Selenium 

Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal, more rarely considered a metalloid, with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium. Selenium exists in several allotropic forms, including amorphous selenium, red selenium, and gray or metallic selenium. The most stable and dense form is gray selenium, which has a hexagonal crystal structure similar to that of the black phosphorus.

Selenium is an essential trace element in the diet of many organisms, including humans, but it is required only in small amounts. It plays a crucial role in the functioning of antioxidant enzymes like glutathione peroxidase, as well as in the functioning of the thyroid gland by participating in the synthesis of thyroid hormones. Selenium deficiency can lead to various health problems, but excessive intake can also result in selenium toxicity, known as selenosis.