Question

Density of sodium and potassium follows the order:
A. $K < Na$
B. $K > Na$
C. $Na \approx K$
D. None of these

Answer 1

Density is referred as the mass of the substance per unit volume. It is denoted by the symbol ρ. Density of an atom is the ratio of the mass of an atom and the space or volume of the respective atom.

Complete step by step answer:
Mass of an element depends on the nucleus of the atom. The mass is accounted for by the number of neutrons present in the core of the element. Density of an element depends on the mass of the atom distributed over the volume or space occupied by the element.
Sodium is an element in the periodic table having atomic number $11$ and mass number $23.0u$. The valence shell of sodium is $3$. The electronic configuration of sodium is:
$Na:1{s^2}2{s^2}2{p^6}3{s^1}$
Potassium is an element in the periodic table having atomic number $19$ and mass number $39.1u$. The valence shell of potassium is $4$. The electron configuration of potassium is:
$K:1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}3{d^0}$
In general the atomic size and atomic mass increases down the group. The density of the element also increases down the group.
Sodium and potassium belong to the same group in the periodic table i.e. the alkali metal group. The electrons of the elements are added to the s orbitals. In potassium however an extra orbital is added to the metal i.e. $3d$ orbital.
Due to this the size of the element increases abnormally resulting in increase in volume. Thus the density of the element potassium gets lowered than sodium.

Hence option A is the correct answer, i.e. $K < Na$.

Note:
Along the period density of the element increases as atomic size decreases along the period. Along the group size increases but the mass of the elements also increases, as a result the density of the element increases.