The electrons that dance around an atomic nucleus help determine how the atom bonds with others, but they are notoriously difficult to pin down. Thanks to quantum theory, which says that tiny particles are in multiple places simultaneously, you can never say where a given electron actually is. The best you can do is say how likely it is to be in a given spot.
Not all positions are equally available: electrons can only reside at certain distances from the nucleus, with these distances related to how much energy the electron holds. In principle, the wave function, denoted by Greek letter psi, can be used to reveal these energy levels for any given atom or molecule, although in practice this has only been done for the very simplest – the hydrogen atom and molecule (made of two hydrogen atoms bonded together).