Introduction to Selenium
Selenium is a chemical element denoted by the symbol Se and possesses an atomic number of 34. It is found in Group 16 of the periodic table, alongside oxygen, sulfur, tellurium, and polonium. This element is widely distributed in the Earth’s crust, often associated with sulfide ores of other metals such as copper and lead, and is obtained as a byproduct during the refining processes of these metals.
Elemental Classification
Selenium is classified as a metalloid. It exhibits properties that are intermediate between those of metals and non-metals. For instance, it is a semiconductor, meaning its electrical conductivity lies between that of a conductor (like copper) and an insulator (like sulfur). Its chemical reactivity often resembles that of non-metals like sulfur, while its lustrous appearance in some forms can be metallic.
Physical Appearance and Allotropic Forms
Selenium exists in several allotropic forms, which are different structural arrangements of the same element, each exhibiting distinct physical properties.
Color
The color of selenium varies significantly depending on its allotropic form:
- Amorphous Selenium: This form can appear as a deep red powder when finely divided or as a black, glassy, vitreous solid. This black form is often produced when molten selenium is rapidly cooled.
- Crystalline Selenium: Two main crystalline forms exist. The monoclinic form is typically reddish-brown. The most stable and common allotrope is hexagonal selenium, often referred to as “grey selenium” or “metallic selenium,” which displays a lustrous grey to black appearance.
Texture
The texture of selenium also depends on its allotropic form:
- Amorphous Selenium: The red powder form is fine and soft. The black vitreous form is brittle and glass-like.
- Crystalline Selenium: The grey, hexagonal form is a brittle crystalline solid.
State of Matter
At standard room temperature (approximately 25 °C), selenium exists as a solid in all its allotropic forms.
Thermal Characteristics
The thermal properties, specifically melting and boiling points, are primarily reported for the most stable allotrope, grey hexagonal selenium.
Melting Point
The melting point of grey hexagonal selenium is approximately 221 °C. Other allotropic forms can have slightly different melting characteristics, but this value is for the most common crystalline form.
Boiling Point
Selenium begins to boil and vaporise at approximately 685 °C. At this temperature, it transitions from its liquid state to a gaseous state.