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Selenium is a chemical element; it has the symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elemental state or as pure ore compounds in Earth's crust.
This is a list of chemical elements and their atomic properties, ordered by atomic number (Z).. Since valence electrons are not clearly defined for the d-block and f-block elements, there not being a clear point at which further ionisation becomes unprofitable, a purely formal definition as number of electrons in the outermost shell has been used.
Like the periodic table, the list below organizes the elements by the number of protons in their atoms; it can also be organized by other properties, such as atomic weight, density, and electronegativity. For more detailed information about the origins of element names, see List of chemical element name etymologies.
After 1869, Dmitri Mendeleev proposed his periodic table placing oxygen at the top of "group VI" above sulfur, selenium, and tellurium. [45] Chromium , molybdenum , tungsten , and uranium were sometimes included in this group, but they would be later rearranged as part of group VIB ; uranium would later be moved to the actinide series.
Ⓚ Of 103 elements shown in the image, just ten form anions, all of these being in the p-block: arsenic; the five chalcogens: oxygen, sulfur, selenium, tellurium, polonium; and the four halogens: fluorine, chlorine, bromine, and iodine Ⓛ Anion-only elements are confined to oxygen and fluorine.
A recognisably modern form of the table was reached in 1945 with Glenn T. Seaborg's discovery that the actinides were in fact f-block rather than d-block elements. The periodic table and law are now a central and indispensable part of modern chemistry. The periodic table continues to evolve with the progress of science.
Selenium forms covalent bonds with most other elements, noting it can form ionic selenides with highly electropositive metals. The common oxide of selenium ( SeO 3 ) is strongly acidic. Selenium forms a series of anionic selenites and selenates such as Na 2 SeO 3 , Na 2 Se 2 O 5 , and Na 2 SeO 4 , [ 158 ] as well as Zintl phases such as Cs 4 Se ...
See also: Electronegativities of the elements (data page) There are no reliable sources for Pm, Eu and Yb other than the range of 1.1–1.2; see Pauling, Linus (1960).