Relative Cosmic Abundance of Elements

Naturally most of the universe is hydrogen or helium, the primordial materials. All of the remainder has since been produced by nucleosynthesis in stars. This means that in general lighter elements are more common than heavier elements. Also, because of the interactions of the subatomic particles in natural nucleosynthetic processes, odd numbered elements beyond hydrogen are much less common than even numbered elements of similar weight.

Depending on the past history of the stellar medium from which they condensed, some stars (as well as the planets and other bodies orbiting them) have higher "metallicity" – a greater proportion of elements produced by nucleosynthesis in earlier generations of stars. This does not affect the relative abundance of elements (oxygen will still be orders of magnitude more common than chlorine, for instance). This is true of the composition of stars and also of their largest planets (Saturn- or Jupiter-sized bodies). Smaller bodies, as they condense from debris around young stars, tend to consist primarily of materials in their orbits that are solid at the time of formation. For this reason the inner planets tend to be composed largely of more refractory substances such as rock or metals and are depleted in more volatile substances such as ammonia, water and methane. The cooler outer solar system is much icier. The noble gases, such as helium, neon, and argon, are lost to all but the largest bodies because they do not form compounds. Finally, bodies of any size begin to differentiate once they have formed. Iron, nickel and siderophilic elements (those that mix well with iron and nickel) tend to migrate towards the core of a planet or moon while lithophilic elements (those that tend to combine with silicate rocks) and carbonaceous compounds are found near the surface. Elements which make up more volatile compounds form either a thin film of hydrosphere and atmosphere (as on terrestrial planets) or a thick coating of ice. This may be most of the mass of a moon or small planet in the outer reaches of a system. On geologically active bodies some elements are locally concentrated. This is the case with gold, which is actually less common cosmically than the rare earth elements but is much easier to find on the surface of an earth-like planet because it is concentrated in veins of ore rather than spread out evenly through the planet's crust.

The following table shows the natural abundance of elements in the universe. The element's name is listed first, then its atomic number, then its frequency by weight according to standard references. The last column is the frequency of the element according to the actual number of atoms (calculated by taking the second-to-last column and dividing by the element's average atomic mass). This figure has not been recalculated in parts per billion, but the relative frequencies speak for themselves. After hydrogen and helium, the top dozen or so elements make up the bulk of available materials. Note that oxygen generally combines to make water or aluminosilicates, carbon forms methane or other hydrocarbons and nitrogen is found mostly as ammonia. Iron and nickel combine together as an alloy or form sulphides. The remaining elements tend to be found as compounds within these large classes of materials (nickel-iron, rock or ice).

The least common elements are not listed because they are so vanishingly rare. The list goes as far as it does because some of the elements near the end of the list (i.e. gold and uranium) have been important in Terragen technology in the Industrial Age or even as far back as the Age of Agriculture.

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