SeO4

SeO4 commonly denotes the selenate ion, SeO4^2-, an oxoanion of selenium in the +6 oxidation state. It is the tetrahedral, dianionic analogue of sulfate and occurs in many salts, such as sodium selenate (Na2SeO4) and ammonium selenate ((NH4)2SeO4). In aqueous solution, the selenium center is surrounded by four equivalent oxygen atoms, with Se–O bonds described by resonance among multiple structures.

The anion is isoelectronic with sulfate and exhibits similar tetrahedral geometry. The central selenium atom bears the +6 oxidation state, and the four oxygens carry the negative charge distributed by resonance. SeO4^2- is stable over a range of pH in water, though it can participate in redox processes under reducing conditions.

SeO4^2- forms by oxidation of selenium species, such as selenite (SeO3^2-) or elemental selenium, and can be produced by neutralization of selenic acid (H2SeO4) with a base. In nature, selenates occur in arid and evaporite environments as minerals and as soluble salts with alkali metals or ammonium.

Solubility and reactions vary with the counterion; many selenate salts are highly soluble in water. Under reducing conditions, SeO4^2- can be reduced to selenite SeO3^2- or to elemental selenium, influencing mobility and bioavailability in soils and waters.

Applications include providing selenium in fertilizers and animal feeds, particularly in selenium-deficient regions, as well as roles in analytical chemistry and certain oxidation reactions. Safety concerns arise from the general toxicity of selenium compounds at elevated exposures, and environmental considerations focus on mobility and bioaccumulation in ecosystems. See also selenic acid and selenite.

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