What is the primary reason diamonds are considered harder than graphite?

Diamonds are considered harder than graphite primarily due to their atomic arrangement. In diamonds, carbon atoms are bonded in a three-dimensional tetrahedral structure, where each carbon atom is covalently bonded to four other carbon atoms. This strong and stable arrangement results in a very rigid and durable crystal lattice, which gives diamonds their exceptional hardness.

On the other hand, graphite has a layered structure in which carbon atoms are bonded in flat, two-dimensional planes. Within these planes, the bonding is strong due to covalent bonds, but between the layers, the forces are much weaker (van der Waals forces allow the layers to slide over each other). This difference in atomic arrangement is fundamental to understanding why diamonds are much harder than graphite, as it directly influences the material's mechanical properties.

The other options, such as carbon purity, molecular size, and densities, are less influential in this context. While purity can impact certain properties, and density is a characteristic of the material, it is the unique atomic structure that fundamentally determines hardness.