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Relative Cosmic Abundance of Elements

Elements
Image from Steve Bowers

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, together of course with 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, and for instance oxygen will still be orders of magnitude more common than chlorine. While this is true of the composition of stars and also of their largest gas giant planets, 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. Inner planets will typically be composed largely of more refractory substances such as rock or metals and depleted in more volatile substances such as ammonia, water and methane. The cooler outer solar system is much icier, so the relative proportion of rock and metal will be smaller. 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 that make up more volatile compounds form either a film of hydrosphere and atmosphere, as on terrestrial planets, or a coating of ices. The ices 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 so-called rare earth elements but is much easier to find on the surface of an earth-like planet than they are 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. In both cases the figures have been rounded off for purposes of illustration, so their total is close to but not exactly a billion. 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 typically 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 Agricultural Age.


ELEMENT NUMBER PARTS PER BILLION BY MASS ATOMS PER BILLION
Hydrogen 1 750,000,000 927,000,000
Helium 2 230,000,000 72,000,000
Oxygen 8 10,000,000 780,000
Carbon 6 5,000,000 520,000
Nitrogen 7 1,000,000 88,000
Neon 10 1,300,000 81,000
Silicon 14 700,000 31,000
Magnesium 12 600,000 31,000
Iron 26 1,100,000 25,000
Sulphur 16 500,000 20,000
Argon 18 200,000 6,200
Aluminum 13 50,000 2,400
Calcium 20 70,000 2,200
Nickel 28 60,000 1,200
Sodium 11 20,000 1,100
Chromium 24 15,000 360
Phosphorous 15 7,000 290
Manganese 25 8,000 190
Potassium 19 3,000 93
Titanium 22 3,000 79
Cobalt 27 3,000 64
Chlorine 17 1,000 36
Fluorine 9 400 26
Vanadium 23 1,000 25
Zinc 30 300 5.7
Germanium 32 200 3.4
Copper 29 60 1.2
Lithium
 3
 6 1.1
Scandium 21 30 0.82
Zirconium 40 50 0.69
Krypton 36 40 0.60
Strontium 38 40 0.56
Selenium 34 30 0.47
Beryllium
 4
 2 0.27
Niobium 41 2 0.27
Boron 5 2 0.22
Gallium 31 10 0.17
Rubidium 37 10 0.14
Arsenic 33 8 0.14
Bromine 35 7 0.11
Yttrium 39 7 0.10
Xenon 54 10 0.095
Barium 56 10 0.090
Cerium 58 10 0.088
Tellurium 52 9 0.087
Neodymium 60 10 0.086
Molybdenum 42 5 0.065
Ruthenium 44 5 0.061
Lead 82 10 0.060
Tin 50 4 0.042
Samarium 62 5 0.041
Platinum 78 5 0.032
Palladium 46 2 0.023
Cadmium 48 2 0.022
Osmium 76 3 0.020
Lanthanum 57 2 0.017
Praseodymium 59 2 0.017
Gadolinium 64 2 0.016
Dysprosium 66 2 0.015
Erbium 68 2 0.015
Ytterbium 70 2 0.015
Iridium 77 2 0.012
Iodine 53 1 0.0098
Cesium 55 0.8 0.0075
Rhodium 45 0.6 0.0072
Silver 47 0.6 0.0070
Mercury 80 1 0.0062
Antimony 51 0.5 0.0051
Hafnium 72 0.7 0.0049
Bismuth 83 0.7 0.0041
Europium 63 0.5 0.0041
Terbium 65 0.5 0.0038
Gold 79 0.6 0.0037
Holmium 67 0.5 0.0046
Tungsten 74 0.5 0.0035
Indium 49 0.3 0.0032
Thallium 81 0.5 0.0025
Thorium 90 0.4 0.0021
Rhenium 75 0.2 0.0014
Uranium 92 0.2 0.0010
Thulium 69 0.1 0.00073
Lutetium 71 0.1 0.00071
Tantalum 73 0.08 0.00055
 
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Development Notes
Text by Stephen Inniss
Initially published on 10 September 2005.

 
 
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