Why is the density of potassium less than that of sodium?

Explanation

In a group, the density increases with increase in atomic number. But the density of `._(19)K` is less than `._(11)Na`.
Reason :
Atomic vo lume of K is nearly twice of Na. but its mass (39) is not exac tly double of Na (23).
Since density is inversely related to volume, the greater volume K has lower density than Na.