Which is hard in nature-

Options

A. `Li`
B. K
C. `Na`
D. `Be`

Explanation

To determine which element is hard in nature among lithium, potassium, sodium, and beryllium, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Groups**:
- Lithium (Li), Sodium (Na), and Potassium (K) are alkali metals, which belong to Group 1 of the periodic table.
- Beryllium (Be) is an alkaline earth metal, which belongs to Group 2 of the periodic table.

2. **Understand the Properties**:
- Alkali metals (Group 1) have one valence electron and are generally soft and have low melting points.
- Alkaline earth metals (Group 2) have two valence electrons and are typically harder and have higher melting points than alkali metals.

3. **Compare Sizes**:
- As we move down the group in the periodic table, the atomic size increases. However, alkaline earth metals are generally smaller in size compared to alkali metals in the same period due to their higher nuclear charge.

4. **Density and Hardness**:
- Density tends to increase across a period. Higher density metals are generally harder. Since beryllium is smaller and denser than the alkali metals listed, it is expected to be harder.

5. **Conclusion**:
- Among the given options, beryllium (Be) is the hardest due to its properties as an alkaline earth metal, which is generally harder than alkali metals like lithium, sodium, and potassium.

### Final Answer:
**Beryllium (Be) is hard in nature.**

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