What is the correct order of density ?

Options

A. Li > Na > K
B. K > Na > Li
C. Na > K > Li
D. Na = K < Li

Answer

To determine the correct order of density among the alkali metals (Lithium, Sodium, Potassium, Rubidium, and Cesium), we need to analyze the trends in density as we move down the group in the periodic table.

### Step-by-Step Solution:

1. **Understanding Alkali Metals**:
- Alkali metals are located in Group 1 of the periodic table and include Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), and Cesium (Cs).

2. **Density Concept**:
- Density is defined as mass per unit volume (Density = Mass/Volume). As we move down the group, the atomic mass of the elements increases.

3. **Trends in Atomic Mass and Size**:
- As we go down the group from Lithium to Cesium, both the atomic mass and the size of the atoms increase. Generally, an increase in atomic mass leads to an increase in density.

4. **Density of Individual Alkali Metals**:
- The densities of the alkali metals are as follows:
- Lithium (Li): ~0.53 g/cm³
- Sodium (Na): ~0.97 g/cm³
- Potassium (K): ~0.86 g/cm³
- Rubidium (Rb): ~1.53 g/cm³
- Cesium (Cs): ~1.93 g/cm³

5. **Comparing Densities**:
- From the above data, we can see that:
- Density of Lithium < Density of Potassium < Density of Sodium
- Density of Sodium > Density of Potassium (exceptional case)
- Density of Rubidium < Density of Cesium

6. **Final Order of Density**:
- The correct order of density from lowest to highest is:
- Lithium < Potassium < Sodium < Rubidium < Cesium

7. **Conclusion**:
- Therefore, the correct order of densities is:
- Li < K < Na < Rb < Cs.