Elements of the molybdenum and of the sulphur groups can act as dyads, tetrads, and hexads, and there are corresponding compounds of chromium, manganese, and iron, while sulphur and manganese are also able to form compounds analogous in formula to the perchlorates, termed the permanganates and persulphates. Compounds in which these elements function as dyads, however, have no acid properties; in the case of chromium, manganese, and iron, the dyad oxides have more or less basic properties- that is, their hydroxides are ionised into element and hydroxyl-and they therefore form salts with acids. The oxides MoO₂ and UO₂ are also feebly basic in character, as well as TeO₂; "but MnO₂ is the anhydride of a feeble acid, and SO₂ and SeO₂ form well-defined acids, sulphurous, H₂SO₃, and selenious, H₂SeO_g.