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

The answer is (b) \(\mathrm{Li}<\mathrm{K}<\mathrm{Na}\).

Step 1: Identify element densities

First, we need to know the densities of lithium (Li), sodium (Na), and potassium (K). Typically, density data can be obtained from a periodic table or a reliable source: - Li has a density of about 0.534 g/cm³. - Na has a density of about 0.97 g/cm³. - K has a density of about 0.86 g/cm³.

Step 2: Arrange elements by density

Now, we sort the elements from the lowest to the highest density: 1. Li (0.534 g/cm³) 2. K (0.86 g/cm³) 3. Na (0.97 g/cm³)

Step 3: Match the arrangement with options

We compare our sorted list Li < K < Na with the given options: (a) Na < K < Li (b) Li < K < Na (c) K < Li < Na (d) Li < Na < K The correct order Li < K < Na matches option (b).

Key Concepts

Periodic Table

The periodic table is a fascinating tool that helps us understand the properties of elements at a glance. It arranges all known chemical elements in an organized way according to increasing atomic number—which is the number of protons in an atom's nucleus.
The periodic table is divided into rows, called periods, and columns, called groups.

• Each period represents elements with the same number of atomic shells.
• Each group contains elements with similar chemical properties.

For students delving into chemistry, the periodic table is crucial for quickly finding essential information about elements, such as atomic mass, atomic number, and even density. This comprehensive setup not only helps in comparing elements like lithium (Li), sodium (Na), and potassium (K) but also determines their chemical behavior in reactions.

Densities of Elements

Density, a fundamental property of matter, is defined as mass per unit volume. It's denoted by the formula:\[\text{Density} = \frac{\text{Mass}}{\text{Volume}}\]For elements, density helps us identify how closely packed the atoms are in a given volume. Elements have unique densities depending on the arrangement of their atoms. Typically, you can find this data on the periodic table or reliable scientific resources.Understanding densities helps with tasks like determining the heavier or lighter element by volume. For example, in the exercise given, lithium (Li) with a density of 0.534 g/cm³ is much lighter compared to sodium (Na) at 0.97 g/cm³.

• Lithium (Li): 0.534 g/cm³
• Sodium (Na): 0.97 g/cm³
• Potassium (K): 0.86 g/cm³

Knowing these densities allows us to sort them in order and solve problems accurately.

Alkali Metals Density Order

Alkali metals are a group of elements in the periodic table known for their reactivity and tendency to form +1 ions. They include lithium (Li), sodium (Na), and potassium (K), among others. One characteristic feature of alkali metals is their relatively low densities compared to other metals, making them lightweight. The density order of alkali metals can be non-intuitive. For instance, one might assume density increases steadily with atomic number, but this is not always the case due to variations in atomic structure. In the exercise discussed:

• Lithium (Li) has the lowest density (0.534 g/cm³).
• Potassium (K), despite being heavier than lithium in terms of atomic weight, has a lower density than sodium, at 0.86 g/cm³.
• Sodium (Na) is the densest among these three, at 0.97 g/cm³.

Therefore, the correct order of increasing density among these alkali metals is Li < K < Na. This order may help explain why potassium floats on water more readily than sodium or lithium.