Which is the hardest elements ?

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the hardest element in the periodic table.

2. **Identifying the Element**: In the periodic table, the element known for its hardness is carbon, specifically in its allotrope form known as diamond.

3. **Explaining the Structure of Diamond**: Diamond is a form of carbon where each carbon atom is covalently bonded to four other carbon atoms. This creates a three-dimensional network structure.

4. **Covalent Network Solid**: The structure of diamond is classified as a covalent network solid. In this type of solid, atoms are bonded by strong covalent bonds in a continuous network.

5. **Bonding and Hardness**: The carbon-carbon (C-C) bonds in diamond are very strong, requiring a significant amount of energy to break. This high bond energy contributes to the overall hardness of diamond.

6. **Conclusion**: Therefore, the hardest element is carbon, specifically in the form of diamond due to its strong covalent bonds and network structure.

### Final Answer:
The hardest element is carbon in the form of diamond.

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