Because electrons are microscopic particles with wave-particle duality, their motion state is different from that of macroscopic objects. We use the word atomic orbit to describe the motion state of electrons, but it doesn't mean that the motion of electrons runs in orbit. The equation describing the motion state of electrons is called Schrodinger equation, which has many solutions, but only some specific solutions can describe the spatial motion state of electrons. In quantum mechanics, these constants that determine the motion state of electrons are called principal quantum number N, angular quantum number L and magnetic quantum number M, respectively.

When principal quantum number = 1, electrons move in the region closest to the nucleus, which is called K layer, and so on.