Assume that the de-Broglie wave associated with an electron can form a standing wave between the atoms arranged in a one-dimensional array with nodes at each of the atomic sites. It is found that one such standing wave is formed if the distance d between the atoms of the array is  $1.2\stackrel{\circ }{\text{A}}$. A similar standing wave is again formed if d is increased to $1.5\stackrel{\circ }{\text{A}}$ but not for any intermediate value of d. Least value of d in $\stackrel{\circ }{\text{A}}$ is equal to