Phosphides are binary compounds of phosphorus and a metal. The phosphide ion is P3-; phosphides of almost every metal in the periodic system are known. Because they exhibit a wide range of chemical and physical properties, it is difficult to place them into classes. On the basis of stoichiometry, three categories have been suggested: (1) phosphorus-rich phosphides, in which the metal-to-phosphorus ratio is less than one, (2) metal-rich phosphides, where the metal-to-phosphorus ratio is greater than one, and (3) monophosphides, in which the metal-to-phosphorus ratio is exactly one. Phosphorus-rich phosphides, which tend to have lower thermal stabilities and lower melting points than do those of the other two categories, include phosphides formed with the later transition metals (e.g., RuP2, PdP3, and NiP3). The great variety of structural types among phosphides appears to depend on both steric and electronic effects. (Steric effects are concerned with the spatial disposition of atoms.) Metal-rich phosphides, which exhibit properties that are metallic in nature, are hard, brittle, high-melting, and chemically inert. These have the appearance of a metal and have high thermal and electrical conductivities. The size of the metal apparently determines the structures of the compound. Ni5P2 and Ir2P are examples of metal-rich phosphides. The phosphides of the electropositive alkali metals and alkaline earth metals, which exhibit what is very close to ionic bonding, readily react with water or dilute acid to produce phosphine, PH3.