Problem 26 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following arrangement is in the order of increasing density? (a) \(\mathrm{Na}<\mathrm{K}<\mathrm{Li}\) (b) \(\mathrm{Li}<\mathrm{K}<\mathrm{Na}\) (c) \(\mathrm{K}<\mathrm{Li}<\mathrm{Na}\) (d) \(\mathrm{Li}<\mathrm{Na}<\mathrm{K}\)

Step-by-Step Solution

Answer

Option (b) \( \mathrm{Li} < \mathrm{K} < \mathrm{Na} \) is the correct answer.

Step 1: Recall Density Trend

Recall the periodic table trend that typically lighter alkali metals occupy higher positions in the group and have lower densities than the heavier alkali metals.

Step 2: Compare Densities of Elements

The densities for the elements in question are: Lithium (Li: 0.534 g/cm³), Sodium (Na: 0.968 g/cm³), and Potassium (K: 0.862 g/cm³). We need to arrange these from lowest to highest.

Step 3: Arrange Elements Based on Density Values

Since Lithium has the lowest density and Sodium has the highest among these, the correct order based on increasing density is Li < K < Na.

Step 4: Match with Given Options

Compare the arrangement Li < K < Na with the options provided. The correct option that matches is (b) \( \mathrm{Li} < \mathrm{K} < \mathrm{Na} \).

Key Concepts

Periodic Table Trends

The periodic table is a powerful tool that displays elements in a systematic order based on their atomic number. Such ordering allows chemists to predict the properties and behavior of elements, including trends in density. Typically, as you move down a group in the periodic table, the atomic mass increases, which often results in a higher density. However, this is not an absolute rule, especially among the alkali metals. These metals demonstrate an intriguing periodic trend where density does not necessarily increase as expected. Alkali metals, for instance, become softer and less dense when moving down the group, making their periodic trends unique in comparison to other element groups. Understanding these trends helps to predict and explain chemical behavior effectively.

Alkali Metals

Alkali metals form the first group in the periodic table and include elements like Lithium (Li), Sodium (Na), and Potassium (K). These metals are characterized by their shiny appearance and high reactivity, especially with water. They have one electron in their outer shell, which explains their strong tendency to lose this electron and form positive ions. Interestingly, as we move down the group from Lithium to Potassium:

• The reactivity increases, making Potassium more reactive than Sodium and Lithium.
• The atomic size increases due to the addition of extra electron shells.
• The metals become softer and can even be cut with a knife.

Despite the increasing atomic mass, the density does not follow the typical trend seen in other groups, with Potassium actually being less dense than the heavier Sodium.

Comparative Densities

In comparing densities, it is essential to have a clear understanding of the actual values, as shown in the periodic table exercise. Lithium (Li) has a density of 0.534 g/cm³, making it the least dense of the three elements in question. Sodium (Na) follows with a higher density of 0.968 g/cm³, and then Potassium (K) with a density of 0.862 g/cm³. This comparison reveals a non-linear increase in density as one might predict from atomic mass or position in the periodic table. Rather, Potassium has a lower density than Sodium despite being further down the group. These nuances highlight the importance of analyzing specific density values rather than relying solely on general trends. Correctly ordering these densities is imperative for accurate comparisons, especially when dealing with elements from the alkali metal group.