Question

Which one is heavier 10 gram of sodium or potassium?

Answer 1

DENSITY Gradually increases in moving down from Li to Cs. Because the increase in mass dominates the increase in volume.
But Potassium is less dense when compared to Sodium. The reason is, the unusual increase in atomic size i. e., atomic volume. This happens because of the addition of empty 3d orbitals. This increases the atomic volume of the atom. Hence we see that anomaly.