共价晶体Network covalent bonding

熔化共价晶体必须断裂大量强共价键，因此熔点很高：金刚石在高压下约 4000 °C 以上，石英约 1713 °C，碳化硅约 2730 °C 分解。共价键的方向性使原子位置固定，晶体坚硬但较脆。硬度与键能和键密度相关：金刚石 C–C 键短且强，莫氏硬度为 10；Si–Si 键较长较弱，硅的硬度约 7。

Melting a network solid means breaking many strong covalent bonds, so melting points are very high: diamond above about 4000 °C under pressure, quartz about 1713 °C, and silicon carbide decomposing near 2730 °C. Directional covalent bonds fix atom positions, making these crystals hard but brittle. Hardness tracks bond strength and density: diamond's short, strong C–C bonds give Mohs hardness 10, while silicon's longer, weaker Si–Si bonds give about 7.

金刚石中每个碳 sp³ 杂化与 4 个碳形成四面体，晶体硅、锗、灰锡有相同结构；SiO₂ 中每个 Si 与 4 个 O 相连、每个 O 桥连 2 个 Si；立方氮化硼（c-BN）硬度仅次于金刚石。石墨层内是共价网络、层间是范德华力，属于混合型晶体。

In diamond each carbon is sp³ hybridised and bonded tetrahedrally to four others, a structure shared by silicon, germanium and grey tin; in SiO₂ each Si bonds to four O and each O bridges two Si; cubic boron nitride (c-BN) is second only to diamond in hardness. Graphite, with covalent networks within layers and van der Waals forces between them, is a mixed-type crystal.

金刚石和立方氮化硼用作切削、磨削工具；碳化硅用于磨料、高温结构陶瓷和功率半导体器件；石英用于光学元件和石英振子；单晶硅是集成电路和太阳能电池的基础材料。

Diamond and cubic boron nitride make cutting and grinding tools; silicon carbide serves as abrasive, high-temperature structural ceramic and power semiconductor; quartz goes into optics and crystal oscillators; single-crystal silicon is the foundation of integrated circuits and solar cells.