Such an arrangement of elements is called as triad. Triad of elements was proposed by Dobereiner. He arranged elements in triads. In a triad, the atomic mass of the middle element was equal to the average of atomic massed of the other two elements.  Lithium, Sodium and Potassium make a triad. Atomic massed of Li. Na and K are 6.9, 23.0 and 39.0 respectively. Average of masses of Li and K is approximately 23.0 which is equal to the atomic mass of Na.