Most Abundant Elements: In the Universe, Earth, and Human Body

Abundance in the Universe

Hydrogen accounts for roughly 73 percent of all ordinary (baryonic) matter in the universe by mass, with helium making up about 25 percent. Together they constitute approximately 98 percent of all atomic matter. This composition is a direct legacy of Big Bang nucleosynthesis, which occurred in the first few minutes after the Big Bang when temperatures were high enough for nuclear fusion but cooling rapidly. The process produced hydrogen, helium, and trace amounts of lithium and beryllium, but nothing heavier.

All elements heavier than lithium were forged in stars. Stellar nucleosynthesis in main-sequence stars fuses hydrogen into helium, then helium into carbon and oxygen. More massive stars continue building heavier elements up to iron (element 26) through successive fusion stages. Iron is the endpoint of energy-releasing fusion because its nucleus has the highest binding energy per nucleon. Elements heavier than iron are produced primarily in supernova explosions and neutron star mergers, where extreme conditions drive rapid neutron capture (the r-process) that builds heavy nuclei up to uranium and beyond.

After hydrogen and helium, the most abundant elements in the universe by mass are oxygen, carbon, neon, iron, nitrogen, silicon, magnesium, and sulfur, roughly in that order. This ranking reflects stellar burning stages: carbon and oxygen are produced in enormous quantities by helium burning in all stars massive enough to reach that stage, while iron accumulates as the ash of silicon burning in the most massive stars.

Abundance in Earth's Crust

Earth's crustal composition is strikingly different from the cosmic average because geological processes have differentiated the planet. Oxygen is the most abundant element in the crust at 46 percent by mass, present in virtually every common mineral as oxide or silicate. Silicon is second at 28 percent, bonded to oxygen in the silicate minerals that form the vast majority of rocks. Together, oxygen and silicon account for nearly three quarters of the crust.

The eight most abundant crustal elements, in order, are: oxygen (46.1%), silicon (28.2%), aluminum (8.2%), iron (5.6%), calcium (4.1%), sodium (2.3%), magnesium (2.3%), and potassium (2.1%). These eight elements make up over 98 percent of the crust by mass. All are common components of silicate and aluminosilicate minerals, the feldspars, quartz, micas, and clays that compose most rocks.

Some elements abundant in the universe are rare in the crust. Hydrogen and helium, the cosmic champions, are minor crustal components because hydrogen mostly resides in the oceans (as water) and helium, being a noble gas, largely escapes to space. Neon, the fifth most abundant element cosmically, has virtually no crustal presence for the same reason. Conversely, aluminum, rare in the cosmos, is concentrated in the crust because its chemistry favors oxide and silicate minerals that float to the surface during planetary differentiation.

Abundance in Earth as a Whole

When considering the entire Earth (including core and mantle), iron dominates at roughly 32 percent by mass, concentrated in the dense metallic core. Oxygen is second at about 30 percent, mostly in mantle silicates. Silicon (15%), magnesium (14%), sulfur, nickel, calcium, and aluminum follow. The difference between whole-Earth and crustal composition reflects the gravitational separation that occurred early in Earth's history: dense iron and nickel sank to form the core, while lighter silicates rose to form the mantle and crust.

Abundance in the Oceans

Seawater is 96.5 percent water by mass, making hydrogen and oxygen the dominant elements. The dissolved solids are primarily sodium and chlorine (as dissolved sodium chloride), which together account for about 85 percent of all dissolved salts. Magnesium, sulfur, calcium, and potassium are the next most abundant dissolved elements. The ocean contains measurable quantities of virtually every naturally occurring element, though most are present only in parts per billion or less.

Some elements are enriched in seawater relative to their crustal abundance. Bromine, for example, is far more concentrated in the ocean than in rocks and was historically extracted from seawater. Others, like aluminum, are abundant in rocks but nearly insoluble in seawater and are found at extremely low oceanic concentrations.

Abundance in the Human Body

The human body composition reflects its aqueous, organic chemistry. By mass, the body is approximately 65 percent oxygen (mostly in water), 18 percent carbon (in organic molecules), 10 percent hydrogen (in water and organic molecules), 3 percent nitrogen (in proteins and nucleic acids), 1.5 percent calcium (in bones), and 1 percent phosphorus (in bones, DNA, and ATP). These six elements account for over 98 percent of body mass.

By atom count, the picture changes dramatically. Hydrogen atoms are by far the most numerous because water molecules (H2O) each contain two hydrogen atoms, and most organic molecules are rich in hydrogen. Roughly 60 percent of the atoms in the human body are hydrogen, followed by oxygen at 26 percent and carbon at 11 percent. Despite being only 0.2 percent of body mass, trace elements like iron, zinc, and iodine are biologically critical, as the essential elements guide explains.

Why Abundance Differs Across Scales

The cosmic abundance pattern is set by nuclear physics: which fusion reactions occur in stars and how much energy each releases. The crustal pattern is shaped by geochemistry: which elements form stable minerals at Earth's surface conditions and which were separated during planetary differentiation. The biological pattern is determined by biochemistry: which elements dissolve in water, form useful bonds, and were available in the environment where life evolved.

The element iron illustrates these differences well. Cosmically, iron is moderately abundant as the endpoint of stellar fusion. In Earth's crust, iron is the fourth most abundant element but represents a small fraction of the whole Earth's iron, most of which is in the core. In biology, iron is a trace element by mass but is absolutely essential for oxygen transport and energy metabolism. The same element plays fundamentally different roles at each scale because different processes govern its distribution.