carbides

We meet carbon mostly bonded to hydrogen and oxygen, but carbon also combines directly with metals and metalloids to make carbides, a class that includes everything from the gas-generating lumps that once lit miners' lamps to the cutting tips harder than hardened steel. The word covers a surprisingly diverse family, and the diversity tells you a lot about how electronegative carbon's partner is.

Carbides are binary compounds of carbon with a less electronegative element, and they fall into three structural types. Ionic (saltlike) carbides form with very electropositive metals: calcium carbide, CaC2, contains the acetylide ion C2^2- and reacts with water to release acetylene (ethyne), which is why it was used in carbide lamps; some, like Al4C3, contain isolated C^4- and give methane with water. Covalent (giant-molecular) carbides form with elements of similar electronegativity to carbon: silicon carbide (SiC, carborundum) and boron carbide are diamond-like networks, extremely hard, high-melting, and inert. Interstitial carbides form when small carbon atoms slip into the holes of a transition-metal lattice: tungsten carbide (WC) and iron carbide (cementite, Fe3C) keep metallic character but gain great hardness; these are nonstoichiometric in many cases.

Carbides matter across heavy industry. Calcium carbide is a route to acetylene and, historically, to fertilizer (via calcium cyanamide). Silicon carbide and boron carbide are abrasives, refractories, and armor materials. Tungsten carbide tips machine tools and drill bits. And iron carbide is central to the metallurgy of steel, its formation and distribution governing hardness. The three carbide types are a clean lesson in how bonding type, ionic, covalent, or interstitial-metallic, follows directly from the electronegativity difference between carbon and its partner.