Assume that the de Broglie wave associated with an electron can from a standing wave between the atom arrange in a one dimensional array with nodes at each of the atomic sites. It is found that one such standing wave if the distance d between the atoms of the array is `2 A^0 A `similar standing wave is again formed if d is increased to `2.5overset @A` . Find the energy of the electrons in electron volts and the least value of d for which the standing wave type described above can from .