I heard that an electron moving in a certain orbital is actually perpetual mobile - is this true?

Asked by: Tony

Answer

Yes, and no, and maybe. The real problem here is that at the subatomic scales, the behaviour of matter does not resemble our daily experience at all. As Feynman (?) said, if we keep picturing electrons as little steel balls, we're always going to have trouble understanding what is happening at the quantum level.

In the current understanding of the atom, an orbital is not something like the orbit of a planet around the sun. It is a probability distribution in space. And, the electron can not be said to be really 'moving' -- an orbital is a stationary state.

There is a very good example for this. The ground state of a hydrogen atom is actually spherically symmetrical. And (therefore) it has zero angular momentum. Which, if we try to interpret in classical terms, means that the electron only ever moves radially -- in and out.... But yet it covers the whole angular range. So in fact it defies 'steel ball' or 'classical' interpretations.

So, it is not good sense to interpret the state of an electron in a stationary orbital as perpetual motion. However, people still like to imagine electrons as little steel marbles orbiting around tennis-ball nuclei, like in the Bohr model of the atom (which, while incorrect, is a very useful model) and tend to think of it as perpetual motion. If I wanted to see perpetual motion, I'd look at the planets going around the sun. Which, by the way is not really perpetual motion either - but comparing with the human lifespan, it is good enough for all practical purposes.
Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey