The abundance of the chemical elements is
a measure of the occurrence of the chemical
elements relative to all other elements in
a given environment. Abundance is measured
in one of three ways: by the mass-fraction
(the same as weight fraction); by the mole-fraction
(fraction of atoms by numerical count, or
sometimes fraction of molecules in gases);
or by the 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.
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 33.3333...%
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 (atomic) 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 (fraction of total gas molecules),
as well as the fraction of atmosphere by volume,
of hydrogen to about 86%, and of helium to
13%.The abundance of chemical elements in
the universe is dominated by the large amounts
of hydrogen and helium which were produced
in the Big Bang. Remaining elements, making
up only about 2% of the universe, were largely
produced by supernovae and certain red giant
stars. Lithium, beryllium and boron are rare
because although they are produced by nuclear
fusion, they are then destroyed by other reactions
in the stars. 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
number than iron (element 26) 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.
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 (which volatilizes as methane).
The crust, mantle, and core of the Earth show
evidence of chemical segregation plus some
sequestration by density. Lighter silicates
of aluminum 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, or oceans, or the human body,
are primarily a product of chemical interactions
with the medium in which they reside.
== Universe ==
The elements – that is, ordinary (baryonic)
matter made of protons, neutrons, and electrons,
are only a small part of the content of the
Universe. Cosmological observations suggest
that only 4.6% of the universe's energy (including
the mass contributed by energy, E = mc² ↔ m
= E / c²) comprises the visible baryonic
matter that constitutes stars, planets, and
living beings. The rest is thought to be made
up of dark energy (68%) and dark matter (27%).
These are forms of matter and energy believed
to exist on the basis of scientific theory
and inductive reasoning based on observations,
but they have not been directly observed and
their nature is not well understood.
Most standard (baryonic) matter is found in
intergalactic gas, stars, and interstellar
clouds, in the form of atoms or ions (plasma),
although it can be found in degenerate forms
in extreme astrophysical settings, such as
the high densities inside white dwarfs and
neutron stars.
Hydrogen is the most abundant element in the
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 of stars.
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 or helium. This
distinction is significant because hydrogen
and helium are the only elements that were
produced in significant quantities in the
Big Bang. Thus, the metallicity of a galaxy
or other object is an indication of stellar
activity, after the Big Bang.
In general, elements up to iron are made in
large stars in the process of becoming supernovae.
Iron-56 is particularly common, since it is
the most stable element that can easily be
made from alpha particles (being a product
of decay of radioactive nickel-56, ultimately
made from 14 helium nuclei). Elements heavier
than iron are made in energy-absorbing processes
in large stars, and their abundance in the
universe (and on Earth) generally decreases
with increasing atomic number.
=== Solar system ===
The following graph (note log scale) shows
abundance of elements in the Solar System.
The table shows the twelve most common elements
in our galaxy (estimated spectroscopically),
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 uniform throughout the universe, however,
it is expected that these galaxies will likewise
have evolved similar abundances of elements.
The abundance of elements is in keeping with
their origin from the Big Bang and nucleosynthesis
in a number of progenitor supernova stars.
Very abundant hydrogen and helium are products
of the Big Bang, while the next three elements
are rare since they had little time to form
in the Big Bang and are not made in stars
(they are, however, produced in small quantities
by breakup of heavier elements in interstellar
dust, as a result of impact by cosmic rays).
Beginning with carbon, elements have been
produced in stars by buildup from alpha particles
(helium nuclei), resulting in an alternatingly
larger abundance of elements with even atomic
numbers (these are also more stable). 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.
=== 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 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 with 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 overall shape of the curve of nuclear
binding energy.
== Earth ==
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. In bulk, by mass, 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.The bulk composition of
the Earth by elemental-mass is roughly similar
to the gross composition of the solar system,
with the major differences being that Earth
is missing a great deal of the volatile elements
hydrogen, helium, neon, and nitrogen, as well
as carbon which has been lost as volatile
hydrocarbons. The remaining elemental composition
is roughly typical of the "rocky" inner planets,
which formed in the thermal zone where solar
heat drove volatile compounds into space.
The Earth retains oxygen as the second-largest
component of its mass (and largest atomic-fraction),
mainly from this element being retained in
silicate minerals which have a very high melting
point and low vapor pressure.
=== Crust ===
The mass-abundance of the nine most abundant
elements in the Earth's crust is approximately:
oxygen 46%, silicon 28%, aluminum 8.2%, iron
5.6%, calcium 4.2%, sodium 2.5%, magnesium
2.4%, potassium 2.0%, and titanium 0.61%.
Other elements occur at less than 0.15%. For
a complete list, see abundance of elements
in Earth's crust.
The graph at right illustrates the relative
atomic-abundance of the chemical elements
in Earth's upper continental crust— the
part that is relatively accessible for measurements
and estimation.
Many of the elements shown in the graph are
classified into (partially overlapping) categories:
rock-forming elements (major elements in green
field, and minor elements in light green field);
rare earth elements (lanthanides, La-Lu, and
Y; labeled in blue);
major industrial metals (global production
>~3×107 kg/year; labeled in red);
precious metals (labeled in purple);
the nine rarest "metals" — the six platinum
group elements plus Au, Re, and Te (a metalloid)
— in the yellow field. These are rare in
the crust from being soluble in iron and thus
concentrated in the Earth's core. Tellurium
is the single most depleted element in the
silicate Earth relative to cosmic abundance,
because in addition to being concentrated
as dense chalcogenides in the core it was
severely depleted by preaccretional sorting
in the nebula as volatile hydrogen telluride.Note
that there are two breaks where the unstable
(radioactive) elements technetium (atomic
number 43) and promethium (atomic number 61)
would be. These elements are surrounded by
stable elements, yet both have relatively
short half lives (~ 4 million years and ~ 18
years respectively). These are thus extremely
rare, since any primordial initial fractions
of these in pre-Solar System materials have
long since decayed. These two elements are
now only produced naturally 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 technetium
and promethium have been identified spectroscopically
in the atmospheres of stars, where they are
produced by ongoing nucleosynthetic processes.
There are also breaks in the abundance graph
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 in the crust,
and are therefore extremely rare there.
The eight naturally occurring very rare, highly
radioactive elements (polonium, astatine,
francium, radium, actinium, protactinium,
neptunium, and plutonium) 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 and is only produced from the radioactive
decay of uranium and thorium.
Oxygen and silicon are notably the most common
elements in the crust. On Earth and in rocky
planets in general, silicon and oxygen are
far more common than their cosmic abundance.
The reason is that they combine with each
other to form silicate minerals. In this way,
they are the lightest of all of the two-percent
"astronomical metals" (i.e., non-hydrogen
and helium elements) to form a solid that
is refractory to the Sun's heat, and thus
cannot boil away into space. All elements
lighter than oxygen have been removed from
the crust in this way, as have the heavier
chalcogens sulfur, selenium and tellurium.
==== Rare-earth elements ====
"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 similarly concentrated
in the crust compared 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.
=== Mantle ===
The mass-abundance of the eight most abundant
elements in the Earth's mantle (see main article
above) is approximately: oxygen 45%, magnesium
23%, silicon 22%, iron 5.8%, calcium 2.3%,
aluminum 2.2%, sodium 0.3%, potassium 0.3%.
The mantle differs in elemental composition
from the crust in having a great deal more
magnesium and significantly more iron, while
having much less aluminum and sodium.
=== Core ===
Due to mass segregation, the core of the Earth
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.
=== Ocean ===
The most abundant elements in the ocean by
proportion of mass in percent are oxygen (85.84),
hydrogen (10.82), chlorine (1.94), sodium
(1.08), magnesium (0.1292), sulfur (0.091),
calcium (0.04), potassium (0.04), bromine
(0.0067), carbon (0.0028), and boron (0.00043).
=== Atmosphere ===
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.
=== Urban soils ===
For a complete list of the abundance of elements
in urban soils, see Abundances of the elements
(data page)#Urban soils.
== 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: hydrogen (H), carbon (C),
nitrogen (N), oxygen (O), calcium (Ca), and
phosphorus (P). The next 0.75% is made up
of the next five elements: potassium (K),
sulfur (S), chlorine (Cl), sodium (Na), and
magnesium (Mg). CHNOPS for short. Only 17
elements are known for certain to be necessary
to human life, with one additional element
(fluorine) thought to be helpful for tooth
enamel strength. A few more trace elements
may play some role in the health of mammals.
Boron and silicon are notably necessary for
plants but have uncertain roles in animals.
The elements aluminium and silicon, although
very common in the earth's crust, are conspicuously
rare in the human body.Below is a periodic
table highlighting nutritional elements.
== See also ==
Abundances of the elements (data page)
Abundance of elements in Earth's crust
Natural abundance (isotopic abundance)
Goldschmidt classification
Primordial nuclide
List of data references for chemical elements
