Any of various compounds of carbon with metals or other more electropositive elements. True carbides contain the ion C⁴⁻ as in Al₄C₃. These are saltlike compounds giving methane on hydrolysis, and were formerly called methanides. Compounds containing the ion C₂²⁻ are also saltlike and are known as dicarbides. They yield ethyne (acetylene) on hydrolysis and were formerly called acetylides. The above types of compound are ionic but have partially covalent bond character, but boron and silicon form true covalent carbides, with giant molecular structures. In addition, the transition metals form a range of interstitial carbides in which the carbon atoms occupy interstitial positions in the metal lattice. These substances are generally hard materials with metallic conductivity. Some transition metals (e.g. Cr, Mn, Fe, Co, and Ni) have atomic radii that are too small to allow individual carbon atoms in the interstitial holes. These form carbides in which the metal lattice is distorted and chains of carbon atoms exist (e.g. Cr₃C₂, Fe₃C). Such compounds are intermediate in character between interstitial carbides and ionic carbides. They give mixtures of hydrocarbons on hydrolysis with water or acids.