The physical properties of any solid material are determined fundamentally by the nature of the chemical bonds that hold it together. Malleability is the ability of a substance to be deformed, or hammered into a thin sheet, without fracturing. This property describes a material’s capacity for plastic deformation under compressive stress. Determining if molecular solids possess this characteristic requires an examination of their unique underlying structure and the forces at play within them.
Understanding Molecular Solids
Molecular solids are composed of discrete, neutral molecules, rather than individual atoms or ions. These molecules are typically formed by atoms held together by strong covalent bonds. Examples of familiar molecular solids include ice (solid water), dry ice (solid carbon dioxide), and table sugar (sucrose).
The defining characteristic of this solid type is the significant difference in strength between the bonds within the molecules and the forces between them. While the intramolecular covalent bonds are quite strong, the intermolecular forces—such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds—are comparatively weak. These weak intermolecular attractions, collectively referred to as van der Waals forces, hold the entire solid structure together in a crystalline lattice.
Because only a small amount of energy is required to disrupt these weak intermolecular forces, molecular solids exhibit relatively low melting and boiling points. The low-energy nature of the binding forces between the molecules dictates their mechanical behavior when external stress is applied.
The Physical Basis of Malleability
A material that is malleable must possess a structural mechanism that allows it to change shape permanently without breaking apart. This mechanical property, known as plastic deformation, involves the rearrangement of atoms or molecules within the solid structure. When a solid is subjected to a compressive force, its internal layers must be able to slide past one another.
This ability to slide depends on the solid’s internal arrangement, often described as a crystal lattice, and the nature of the forces connecting the layers. For a material to be malleable, the forces between the structural units must be non-directional and uniform throughout the structure. The shifting of atomic planes is typically accommodated by the movement of structural defects called dislocations.
The successful deformation of a malleable material results in a permanent change in shape because the bonds are reformed in new positions, maintaining the material’s integrity. The energy required to cause this permanent slip is lower than the energy required to cause a catastrophic fracture.
Why Molecular Solids Are Brittle
Molecular solids are not malleable; instead, they are characterized by brittleness. Brittleness describes a material that fractures or shatters immediately upon reaching its yield stress, showing little or no plastic deformation. This response is a direct consequence of the weak intermolecular forces that govern the solid’s structure.
When a molecular solid is pressed or hammered, the applied stress attempts to shift the layers of molecules relative to one another. The weak van der Waals forces holding these layers together cannot withstand the shearing force required for the molecules to slip into new, stable positions. Instead of allowing for a smooth deformation, the small amount of energy transferred by the impact is sufficient to instantaneously overcome and break these weak forces between the molecules.
This sudden bond breakage leads to the rapid propagation of cracks and subsequent fragmentation of the solid. The energy required to break the weak intermolecular bonds is far less than the energy needed to permanently deform the structure. The inherent softness and low binding energy of the lattice results in a material that is easily cleaved, confirming its brittle nature.
Comparison to Metallic and Ionic Solids
The brittleness of molecular solids stands in contrast to the properties of other major solid types, particularly metallic solids. Metallic solids, such as copper and gold, are highly malleable because they are held together by metallic bonds, which involve a “sea” of delocalized electrons. When a compressive force is applied, the non-directional nature of this electron sea allows layers of metal atoms to slide past one another easily, with the mobile electrons flowing to accommodate the new atomic positions without causing a fracture.
Ionic solids, like table salt (sodium chloride), are also brittle, but for a different reason than molecular solids. These materials are held together by strong, directional electrostatic attractions between positively and negatively charged ions arranged in a rigid lattice. When the layers of an ionic solid are forced to shift, the sliding motion brings ions of the same charge into alignment.
The resulting strong electrostatic repulsion between the like-charged ions creates a massive internal stress, which causes the crystal structure to cleave or shatter immediately. While both molecular and ionic solids are brittle, molecular solids fracture due to the weakness of their bonds, whereas ionic solids break due to the repulsion caused by the strong, but misaligned, electrostatic forces.