Lithium, sodium and potassium are said to be one of the triads in the Mendeleev’s periodic table. Lithium (atomic mass 7) is the first element in the triad and sodium being in the middle has atomic mass of 23 and third element is potassium (atomic mass 39).
Atomic mass of sodium = (At. Mass of Li + At. Mass of K) / 2
= (7+39)/2
=23