Silicon nitride 1Si3N42, a high-temperature ceramic useful for making engine components, is a covalent network solid in which each Si atom is bonded to four N atoms and each N atom is bonded to three Si atoms. Explain why silicon nitride is more brittle than a metal like copper.

Covalent network solids, like silicon nitride, consist of a continuous network of covalent bonds, creating a rigid structure. This extensive bonding leads to high melting points and hardness but also results in brittleness, as the strong bonds do not allow for easy deformation. When stress is applied, these materials tend to fracture rather than bend.

In metals like copper, atoms are held together by metallic bonds, which involve a 'sea of electrons' that are free to move. This electron mobility allows metals to deform plastically under stress, making them malleable and ductile. The ability to absorb energy through deformation contributes to the toughness of metals compared to covalent network solids.

Brittleness refers to a material's tendency to break or shatter under stress without significant deformation, while ductility is the ability to stretch or deform without breaking. Silicon nitride's strong covalent bonds contribute to its brittleness, as it cannot undergo plastic deformation like ductile metals, which can absorb and redistribute stress through movement of dislocations.