selen(o)-²

Also seleni‑.

Selenium.

English selenium.

The non-metal selenium, related to sulphur, was named by its discoverer Berzelius (1779–1848) after the moon (see seleno‑¹) to indicate how close its properties are to those of tellurium (Latin tellus, tellur‑, earth). The lower combining power of selenium, divalent or tetravalent, is referred to as the selenous or selenious form (as in selenious acid, H₂SeO₃, whose salts are selenites, such as sodium selenite, a dietary supplement providing selenium), while the hexavalent form is termed selenic (as in selenic acid, H₂SeO₄, analogous to sulphuric acid, whose compounds are selenates). The compound selenomethionine is a marker used in nuclear medicine for detecting tumours.