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Oxygen is a chemical element; it has symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is the most abundant element in Earth's crust, and after hydrogen and helium, it ...
The abundance of the chemical elements is a measure of the occurrence of the chemical elements relative to all other elements in a given environment. Abundance is measured in one of three ways: by mass fraction (in commercial contexts often called weight fraction), by mole fraction (fraction of atoms by numerical count, or sometimes fraction of molecules in gases), or by volume fraction.
Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of atomic number; [5] siderophiles shown in yellow. Graphs of abundance against atomic number can reveal patterns relating abundance to stellar nucleosynthesis and geochemistry. The alternation of abundance between even and odd atomic number is ...
Oxygen compounds. Water (H 2 O) is the most familiar oxygen compound. The oxidation state of oxygen is −2 in almost all known compounds of oxygen. The oxidation state −1 is found in a few compounds such as peroxides. Compounds containing oxygen in other oxidation states are very uncommon: − 1⁄2 (superoxides), − 1⁄3 (ozonides), 0 ...
Elements. The main elements that comprise the human body (including water) can be summarized as CHNOPS. About 99% of the mass of the human body is made up of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Only about 0.85% is composed of another five elements: potassium, sulfur, sodium, chlorine, and magnesium.
Chemical oxygen demand. In environmental chemistry, the chemical oxygen demand (COD) is an indicative measure of the amount of oxygen that can be consumed by reactions in a measured solution. It is commonly expressed in mass of oxygen consumed over volume of solution, which in SI units is milligrams per liter (mg / L).
Oxygen-15 is a radioisotope, often used in positron emission tomography (PET). It can be used in, among other things, water for PET myocardial perfusion imaging and for brain imaging. [ 20 ] [ 21 ] It has an atomic mass of 15.003 0656 (5) , and a half-life of 122.266(43) s .
Effective concentration (activity) 1 mol/L for each aqueous or amalgamated (mercury-alloyed) species; Unit activity for each solvent and pure solid or liquid species; and; Absolute partial pressure 101.325 kPa (1.00000 atm; 1.01325 bar) for each gaseous reagent — the convention in most literature data but not the current standard state (100 kPa).