Abundance of the chemical elements

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File:080998 Universe Content 240.jpg

Estimated proportions of matter, dark matter and dark energy in the universe. Only the fraction of the mass and energy in the universe labeled "atoms" is composed of chemical elements.

The abundance of a chemical element measures how relatively common (or rare) the element is, or how much of the element is present in a given environment by comparison to all other elements. Abundance may be variously measured by the mass-fraction (the same as weight fraction), or mole-fraction (fraction of atoms by numerical count, or sometimes fraction of molecules in gases), or by volume-fraction. Measurement by volume-fraction is a common abundance measure in mixed gases such as planetary atmospheres, and is close to molecular mole-fraction for ideal gas mixtures (i.e., gas mixtures at relatively low densities and pressures).

For example, the mass-fraction abundance of oxygen in water is about 89%, because that is the fraction of water's mass which is oxygen. However, the mole-fraction abundance of oxygen in water is only 33% because only 1 atom of 3 in water is an oxygen atom. In the universe as a whole, and in the atmospheres of gas-giant planets such as Jupiter, the mass-fraction abundances of hydrogen and helium are about 74% and 23–25% respectively, while the (atomic) mole-fractions of these elements are closer to 92% and 8%. However, since hydrogen is diatomic while helium is not, in the conditions of Jupiter's outer atmosphere, the molecular mole-fraction (fraction of total gas molecules, or fraction of atmosphere by volume) of hydrogen in the outer atmosphere of Jupiter is about 86%, and for helium, 13%.

Most abundances in this article are given as mass-fraction abundances.

Abundance of elements in the Universe

Ten most common elements in the Milky Way Galaxy estimated spectroscopically

Z	Element	Mass fraction in parts per million
1	Hydrogen	739,000	71 × mass of Oxygen (red bar)
2	Helium	240,000	23 × mass of Oxygen (red bar)
8	Oxygen	Template:Bartable
6	Carbon	Template:Bartable
10	Neon	Template:Bartable
26	Iron	Template:Bartable
7	Nitrogen	Template:Bartable
14	Silicon	Template:Bartable
12	Magnesium	Template:Bartable
16	Sulfur	Template:Bartable

See also: Stellar population, Cosmochemistry, and Astrochemistry

The elements – that is, ordinary (baryonic) matter made out of protons and neutrons (as well as electrons) – are only a small part of the content of the Universe. Cosmological observations suggest that only 4.6% of the universe comprises the visible baryonic matter which constitutes stars, planets and living beings. The rest is made up of dark energy (72%) and dark matter (23%). The latter are forms of matter and energy believed to exist on the basis of theory and observational deductions, but their details are still the subject of research. They have not yet been directly observed and are not well understood.

Most standard (baryonic) matter is found in stars and interstellar clouds, in the form of atoms or ions (plasma), although other unusual kinds of matter can be found in astrophysical settings, such as the high densities inside white dwarfs and neutron stars.

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 substantially less common.

The abundance of the lightest elements is well predicted by the standard cosmological model, since they were mostly produced shortly (i.e., within a few hundred seconds) after the Big Bang, in a process known as Big Bang nucleosynthesis. Heavier elements were mostly produced much later, inside stars.

Helium-3 is rare on Earth and sought-after for use in nuclear fusion research. More abundant helium-3 is thought to exist on the Moon. Additional helium is produced by the fusion of hydrogen inside stellar cores by a variety of processes including the proton-proton chain and the CNO cycle.

Hydrogen and helium are estimated to make up roughly 74% and 24% of all baryonic 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% (by mass) of the Milky Way galaxy's disk is composed of heavy elements.

These other elements are generated by stellar processes. In astronomy, a "metal" is any element other than hydrogen, helium or lithium. 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.

Most abundant isotopes in the Solar System

Isotope	A	Mass fraction in parts per million	Atom fraction in parts per million
Hydrogen-1	1	705,700	909,964
Helium-4	4	275,200	88,714
Oxygen-16	16	5,920	477
Carbon-12	12	3,032	326
Nitrogen-14	14	1,105	102
Neon-20	20	1,548	100
Other elements:		3,879	149
Silicon-28	28	653	30
Magnesium-24	24	513	28
Iron-56	56	1,169	27
Sulfur-32	32	396	16
Helium-3	3	35	15
Hydrogen-2	2	23	15
Neon-22	22	208	12
Magnesium-26	26	79	4
Carbon-13	13	37	4
Magnesium-25	25	69	4
Aluminum-27	27	58	3
Argon-36	36	77	3
Calcium-40	40	60	2
Sodium-23	23	33	2
Iron-54	54	72	2
Silicon-29	29	34	2
Nickel-58	58	49	1
Silicon-30	30	23	1
Iron-57	57	28	1

Elemental abundance and nuclear binding energy

Loose correlations have been observed between estimated elemental abundances in the universe and the nuclear binding energy curve. Roughly speaking, the relative stability of various atomic isotopes has exerted a strong influence on the relative abundance of elements formed in the Big Bang, and during the development of the universe thereafter. See the article about nucleosynthesis for the explanation on how certain nuclear fusion processes in stars (such as carbon burning, etc.) create the elements heavier than hydrogen and helium.

A further observed peculiarity is the jagged alternation between relative abundance and scarcity of adjacent atomic numbers in the elemental abundance curve, and a similar pattern of energy levels in the nuclear binding energy curve. This alternation is caused by the higher relative binding energy (corresponding to relative stability) of even atomic numbers compared to odd atomic numbers, and is explained by the Pauli Exclusion Principle. The semi-empirical mass formula (SEMF), also called Weizsäcker's formula or the Bethe-Weizsäcker mass formula, gives a theoretical explanation of the curve of nuclear binding energy.

File:SolarSystemAbundances.png

Estimated abundances of the chemical elements in the Solar system. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers, and (2) a general decrease in abundance, as elements become heavier.

Abundance of elements in the Earth

See also: Earth#Chemical composition

The Earth formed from the same cloud of matter that formed the Sun, but the planets acquired different compositions during the formation and evolution of the solar system. In turn, the natural history of the Earth caused parts of this planet to have differing concentrations of the elements.

The mass of the Earth is approximately 5.98×10²⁴  kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.

Earth's bulk (total) elemental abundance

Click "show" at right, to show numerical values in full table:

Number	Name	Symbol	ppm (μg/g)	ppb (atoms)[citation needed]
8	oxygen	O	297000	482,000,000.00
12	magnesium	Mg	154000	164,000,000.00
14	silicon	Si	161000	150,000,000.00
26	iron	Fe	319000	148,000,000.00
13	aluminum	Al	15900	15,300,000.00
20	calcium	Ca	17100	11,100,000.00
28	nickel	Ni	18220	8,010,000.00
1	hydrogen	H	260	6,700,000.00
16	sulfur	S	6350	5,150,000.00
24	chromium	Cr	4700	2,300,000.00
11	sodium	Na	1800	2,000,000.00
6	carbon	C	730	1,600,000.00
15	phosphorus	P	1210	1,020,000.00
25	manganese	Mn	1700	800,000.00
22	titanium	Ti	810	440,000.00
27	cobalt	Co	880	390,000.00
19	potassium	K	160	110,000.00
17	chlorine	Cl	76	56,000.00
23	vanadium	V	105	53,600.00
7	nitrogen	N	25	46,000.00
29	copper	Cu	60	25,000.00
30	zinc	Zn	40	16,000.00
9	fluorine	F	10	14,000.00
21	scandium	Sc	11	6,300.00
3	lithium	Li	1.10	4,100.00
38	strontium	Sr	13	3,900.00
32	germanium	Ge	7.00	2,500.00
40	zirconium	Zr	7.10	2,000.00
31	gallium	Ga	3.00	1,000.00
34	selenium	Se	2.70	890
56	barium	Ba	4.50	850
39	yttrium	Y	2.90	850
33	arsenic	As	1.70	590
5	boron	B	0.20	480
42	molybdenum	Mo	1.70	460
44	ruthenium	Ru	1.30	330
78	platinum	Pt	1.90	250
46	palladium	Pd	1.00	240
58	cerium	Ce	1.13	210
60	neodymium	Nd	0.84	150
4	beryllium	Be	0.05	140
41	niobium	Nb	0.44	120
76	osmium	Os	0.90	120
77	iridium	Ir	0.90	120
37	rubidium	Rb	0.40	120
35	bromine	Br	0.30	97
57	lanthanum	La	0.44	82
66	dysprosium	Dy	0.46	74
64	gadolinium	Gd	0.37	61
52	tellurium	Te	0.30	61
45	rhodium	Rh	0.24	61
50	tin	Sn	0.25	55
62	samarium	Sm	0.27	47
68	erbium	Er	0.30	47
70	ytterbium	Yb	0.30	45
59	praseodymium	Pr	0.17	31
82	lead	Pb	0.23	29
72	hafnium	Hf	0.19	28
74	tungsten	W	0.17	24
79	gold	Au	0.16	21
48	cadmium	Cd	0.08	18
63	europium	Eu	0.10	17
67	holmium	Ho	0.10	16
47	silver	Ag	0.05	12
65	terbium	Tb	0.07	11
51	antimony	Sb	0.05	11
75	rhenium	Re	0.08	10
53	iodine	I	0.05	10
69	thulium	Tm	0.05	7
55	cesium	Cs	0.04	7
71	lutetium	Lu	0.05	7
90	thorium	Th	0.06	6
73	tantalum	Ta	0.03	4
80	mercury	Hg	0.02	3
92	uranium	U	0.02	0.002
49	indium	In	0.01	0.002
81	thallium	Tl	0.01	0.002
83	bismuth	Bi	0.01	0.001

An estimate of the elemental abundancies in the total mass of the Earth. Note that numbers are estimates, and they will vary depending on source and method of estimation. Order of magnitude of data can roughly be relied upon. ppb (atoms) is parts per billion, meaning that is the number of atoms of a given element in every billion atoms in the Earth.

Earth's crustal elemental abundance

Main article: Abundance of elements in Earth's crust

File:Elemental abundances.svg

Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of atomic number. The rarest elements in the crust (shown in yellow) are the most dense. They were further rarefied in the crust by being siderophile (iron-loving) elements, in the Goldschmidt classification of elements. Siderophiles were depleted by being relocated into the Earth's core. Their abundance in meteoroids materials is relatively higher. Additionally, tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.

This graph illustrates the relative abundance of the chemical elements in Earth's upper continental crust, which is relatively accessible for measurements and estimation. Many of the elements shown in the graph 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 red);
4. precious metals (labeled in purple);
5. the nine rarest "metals" — the six platinum group elements plus Au, Re, and Te (a metalloid) — in the yellow field.

Note that there are two breaks where the unstable elements technetium (atomic number: 43) and promethium (atomic number: 61) would be. These are both extremely rare, since on Earth they are only produced through the spontaneous fission of very heavy radioactive elements (for example, uranium, thorium, or the trace amounts of plutonium that exist in uranium ores), or by the interaction of certain other elements with cosmic rays. Both of the first two of these elements have been identified spectroscopically in the atmospheres of stars, where they are produced by ongoing nucleosynthetic processes. There are also breaks where the six noble gases would be, since they are not chemically bound in the Earth's crust, and they are only generated by decay chains from radioactive elements and are therefore extremely rare there. The six very rare, highly radioactive elements (polonium, astatine, francium, radium, actinium, and protactinium) are not included, since any of these elements that were present at the formation of the Earth have decayed away eons ago, and their quantity today is negligible.

Oxygen and silicon are notably quite common elements. They have frequently combined with each other to form common silicate minerals.

Rare-earth elemental abundance

"Rare" earth elements is a historical misnomer. The 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. The two least abundant rare earth elements (thulium and lutetium) are nearly 200 times more common than gold. However, in contrast to the 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. Furthermore, the rare earth metals are all quite chemically similar to each other, and they are thus quite difficult to separate into quantities of the pure elements.

Differences in abundances of individual rare earth elements in the upper continental crust of the Earth represent the superposition of two effects, one nuclear and one geochemical. First, the rare earth elements with even atomic numbers (₅₈Ce, ₆₀Nd, ...) have greater cosmic and terrestrial abundances than the 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 ore deposits, the first four rare earth elements – lanthanum, cerium, praseodymium, and neodymium – constitute 80% to 99% of the total amount of rare earth metal that can be found in the ore.

Oceanic elemental abundance

Earth's ocean water elemental abundance

Element	Proportion (by mass)
Oxygen	Template:Bartable
Hydrogen	Template:Bartable
Chlorine	Template:Bartable
Sodium	Template:Bartable
Magnesium	Template:Bartable
Sulfur	Template:Bartable
Calcium	Template:Bartable
Potassium	Template:Bartable
Bromine	Template:Bartable
Carbon	Template:Bartable

For a complete list of the abundance of elements in the ocean, see Abundances of the elements (data page)#Sea water.

Atmospheric elemental abundance

The order of elements by volume-fraction (which is approximately molecular mole-fraction) in the atmosphere is nitrogen (78.1%), oxygen (20.9%), argon (0.96%), followed by (in uncertain order) carbon and hydrogen because water vapor and carbon dioxide, which represent most of these two elements in the air, are variable components. Sulfur, phosphorus, and all other elements are present in significantly lower proportions.

According to the abundance curve graph (above right), argon, a significant if not major component of the atmosphere, does not appear in the crust at all. This is because the atmosphere has a far smaller mass than the crust, so argon remaining in the crust contributes little to mass-fraction there, while at the same time buildup of argon in the atmosphere has become large enough to be significant.

Human body elemental abundance

Element	Proportion (by mass)
Oxygen	Template:Bartable
Carbon	Template:Bartable
Hydrogen	Template:Bartable
Nitrogen	Template:Bartable
Calcium	Template:Bartable
Phosphorus	Template:Bartable
Potassium	Template:Bartable
Sulfur	Template:Bartable
Chlorine	Template:Bartable
Sodium	Template:Bartable
Magnesium	Template:Bartable
Iron	< 0.05%
Cobalt	< 0.05%
Copper	< 0.05%
Zinc	< 0.05%
Iodine	< 0.05%
Selenium	< 0.01%
Fluorine	< 0.01%

Main article: Chemical makeup of the human body

By mass, human cells consist of 65–90% water (H₂O), and a significant portion of the remainder is composed of carbon-containing organic molecules. Oxygen therefore contributes a majority of a human body's mass, followed by carbon. Almost 99% of the mass of the human body is made up of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. The next 0.75% is made up of the next five elements: potassium, sulfur, chlorine, sodium, and magnesium. Only 17 elements are known for certain to be necessary to human life, with one additional element (fluoride) thought to be helpful for tooth enamel strength. A few more trace elements appear to be necessary to mammals in carefully-dust free conditions. Boron and silicon are notably necessary for plants but have uncertain roles in animals. The elements aluminum and silicon, although very common in the earth's crust, are conspicuously rare in the human body."

See also

• Abundances of the elements (data page)
• Natural abundance (isotopic abundance)

References

Notations

• http://geopubs.wr.usgs.gov/fact-sheet/fs087-02/
• http://imagine.gsfc.nasa.gov/docs/dict_ei.html

External links

• List of elements in order of abundance in the Earth's crust (only correct for the twenty most common elements)
• Cosmic abundance of the elements and nucleosynthesis
• webelements.com Lists of elemental abundances for the Universe, Sun, meteorites, Earth, ocean, streamwater