Abundance of the chemical elements

The abundance of the chemical elements is a measure of the occurrences of the chemical elements relative to all other elements in a given environment. Abundance is measured in one of three ways: by mass fraction, by mole fraction, or by volume fraction. Volume fraction is a common abundance measure in mixed gases such as planetary atmospheres, and is similar in value to molecular mole fraction for gas mixtures at relatively low densities and pressures, and ideal gas mixtures. Most abundance values in this article are given as mass fractions.
The abundance of chemical elements in the universe is dominated by the large amounts of hydrogen and helium which were produced during Big Bang nucleosynthesis. Remaining elements, making up only about 2% of the universe, were largely produced by supernova nucleosynthesis. Elements with even atomic numbers are generally more common than their neighbors in the periodic table, due to their favorable energetics of formation, described by the Oddo–Harkins rule.
The abundance of elements in the Sun and outer planets is similar to that in the universe. Due to solar heating, the elements of Earth and the inner rocky planets of the Solar System have undergone an additional depletion of volatile hydrogen, helium, neon, nitrogen, and carbon. The crust, mantle, and core of the Earth show evidence of chemical segregation plus some sequestration by density. Lighter silicates of aluminium are found in the crust, with more magnesium silicate in the mantle, while metallic iron and nickel compose the core. The abundance of elements in specialized environments, such as atmospheres, oceans, or the human body, are primarily a product of chemical interactions with the medium in which they reside.

Abundance values

Abundance of each element is expressed as a relative number. Astronomy uses a logarithmic scale for abundance of element X relative to hydrogen, defined by
for number density ; on this scale. Another scale is mass fraction or, equivalently, percent by mass.
For example, the abundance of oxygen in pure water can be measured in two ways: the mass fraction is about 89%, because that is the fraction of water's mass which is oxygen. However, the mole fraction is about 33% because only 1 atom of 3 in water, H₂O, is oxygen. As another example, looking at the mass fraction abundance of hydrogen and helium in both the universe as a whole and in the atmospheres of gas-giant planets such as Jupiter, it is 74% for hydrogen and 23–25% for helium; while the mole fraction for hydrogen is 92%, and for helium is 8%, in these environments. Changing the given environment to Jupiter's outer atmosphere, where hydrogen is diatomic while helium is not, changes the molecular mole fraction, as well as the fraction of atmosphere by volume, of hydrogen to about 86%, and of helium to 13%. Below Jupiter's outer atmosphere, volume fractions are significantly different from mole fractions due to high temperatures and high density, where the ideal gas law is inapplicable.

Universe

Z	Element	Mass fraction
1	Hydrogen	73.97%
2	Helium	24.02%
8	Oxygen	1.04%
6	Carbon	0.46%
10	Neon	0.134%
26	Iron	0.109%
7	Nitrogen	0.096%
14	Silicon	0.065%
12	Magnesium	0.058%
16	Sulfur	0.044%

The abundance of chemical elements in the universe is dominated by the large amounts of hydrogen and helium which were produced during Big Bang nucleosynthesis. Remaining elements, making up only about 2% of the universe, were largely produced by supernovae and certain red giant stars. Lithium, beryllium, and boron, despite their low atomic number, are rare because, although they are produced by nuclear fusion, they are destroyed by other reactions in the stars. Their natural occurrence is the result of cosmic ray spallation of carbon, nitrogen and oxygen in a type of nuclear fission reaction. The elements from carbon to iron are relatively more abundant in the universe because of the ease of making them in supernova nucleosynthesis. Elements of higher atomic numbers than iron become progressively rarer in the universe, because they increasingly absorb stellar energy in their production. Also, elements with even atomic numbers are generally more common than their neighbors in the periodic table, due to favorable energetics of formation, and among the lightest nuclides helium through sulfur the most abundant isotopes of equal number of protons and neutrons.
Hydrogen is the most abundant element in the Universe; helium is second. All others are orders of magnitude less common. After this, the rank of abundance does not continue to correspond to the atomic number. Oxygen has abundance rank 3, but atomic number 8.
There are 80 known stable elements, and the lightest 16 comprise 99.9% of the ordinary matter of the universe. These same 16 elements, hydrogen through sulfur, fall on the initial linear portion of the table of nuclides, a plot of the proton versus neutron numbers of all matter both ordinary and exotic, containing hundreds of stable isotopes and thousands more that are unstable. The Segrè plot is initially linear because the vast majority of ordinary matter contains an equal number of protons and neutrons.
The abundance of the elements up to lithium is well predicted by the standard cosmological model, since they were mostly produced shortly after the Big Bang, in a process known as Big Bang nucleosynthesis. Heavier elements were mostly produced much later, in stellar nucleosynthesis.
In general, elements up to iron are made by large stars in the process of becoming supernovae, or by smaller stars in the process of dying. Iron-56 is particularly common, since it is the most stable nuclide and can easily be "built up" from alpha particles. Elements heavier than iron are made in energy-absorbing processes in large stars, and their abundance in the universe generally decreases with increasing atomic number.
The table shows the ten most common elements in our galaxy, as measured in parts per million, by mass.
Nearby galaxies that have evolved along similar lines have a corresponding enrichment of elements heavier than hydrogen and helium. The more distant galaxies are being viewed as they appeared in the past, so their abundances of elements appear closer to the primordial mixture. Since physical laws and processes are apparently uniform throughout the universe, however, it is expected that these galaxies will likewise have evolved similar abundances of elements.
As shown in the periodic table, the abundance of elements is in keeping with their origin. Very abundant hydrogen and helium are products of the Big Bang. The next three elements in the periodic table are rare, despite their low atomic number. They had little time to form in the Big Bang. They are produced in small quantities by nuclear fusion in dying stars or by breakup of heavier elements in interstellar dust, caused by cosmic ray spallation. In supernova stars, they are produced by nuclear fusion, but then destroyed by other reactions.
Heavier elements, beginning with carbon, have been produced in dying or supernova stars by buildup from alpha particles, contributing to an alternatingly larger abundance of elements with even atomic numbers. The effect of odd-numbered chemical elements generally being more rare in the universe was empirically noticed in 1914, and is known as the Oddo–Harkins rule.
The following graph shows abundance of elements in the Solar System.

Relation to 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 nuclides in withstanding the extremely energetic conditions of Big Bang nucleosynthesis 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 an explanation of how certain nuclear fusion processes in stars 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 estimated abundances of the chemical elements in which the relative abundance of even atomic numbers is roughly 2 orders of magnitude greater than the relative abundance of odd atomic numbers. A similar alternation between even and odd atomic numbers can be observed in the nuclear binding energy curve in the neighborhood of carbon and oxygen, but here the loose correlation between relative abundance and binding energy ends. The binding energy for beryllium, for example, is less than the binding energy for boron, as illustrated in the nuclear binding energy curve. Additionally, the alternation in the nuclear binding energy between even and odd atomic numbers resolves above oxygen as the graph increases steadily up to its peak at iron. The semi-empirical mass formula, also called Weizsäcker's formula or the Bethe-Weizsäcker mass formula, gives a theoretical explanation of the overall shape of the curve of nuclear binding energy.

Sun

Modern astronomy relies on understanding the abundance of elements in the Sun as part of cosmological models. Abundance values are difficult to obtain: even photospheric or observational abundances depend upon models of solar atmospherics and radiation coupling. These astronomical abundance values are reported as logarithms of the ratio with hydrogen. Hydrogen is set to an abundance of 12 on this scale.
The Sun's photosphere consists mostly of hydrogen and helium; the helium abundance varies between about 10.3 and 10.5 depending on the phase of the solar cycle; carbon is 8.47, neon is 8.29, oxygen is 7.69 and iron is estimated at 7.62.