# Abundance of the chemical elements The abundance of the chemical elements is the census of matter: how much of each [[Chemical_element]] exists in a given reservoir, whether the universe, the [[Sun]], Earth's [[Crust_(geology)|crust]], or a human body. Its headline is lopsided — [[Hydrogen]] and [[Helium]] together are ~98% of ordinary matter by mass — and its fine structure is a fossil record of [[Nucleosynthesis]]: every bump and gap in the abundance curve maps onto a nuclear process, from the first minutes of the universe to neutron-star collisions. Read correctly, the table explains why [[Oxygen]] and [[Silicon]] dominate rock, why [[Iron]] sits at a peak, why [[Gold]] is scarce and [[Lithium]] scarcer than it should be, and why helium — second most abundant substance in the cosmos — has to be mined from [[Natural_gas]] wells on Earth. ## Three ledgers that disagree Abundance depends on where you count and in what units (mass fraction versus [[Atom|atom]] fraction — hydrogen wins any count by atoms). By mass, the universe runs roughly H 74%, He 24%, with everything else — "metals," in astronomers' dismissive shorthand — near 2%, led by [[Oxygen]] (~1%) and [[Carbon]] (~0.5%). Earth's crust inverts the list: O ≈ 46%, [[Silicon]] 28%, [[Aluminium]] 8.2%, [[Iron]] 5.6%, [[Calcium]] 4.2%, [[Sodium]] 2.4%, [[Magnesium]] 2.3%, [[Potassium]] 2.1% — oxygen bound into silicate frameworks ([[Silicon_dioxide]] and its relatives), hydrogen a trace. Living tissue keeps different books again: O 65%, C 18%, H 10%, [[Nitrogen]] 3% by mass, because life is mostly [[Water]] plus carbon chemistry. None of the three ledgers contradicts the others; they differ because gravity, chemistry, and biology each sort the same primordial stock by different rules. ## Why hydrogen and helium own the universe The opening inventory was fixed in the first ~20 minutes: as the hot early universe expanded and cooled, essentially all surviving neutrons were locked into ⁴He, yielding ~75% H and ~25% He by mass, plus traces of deuterium, [[Helium-3]], and ⁷[[Lithium|Li]] — and nothing heavier, because no stable nucleus of mass 5 or 8 exists to bridge the gap. That bottleneck stalled element-building for the ~100 million years until stars formed, where the [[Triple-alpha_process]] finally vaults it by fusing three ⁴He into [[Carbon]]. The primordial ratio is still legible today: helium was identified in the solar spectrum in 1868, 27 years before it was isolated on Earth (1895), and radio astronomy maps galactic hydrogen through the 21-cm [[Hydrogen_line]] emitted across the [[Interstellar_medium]]. Stars have since converted only a few percent of the hydrogen stock — the universe remains, materially, a hydrogen phenomenon, which is also why [[Nuclear_fusion]] of hydrogen is the energy source that lights everything visible. ## The sawtooth and the iron peak Plot abundance against [[Atomic_number]] and three features leap out. First, a steep overall decline: each rung of stellar burning processes less material. Second, a sawtooth — even-Z elements are systematically ~10× more abundant than their odd-Z neighbors (the Oddo–Harkins rule, 1914–17), because proton pairing adds [[Binding_energy]] and because the alpha-ladder of stellar fusion deposits matter at even Z ([[Carbon]], [[Oxygen]], [[Neon]], [[Magnesium]], [[Silicon]]… [[Calcium]]). Third, a peak at [[Iron]]: binding energy per nucleon maxes out near ⁵⁶Fe (~8.8 MeV), so fusion is exothermic only up to the iron group ([[Chromium]], [[Manganese]], Fe, [[Cobalt]], [[Nickel]]) and stars pile mass there before collapse. Two anomalies complete the fingerprint. The [[Lithium]]–[[Beryllium]]–[[Boron]] trio sits in a deep valley — fragile nuclei burned inside stars, replenished mainly when cosmic rays shatter heavier nuclei in flight. And everything beyond iron owes its existence to neutron capture: the slow s-process in giant stars, the rapid r-process in supernovae and neutron-star mergers — the 2017 kilonova observation showed such collisions minting heavy elements at planetary-mass scale, settling where much of the [[Gold]], [[Platinum]], and [[Uranium]] budget comes from. ## Planetary sorting: why Earth's list differs Earth is a residue, not a sample. Forming close to the young [[Sun]], too small and too warm to hold the light gases against thermal escape and early solar wind, it kept refractories and lost volatiles — hence a rocky planet poor in H and He while [[Jupiter]] kept the cosmic ratio nearly intact. Differentiation then sorted the remainder by [[Density]] and chemical affinity: molten [[Iron]] sank to the core, dragging metal-loving elements ([[Nickel]], [[Gold]], [[Platinum]]) with it — one reason crustal gold is rare and why some accessible precious metal likely arrived in later impacts. The [[Atmosphere_of_Earth]] is a third-generation artifact: [[Nitrogen]] 78%, [[Oxygen]] 21% (entirely biological, the exhaust of the [[Oxygen-evolving_complex]] running photosynthesis for ~2.4 billion years), and [[Argon]] 0.9% that is almost all radiogenic ⁴⁰Ar from [[Potassium|potassium-40]] [[Radioactive_decay|decay]]. [[Helium]] tells the sorting story in miniature: primordial helium escaped long ago, so today's supply is [[Alpha_particle]] ash from [[Uranium]] and [[Thorium]], accumulating in gas traps — the deposits behind [[Helium_production_in_the_United_States]] and the [[National_Helium_Reserve]]. ## Using the curve The abundance table is an engineering document. It explains ore economics — [[Iron_ore]] is mined at gigatonne scale because iron is genuinely common, while "rare earths" are misnamed ([[Cerium]] is about as abundant in the crust as [[Copper]]; the problem is concentration, not existence). It sets resource frontiers: [[Helium-3]] at ~1 ppm of already-scarce terrestrial helium motivates studies of solar-wind-loaded lunar [[Regolith]] and [[In_situ_resource_utilization|in-situ extraction]] ([[Lunar_resources]]). And it is the calibration target for all of nuclear astrophysics: any model of [[Nucleosynthesis]] must reproduce the solar curve isotope by isotope, which is why measured [[Natural_abundance|isotopic abundances]] and the inventory of [[Primordial_nuclide|primordial nuclides]] function as the fossil record of the processes that built every [[Atom]] in the room. **On the spine:** [[Hydrogen]] · [[Helium]] · [[Nucleosynthesis]] · [[Crust_(geology)]] · [[Natural_abundance]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements) : [Wikitube](https://en.wikitube.io/wiki/Abundance_of_the_chemical_elements) ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Hydrogen]], [[PORTAL_Oxygen]], [[PORTAL_Helium]]. --- *Repopulated 2026-08-12 · redlink fill · 0 deletions.*