Which of the following has least density?

Options

A. Na
B. Li
C. Mg
D. K

Explanation

To determine which of the given options has the least density, we can follow these steps:

### Step 1: Understand the Concept of Density
Density is defined as mass per unit volume, represented by the formula:
\[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \]

### Step 2: Identify the Elements in the Question
The elements mentioned in the question are:
- Lithium (Li)
- Sodium (Na)
- Potassium (K)
- Magnesium (Mg)

### Step 3: Classify the Elements
- Lithium, Sodium, and Potassium are alkali metals (Group 1).
- Magnesium is an alkaline earth metal (Group 2).

### Step 4: Analyze the Density Trends in Alkali Metals
Alkali metals have low densities due to their large atomic size and weak metallic bonds. The trend in density for alkali metals is that it increases as you move down the group. This means:
- Lithium (Li) < Sodium (Na) < Potassium (K)

### Step 5: Compare the Densities of the Elements
- Lithium is at the top of the alkali metals group, which means it has the smallest atomic size and, consequently, the least density.
- Sodium has a higher density than Lithium.
- Potassium has a higher density than both Lithium and Sodium.
- Magnesium, being an alkaline earth metal, has a density that is generally higher than that of alkali metals.

### Step 6: Conclusion
From the analysis, it is clear that Lithium (Li) has the least density among the given options.

### Final Answer
**Lithium (Li) has the least density.**

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