moving.
To be in a stationary state is important for our understanding why matter is stable, and this is for me the most convincing argument for the invalidity of classical physics when describing matter. If the electron where really moving in some orbit around the nucleus as envisaged in the outdated old quantum theory a la Bohr and Sommerfeld, it should radiate electromagnetic radiation and thus loose energy and finally crash into the nucleus. Nothing like this is fortunately the case, because according to the new quantum theory, valid with utmost precise confirmation by experiments, the electron is not moving in such a naive sense but is in a stationary state, i.e., an energy eigenstate, of the atom.
The only honest half-popular book about quantum theory I know is the corresponding volume in Susskind's "Theoretical Minimum" series. There's no way to understand quantum theory without math, and Susskind provides indeed the minimum needed to starting to really understand quantum theory.