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Why is diamond extremely hard?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — chemical compound Methane, in which four hydrogen atoms are bound to a single carbon atom, is an example of a basic chemical compound. The structures of chemical compounds are influenced by complex factors, such as bond angles and bond length.

Diamond is considered the hardest naturally occurring substance on Earth due to its unique atomic structure and the strength of its chemical bonds.12

In a diamond, every carbon atom is covalently bonded to four other carbon atoms in a rigid, three-dimensional tetrahedral configuration. This arrangement creates an infinite, dense network where each bond is highly directional and localized. Because the covalent bond is the strongest type of chemical bond, the diamond structure can only be destroyed by rupturing a vast number of these strong connections.32

This structural integrity is why diamond achieves a 10 on the Mohs scale of mineral hardness, providing supreme resistance to scratching and making it highly valuable for industrial applications such as cutting and drilling.1