2.1.5 Abundance of the Elements

Overview: What “Abundance” Means

When we talk about the abundance of the elements, we are asking:

• How much of each element exists:
• in the observable universe,
• in our galaxy and the Solar System,
• on Earth as a whole,
• and at the Earth’s surface (crust, oceans, atmosphere, biosphere)?
• In what forms (chemical and physical) those elements typically occur?

Here, “abundance” is usually expressed as:

• a fraction of the total amount (e.g. mass percent, atom percent),
• or relative to a reference element (often hydrogen or silicon).

For beginners, you can think of abundance charts as the “inventory lists” of nature’s elements: which ones are common, which are rare, and roughly by how much.

Cosmic Abundances: The Big Picture

Dominance of Hydrogen and Helium

On the scale of the observable universe (by mass):

• Hydrogen: roughly $\sim 74\%$
• Helium: roughly $\sim 24\%$
• All heavier elements together (“metals” in astronomical language): only a few percent

So, by far:

• most atoms are hydrogen atoms,
• most of the rest are helium,
• everything else is relatively scarce.

These global abundances reflect the earliest stages of element formation and later nuclear processes in stars (treated in detail in the chapters on the Origin of the Elements and Nuclear Synthesis).

The “Odd–Even” Pattern and Peaks

If you make a bar chart of element abundance (e.g. in the Solar System) as a function of atomic number $Z$, you see several striking features:

• Even–odd effect:
• Elements with even atomic numbers (e.g. C, O, Ne, Mg, Si, S, Fe) tend to be more abundant than their odd-$Z$ neighbors (e.g. N, F, Na, Al, P).
• This relates to nuclear stability: nuclei with even numbers of both protons and neutrons are typically more stable, and thus formed more frequently.
• Several abundance peaks:
• Light elements like H and He are very abundant.
• Intermediate-mass elements around carbon (C), oxygen (O), neon (Ne), magnesium (Mg), and silicon (Si) are also relatively abundant.
• Iron (Fe) and nearby elements (Cr, Ni) form a pronounced peak: these nuclei are particularly stable, so they accumulate in stellar processes.
• Drop-off at high $Z$:
• For elements much heavier than iron (e.g. Au, Pb, U), abundances generally decrease with increasing atomic number.
• These elements are produced in more specialized and less frequent nucleosynthesis processes, so less total mass of them is created.

Why Some Elements Are Very Rare

A few notable features:

• Lithium, beryllium, boron (Li, Be, B):
• Surprisingly low abundances compared to their neighbors on the periodic table.
• Their nuclei are relatively fragile in stellar interiors and are easily destroyed (“burned”), so they are not strongly built up.
• Technetium (Tc) and promethium (Pm):
• Have no stable isotopes.
• In nature, they are essentially absent in bulk, appearing only in trace amounts due to ongoing nuclear reactions.
• Their scarcity emphasizes the role of nuclear stability in abundance.
• The radioelements (e.g. Th, U):
• Present in small but measurable amounts on Earth.
• Their current abundances are the residues of once much larger initial amounts, reduced by radioactive decay over billions of years.

Solar System Abundances

Astronomers often use the Sun as a reference, because:

• The Sun contains >99% of the mass of the Solar System.
• Its composition broadly reflects the “average” composition of the material from which the Solar System formed.

Abundance Scale in Astronomy

A common way to express elemental abundances in stars and the Solar System is the “logarithmic scale” relative to hydrogen:

$$
\log_{10} \left(\frac{N(\text{element})}{N(\text{H})} \right) + 12
$$

• Here $N(\text{element})$ is the number of atoms of that element.
• Hydrogen is defined to have a value of 12.00 on this scale by convention.
• For example, if oxygen has value 8.7, then:
  $$
  \frac{N(\text{O})}{N(\text{H})} = 10^{8.7 - 12} = 10^{-3.3}
  $$
  so there is about 1 oxygen atom per 00$ hydrogen atoms.

You do not need to master this scale in detail at this beginner level; it is primarily to understand how astronomers compare abundances.

Main Features of Solar System Abundances

• H and He dominate by both mass and atom count.
• Among heavier elements (“metals”):
• C, N, O are relatively abundant “light metals”.
• Ne, Mg, Si, S, Fe are also abundant and critical for planetary and rocky material.
• Elements with atomic numbers around Fe are relatively common; heavier elements drop off progressively.

These distributions connect directly to the nuclear processes in stars, supernovae, and other astrophysical sites, which are dealt with in detail in the nucleosynthesis chapters.

Element Abundances on Earth

The composition of Earth is not the same as the Solar System average. Processes like condensation, differentiation (core–mantle separation), and volatile loss have shaped Earth’s elemental distribution.

We consider three main “levels”:

• The whole Earth (bulk)
• The Earth’s crust (surface rock)
• Oceans and atmosphere

Each tells a different story about the behavior of elements in geological and geochemical processes.

Bulk Earth and the Core–Mantle Contrast

Indirect evidence (from seismic data, density, and meteorite compositions) suggests that, by mass, the Earth is dominated by:

• Iron (Fe) – especially in the core
• Oxygen (O)
• Silicon (Si)
• Magnesium (Mg)

Siderophile vs Lithophile Elements

Earth’s differentiation into core and mantle led to a chemical partition:

• Siderophile (“iron-loving”) elements:
• Example: Fe, Ni, many platinum-group metals (Pt, Ir, Pd), and some others.
• Prefer metallic iron and therefore concentrate in the core.
• Their abundance at the surface (crust) is much lower than in the bulk Earth.
• Lithophile (“rock-loving”) elements:
• Example: O, Si, Al, Ca, Na, K, Mg, Ti.
• Prefer oxygen-rich silicate minerals and dominate the crust and mantle.

This explains why the Earth’s surface rocks are rich in silicate minerals (composed largely of O, Si, Al, Mg, Fe, Ca, Na, K), while most of the planet’s iron is hidden in the core.

Abundance in the Earth’s Crust

In the continental crust, by mass:

• Oxygen and silicon together make up the majority (over 70%) of the crust’s mass.
• Other abundant elements include Al, Fe, Ca, Na, K, and Mg.

Approximate ranking (mass %):

• O, Si (dominant)
• Al, Fe
• Ca, Na, K, Mg
• Then smaller amounts of Ti, P, Mn, etc.

This abundance pattern is closely tied to:

• the dominance of silicate minerals (built from $ \text{SiO}_4^{4-} $ units),
• the conditions under which the crust formed and evolved.

Elements like Au (gold), Pt (platinum), and rare earth elements have very low crustal abundances, which is a major reason for their high economic value.

Atmosphere and Hydrosphere

Different chemical and physical properties cause very different dominant elements in air and water:

Atmosphere

The main components (by volume) of Earth’s dry atmosphere:

• Nitrogen ($\text{N}_2$) ~78%
• Oxygen ($\text{O}_2$) ~21%
• Argon (Ar) ~0.9%
• Carbon dioxide ($\text{CO}_2$) ~0.04% (but very important climatically)

So, in terms of elemental abundance in the atmosphere, N and O dominate, with Ar (a noble gas) as a significant minor component. Other gases (Ne, He, CH$_4$, etc.) are present as traces.

Oceans (Hydrosphere)

Sea water is mostly:

• Hydrogen and oxygen (in H$_2$O) by atom count.
• If you look at dissolved ions:
• Na$^+$ and Cl$^-$ are the most abundant (hence the salty taste),
• followed by Mg$^{2+}$, SO$_4^{2-}$, Ca$^{2+}$, K$^+$.

This reflects the long-term interaction between water and rocks, volcanic inputs, and the solubility and mobility of different elements.

Biosphere (Living Matter)

Living organisms are made up mostly of a relatively small set of elements:

• Major biogenic elements:
• C, H, O, N (the overwhelming majority of biomass)
• Important macronutrients:
• P, S, K, Ca, Mg, Na, Cl
• Trace elements (micronutrients):
• Fe, Zn, Cu, Mn, Se, I, Mo, Co, etc.

Many elements that are rare in the crust or ocean can still be essential in tiny amounts for biochemical processes. For example:

• Iron (Fe) is crucial in hemoglobin, despite being a minor constituent of the whole body by mass.
• Iodine (I) is required in very small quantities for thyroid hormones.

Thus, “abundance in the biosphere” is not simply a copy of crustal or oceanic abundance; it is strongly shaped by biological selection and function.

Patterns and Their Implications

Geochemical Behavior and Element Distribution

Element abundances in different Earth reservoirs (core, mantle, crust, oceans, atmosphere, biosphere) are governed by:

• Chemical affinity (who bonds with whom):
• Metals that form stable oxides or silicates (lithophile) vs. metals that prefer metallic or sulfide phases (siderophile, chalcophile).
• Volatility:
• Elements with low boiling/condensation temperatures (e.g. noble gases, some light elements) can be lost more easily from planetary surfaces or atmospheres.
• Solubility and complex formation:
• Determine how elements move into, and remain in, water or biological systems.

This is why the same element can be abundant in one reservoir but rare in another.

Abundance and Resource Availability

From a human perspective:

• Common crustal elements (e.g. Fe, Al, Si, Ca) are used in massive quantities for construction and industry.
• Rare elements (e.g. Au, Pt, rare earth elements) are critical in high-technology applications but found only in low concentrations.
• Extraction and refining of very rare elements can be resource-intensive and environmentally significant, even if only small masses are used.

Understanding where elements are abundant (and in what chemical forms) is central to:

• mining and resource geology,
• materials science,
• environmental impact assessments.

Measuring Elemental Abundances

Determining abundances is an analytical and observational task. Methods include:

• In astronomy:
• Spectroscopic analysis of light from stars, gas clouds, and galaxies.
• Comparing observed spectral line strengths with models to infer relative abundances.
• In geology and geochemistry:
• Classical and instrumental chemical analysis of rocks, soils, and waters.
• Techniques such as:
• Mass spectrometry (e.g. ICP-MS),
• X-ray fluorescence (XRF),
• Neutron activation analysis, and others (detailed in the analytical methods chapters).
• In environmental and biological contexts:
• Chemical analysis of air, water, tissues, and organisms to determine trace and major element concentrations.

These measurements, collected across many environments and over time, are combined to build the abundance tables and charts used in chemistry, geology, astronomy, and environmental science.

Linking Abundance to Nuclear Origins

Even though the nuclear formation of elements is dealt with elsewhere, it is useful to remember the broad connection:

• The cosmic abundance pattern (e.g. dominance of H and He, peaks near C/O and Fe, odd–even effects, rarity of Li–Be–B, scarcity of very heavy elements) reflects:
• the conditions and pathways of nucleosynthesis in the early universe and in stars,
• the relative stability of certain nuclei.
• The Solar System and Earth-specific patterns add additional layers of sorting:
• Condensation of solids from a cooling gas,
• Planet formation,
• Planetary differentiation (core–mantle–crust),
• Chemical weathering, erosion, and sedimentation,
• Biological uptake and concentration.

Abundance, therefore, is the visible outcome of both nuclear processes and later chemical and physical processes acting over billions of years.

Summary

• Hydrogen and helium dominate the universe; heavier elements are present in much smaller amounts, with characteristic peaks and patterns.
• Solar System abundances largely mirror cosmic ones, with H, He, C, O, Ne, Mg, Si, S, and Fe being especially important.
• On Earth, abundances differ significantly from cosmic averages due to planetary differentiation and geochemical behavior:
• Iron and other siderophiles concentrate in the core.
• Oxygen, silicon, and other lithophiles dominate the crust.
• The atmosphere and oceans have compositions controlled by volatility, solubility, and surface processes.
• The biosphere selectively uses a subset of elements in specific proportions, emphasizing function over crustal abundance.
• Measuring abundances relies on a variety of spectroscopic and analytical techniques, and the observed patterns are key evidence for the nuclear and geochemical histories of matter.