In chemistry, an arsenide is a compound of arsenic with a less electronegative element or elements. Most metals form arsenides. There are a large number of different stoichiometries and in this respect arsenides are similar to phosphides, borides, silicides and nitrides. The group 1 alkali metals and the group 2, alkaline earth metals, form arsenides with isolated arsenic atoms which can be considered to be the As^3- anion, for example heating arsenic powder with excess sodium gives sodium arsenide (Na₃As). The As^3- ion is formed by the reduction of arsenic by three electrons. The structure of Na₃As is complex with unusually short Na–Na distances of 328–330 pm which are shorter than in sodium metal, and this is indicates that even with these electropositive metals the bonding cannot be straightforward ionic. The compound LiAs, has a metallic lustre and electrical conductivity indicating some metallic bonding. A number of arsenide minerals are known, for example nickeline, NiAs, skutterudite, CoAs₃ (a diamagnetic semiconductor) Assigning formal oxidation numbers (states) is difficult unless the structure is known, for example the mineral sperrylite (PtAs₂), platinum arsenide, has a formal oxidation state for arsenic of –2. as the solid is usually described as Pt⁴⁺,As₂^4-. The description of gallium arsenide (GaAs) is more straightforward since it features isolated arsenic centers, however as it is a III–V semiconductor with a wurtzite structure the bonding is predominantly covalent.