Hardness of Solid Substances - Diamond Scratching Glass

The hardness of a substance depends on the nature and strength of the bonds that bind the atoms, molecules or ions in a crystal lattice. Three-dimensional covalent network solids such as diamond and silicon are hard and brittle because the bonds holding the crystal together are quite strong. Since the strength of these bonds depends on good overlap of the valence orbitals, attempts to deform the solids will rupture the bonds and result in a breaking of the crystal, as is seen in the case of silicon. Graphite is a two-dimensional covalent network solid. Although the bonds within the hexagonal layers are very strong, only weak London forces must be overcome when sliding the layers past one another; hence graphite is a soft material. Similarly, the intermolecular forces holding the helical chains of selenium together are weak, so this substance is also soft. Like most molecular substances, sulfur and iodine are easily crushed.

In molybdenum disulfide the covalent bonds between molybdenum and sulfur atoms are strong, but the sulfur atoms on adjacent layers are bound only by weak London forces. Thus, it is easy to peel MoS₂ into very thin sheets.

Although metal atoms are bound by a sea of delocalized electrons, the strengths of the bonds vary widely, depending in part on the size of the metal atoms and the number of bonding electrons. Thus, lithium is easily cut, lead and tin may be flattened when pressure is applied, while magnesium, iron and zinc are so hard that they cannot be ground with a mortar and pestle. Lead and tin may be deformed without breaking, since the strengths of metallic bonds are much less sensitive to the positions of the atoms than are the strengths of covalent bonds. The hardness of a metal is not related to its crystal structure. Although lithium is soft and iron is hard, both metals have body centered cubic lattices. A face centered cubic (cubic closest packing) unit cell of lead is also shown.