Relative Cosmic Abundance of Elements

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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

Element - Text by M. Alan Kazlev
A chemically pure substance composed of atoms of a single type.

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Development Notes

Text by Stephen Inniss
Initially published on 10 September 2005.

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