Why is potassium lighter than sodium?

Though there is a trend in increasing density down the groups in the periodic table, there is an exception in the density trend between sodium and potassium. read more

the potassium is lighter than sodium because increase in atomic volume overweighs the increase in atomic mass. There is an exception in the density trend between sodium and potassium. The exception is that although the atomic mass increases and the number of protons increase for potassium, its density is less than that for sodium. read more

This is because the density of Na is greater than that of K. We know that density (mass/Volume) down the group generally increases. This is because of the reason that the atomic masses of the elements increase and the effect of increase in atomic mass is more pronounced as compared to increase in atomic size. read more

as we know, on moving down the group, generally there is increase in density. But moving from sodium to potassium, there is increase in charge by 8 units, but atomic radius of potassium got much increase due to presence of 3d subshell but in case of sodium, there is no presence of d subshell. read more