Question

The density of –
[1] Na > K
[2] Na = K
[C] K > Na
[D] Li > K

Answer 1

Hint: In the given question, we have sodium i.e. Na, potassium i.e. K and lithium i.e. Li. They belong to the same group in the periodic table. Remember than density depends upon mass and volume. You should consider the factor of atomic radii to answer this.

Complete answer:
To answer this, let us discuss their densities. Since they belong to the same group thus they have comparable properties and so we can arrange their densities accordingly.
We know density depends upon mass and volume. These properties i.e. mass and volume both changes when we move down the group.
Now let us see how these factors affect the density.
 We know that atoms are packed in the same way so the factors that we have to consider are the number of atoms packed in the given volume (that depends upon the volume of each atom) and also the mass of each individual atom. When we move down the group, atomic radius increases due to addition of an orbital. With increase in the atomic radii the volume of the atom also increases.
Due to the above mentioned factor, the density increases down the group.
Let’s take an example of sodium and lithium to understand this better. Sodium atoms are larger in size so for a given volume, a lesser number of sodium atoms will fit in as compared to lithium ions. However, when the atomic radius increases, the mass also increases and thus the sodium atom weighs more than lithium atoms. Hence, density increases down the group.
However, potassium is lighter than sodium as the increase in atomic volume outreaches the increase in atomic mass and thus sodium is denser than potassium.

Therefore, the correct answer is option [1] Na > K

Note:
The metal given to us in the above question belongs to the same group in the periodic table and they have similar chemical properties. They are known as alkali metals and they belong to group 1. They are all soft and reactive metals with lower melting point out of which sodium and potassium are highly reactive and explode upon reaction with water.