Abundance of the chemical elements
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The abundance of a chemical element measures how common the element is, or how much of the element there is.
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Abundance of elements in the Universe
Hydrogen is the most abundant element in the known Universe; helium is second. However, after this, the rank of abundance does not continue to correspond to the atomic number; oxygen has abundance rank 3, but atomic number 8. All others are orders of magnitude less common.
Both helium-3 and helium-4 were produced in the Big Bang. Additional helium is produced by the fusion of hydrogen inside stellar cores, via a process called the proton-proton chain.
Hydrogen and helium are estimated to make up roughly 80% and 20% of all the matter in the universe respectively. Despite comprising only a very small fraction of the universe, the remaining "heavy elements" can greatly influence astronomical phenomena. Only about 2% of the Milky Way galaxy's disk is comprised of heavy elements.
These other elements are generated by stellar processes. In astronomy, a "metal" is any element other than hydrogen or helium. This distinction is significant because hydrogen and helium (together with trace amounts of lithium) are the only elements that occur naturally without the nuclear fusion activity of stars. Thus, the "metallicity" of a galaxy or other object is an indication of past stellar activity.
See also: Stellar population
Abundance of elements in the Solar System
The Solar System was created from the remnants of previous stellar systems that went nova, and is hence, relative to the rest of the universe, richer in heavier elements.
Population I stars contain significant amounts of elements heavier than helium ("metals", in the terminology of astronomers). These heavy elements were produced by earlier generations of stars and spread by supernova explosions. Our own Sun is a Population I star. They are common in the spiral arms of the Milky Way galaxy.
Abundance of elements in Earth
Scientists believe that the Earth formed from the same cloud of matter that formed the Sun.
Abundance of elements in Earth's crust
The graphic below illustrates the relative abundance of the chemical elements in Earth's upper continental crust.
Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of atomic number.
Many of the elements shown in the graphic are classified into (partially overlapping) categories:
(1) rock-forming elements (major elements in green field and minor elements in light green field);
(2) rare earth elements (lanthanides, La-Lu, and Y; labeled in blue);
(3) major industrial metals (global production >~3×107 kg/year; labeled in bold);
(4) precious metals (italic); and
(5) the nine rarest "metals" - the six platinum group elements plus Au, Re, and Te (a metalloid).
Note that there is a break where the unstable, and very rare, technetium would be, at atomic number 43.
Oxygen and silicon are notably common because they together are the only components of several common minerals.
"Rare earth" element abundances
"Rare" earth elements is a historical misnomer; persistence of the term reflects unfamiliarity rather than true rarity. The more abundant rare earth elements are each similar in crustal concentration to commonplace industrial metals such as chromium, nickel, copper, zinc, molybdenum, tin, tungsten, or lead. Even the two least abundant rare earth elements (Tm, Lu) are nearly 200 times more common than gold. However, in contrast to ordinary base and precious metals, rare earth elements have very little tendency to become concentrated in exploitable ore deposits. Consequently, most of the world's supply of rare earth elements comes from only a handful of sources.
Differences in abundances of individual rare earth elements in the upper continental crust of Earth represent the super-position of two effects, one nuclear and one geochemical. First, rare earth elements with even atomic numbers (58Ce, 60Nd, ...) have greater cosmic and terrestrial abundances than adjacent rare earth elements with odd atomic numbers (57La, 59Pr, ...). Second, the lighter rare earth elements are more incompatible (because they have larger ionic radii) and therefore more strongly concentrated in the continental crust than the heavier rare earth elements. In most rare earth deposits, the first four rare earth elements - La, Ce, Pr, and Nd - constitute 80 to 99% of the total.
Ocean
See sea water for abundance of elements in the ocean, but note that that list is by mass - a list by molarity would look very different, especially as regards the first 4 elements; specifically, hydrogen would comprise well over half of such a list
Atmosphere
The order of elements in the atmosphere is nitrogen (78.1%), oxygen (20.9%), argon (roughly 1%), followed by (in uncertain order) carbon and hydrogen. The carbon content is variable for a number of reasons, including human activity. Sulfur, phosphorus, and all other elements are present in significantly lower proportions.
According to the above graphic, argon, a significant if not major component of the atmosphere, does not appear in the crust at all.
See also
Parts of this article are taken from the public domain sources at http://geopubs.wr.usgs.gov/fact-sheet/fs087-02/ and http://imagine.gsfc.nasa.gov/docs/dict_ei.html Please update as needed.