The elements lithium, sodium, and potassium form a Dobereiner's triad.  Lithium is the first element of this triad, sodium is the middle element whereas potassium is the third element of the triad. Sodium (middle element) has an atomic mass of $$23$$. According to Dobereiner, $$Atomic  mass  of  sodium=$$ $$ dfrac{Atomic  mass of Li+ Atomic  mass of K}{2} $$ = $$ dfrac{7+39}{2} $$ = $$23$$ This indicates that sodium is half way between lithium and potassium.