Understanding Selenium’s Nature
Selenium (Se) is a non-metallic chemical element belonging to Group 16 of the periodic table, also known as the chalcogens, alongside oxygen, sulfur, tellurium, and polonium. Its atomic number is 34. Like sulfur, selenium can exist in several allotropic forms, including a red amorphous form and a gray crystalline (metallic) form. Its properties are generally intermediate between those of sulfur and tellurium.
Position in the Periodic Table
As a Group 16 element, selenium typically exhibits valencies of -2, +2, +4, and +6 in its compounds. Its outer electron configuration, $3s^2 3p^4$, dictates its tendency to gain two electrons to achieve a stable octet, forming selenide ions ($Se^{2-}$), or to share electrons, leading to covalent compounds.
Chemical Reactivity of Selenium
Reaction with Water
Selenium does not react with water under normal conditions. It is insoluble in water. Even when heated, there is no significant chemical interaction between selenium and water.
Reaction with Air
The reactivity of selenium with air depends on its allotropic form and temperature.
- Amorphous red selenium is relatively stable in air at room temperature.
- Crystalline gray selenium slowly oxidises in air at room temperature.
- When heated in air or oxygen, selenium burns with a blue flame, forming selenium dioxide ($SeO_2$), which is a white solid.
Toxicity
Selenium exhibits a dual nature regarding its biological impact. It is an essential trace element for humans and animals, playing crucial roles in various bodily functions, particularly as a component of the enzyme glutathione peroxidase, which has antioxidant properties. However, selenium is toxic in higher concentrations. Both deficiency and excess of selenium can lead to health problems. For instance, in some parts of India, specific regions might have soil with either too low or too high selenium levels, impacting livestock health through their diet. Symptoms of selenium toxicity (selenosis) can include hair loss, nail brittleness, and neurological issues. The maximum permissible limit for selenium in drinking water in India, as per IS 10500:2012, is 0.01 mg/L.
Radioactivity
Naturally occurring selenium is not radioactive. It consists of six stable isotopes, with Se-80 being the most abundant. While several radioactive isotopes of selenium have been produced artificially for research and medical applications (e.g., Se-75 is used in nuclear medicine imaging), the element as found in nature is not considered radioactive.
Flammability
Yes, selenium is flammable. As mentioned, it readily burns in air or oxygen when heated, producing selenium dioxide ($SeO_2$). This combustion reaction typically involves a blue flame.
Illustrative Chemical Reaction
A common and illustrative chemical reaction involving selenium is its combustion in air or oxygen, forming selenium dioxide.
$Se_{(s)} + O_{2(g)} \xrightarrow{\text{heat}} SeO_{2(s)}$
This reaction demonstrates selenium’s ability to combine with oxygen, characteristic of many non-metals when heated. Selenium dioxide is an acidic oxide, meaning it reacts with water to form selenous acid ($H_2SeO_3$).