Why do electrons move in shells rather than in fixed orbits?
Quantum mechanical objects (ie, very small, like the size of electrons in atoms) don’t have fixed speeds and positions. So you can’t talk about their orbits as you would in classical mechanics. Their properties (such as position) are described by a wave function, a function which obeys a differential equation (Schrodinger’s equation). For an electron in an atom, there are only certain, specific, solutions to that equation that are mathematically possible. So the electron is limited to being in one of these wave functions (or a combination of them–that’s allowed, too), which give you a probability distribution for its position, the shell. The electron isn’t exactly moving in the shell (it may or may not have orbital angular momentum). It’s just hanging out there. It doesn’t need to decide exactly where it is unless you go looking for it.
