Assume that the de Broglie wave associated with an electron can form a standing wave between atoms arranged in a one-dimensional array, with nodes at each atomic site. One such standing wave is formed when the distance d between adjacent atoms is 2A˚. A similar standing wave is again formed when d is increased to 2.5A˚, but not for any intermediate value of d. Find the energy of the electron in eV and the least value of d for which the standing wave can form.