Arrange the alkali metals in increasing order of their density.

Options

A. (a) `Li lt Na lt K lt Rb lt Cs`
B. (b) `Cs lt Rb lt K lt Na lt Li`
C. (c) `Li lt K lt Na lt Rb lt Cs`
D. (d) `K lt Na lt Li lt Rb lt Cs`

Answer

To solve the question of arranging alkali metals in increasing order of their density, we will follow these steps:

### Step 1: Understand the Alkali Metals
The alkali metals include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). They are located in Group 1 of the periodic table.

### Step 2: Analyze the Trend in Density
Density is defined as mass per unit volume. For elements, it can be thought of as atomic mass divided by atomic volume. As we move down the group of alkali metals:
- The atomic mass increases due to the increase in atomic number.
- The atomic size also increases because of the addition of electron shells.

### Step 3: Determine the Effect on Density
While both atomic mass and atomic size increase down the group, the increase in atomic mass tends to dominate. This means that, generally, the density of alkali metals increases as we move down the group.

### Step 4: Identify Anomalies
However, there is an anomaly between sodium and potassium:
- Sodium (Na) has a higher density than potassium (K) despite potassium being lower in the group. This is due to the unusual increase in the atomic size of potassium, which decreases its density.

### Step 5: Arrange the Alkali Metals by Density
Based on the trends and the anomaly:
1. Lithium (Li) - lowest density
2. Potassium (K) - lower density than sodium
3. Sodium (Na) - higher density than potassium
4. Rubidium (Rb) - higher density than sodium
5. Cesium (Cs) - highest density

Thus, the correct order of increasing density is:
**Li < K < Na < Rb < Cs**

### Step 6: Verify Against Options
Now, we can check the provided options:
- Option A: Li < Na < K < Rb < Cs (Incorrect)
- Option B: Cs < Rb < K < Na < Li (Incorrect)
- Option C: Li < K < Na < Rb < Cs (Correct)
- Option D: K < Na < Li < Rb < Cs (Incorrect)

The correct answer is **Option C: Li < K < Na < Rb < Cs**.