Question

The increasing order of the density of alkali metals:
a.) Li < K < Na < Rb < Cs
b.) Li < Na < K < Rb < Cs
c.) Cs < Rb < Na < K < Li
d.) Cs < Rb < K < Na < Li
e.) Li < Na < Rb < K < Cs

Answer 1

Hint: There is an anomaly in what we would expect to be the regular trend due to the metal causing the anomaly in question to have a much greater volume than that of its successor. With this idea in mind, try and figure out the correct order here.
Complete answer:
Let us first try and understand what the alkali metals really are before trying to determine the order of their densities.
An alkali metal is one of any of the six chemical elements that make up Group 1 (Ia) of the periodic table -namely, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). The alkali metals are so called because reaction with water forms alkalis (i.e., strong bases capable of neutralizing acids).
Let us now try and determine the order of their densities.
The alkali metals all have the same crystal structure (body-centred cubic) and thus the only relevant factors are the number of atoms that can fit into a certain volume and the mass of one of the atoms, since density is defined as mass per unit volume. The first factor depends on the volume of the atom and thus the atomic radius, which increases going down the group; thus, the volume of an alkali metal atom increases going down the group. The mass of an alkali metal atom also increases going down the group.
Thus, the trend for the densities of the alkali metals depends on their atomic weights and atomic radii; if figures for these two factors are known, the ratios between the densities of the alkali metals can then be calculated. The resultant trend is that the densities of the alkali metals increase down the table, with an exception at potassium due to an unusual increase in atomic size of potassium.
Therefore, we can safely conclude that the correct increasing order of the density of alkali metals is given by option a).


Note:
Due to having the lowest atomic weight and the largest atomic radius of all the elements in their periods, the alkali metals are the least dense metals in the periodic table with Lithium, Sodium, and Potassium being the only three metals in the periodic table that are less dense than water. In fact, lithium is the least dense known solid at room temperature.