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Oxygen is the third most abundant chemical element in the universe, after hydrogen and helium. [68] About 0.9% of the Sun's mass is oxygen. [19] Oxygen constitutes 49.2% of the Earth's crust by mass [69] as part of oxide compounds such as silicon dioxide and is the most abundant element by mass in the Earth's crust.
The abundance of the chemical elements is a measure of the occurrences 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.
Water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent, and these numbers along with the complementary % numbers for oxygen in water, are the largest contributors to overall mass and atomic composition figures. Because of water content, the human body contains more oxygen by mass than any other element, but more hydrogen by atom ...
Under a molecular orbital theory framework, the oxygen-oxygen bond in triplet dioxygen is better described as one full σ bond plus two π half-bonds, each half-bond accounted for by two-center three-electron (2c-3e) bonding, to give a net bond order of two (1+2× 1 / 2 ), while also accounting for the spin state (S = 1).
Historically, the mole was defined as the amount of substance in 12 grams of the carbon-12 isotope.As a consequence, the mass of one mole of a chemical compound, in grams, is numerically equal (for all practical purposes) to the mass of one molecule or formula unit of the compound, in daltons, and the molar mass of an isotope in grams per mole is approximately equal to the mass number ...
Also carbohydrates, for example, have the same ratio (carbon:hydrogen:oxygen= 1:2:1) (and thus the same empirical formula) but different total numbers of atoms in the molecule. The molecular formula reflects the exact number of atoms that compose the molecule and so characterizes different molecules. However different isomers can have the same ...
The atomic mass of atoms, ions, or atomic nuclei is slightly less than the sum of the masses of their constituent protons, neutrons, and electrons, due to (per E = mc 2). Atomic mass is often measured in dalton (Da) or unified atomic mass unit (u). One dalton is equal to 1 ⁄ 12 the mass of a carbon-12 atom in its natural state.
Before the 2019 revision of the SI, the mole was defined as the amount of substance of a system that contains as many elementary entities as there are atoms in 12 grams of carbon-12 (the most common isotope of carbon). [19] The term gram-molecule was formerly used to mean one mole of molecules, and gram-atom for one mole of atoms. [15]