A chemical element is a pure substance that cannot be broken down into simpler substances by ordinary chemical processes. Each element is distinguished by its unique atomic number, which represents the precise count of protons within the nucleus of its atoms. Elements are the fundamental building blocks of all observable matter, existing in diverse forms throughout the universe. Their distribution is governed by the energetic history of the cosmos and the physical processes that shaped planetary bodies. Understanding where these materials are located reveals the deep connections between cosmic events, planetary geology, and the biology of life itself.
Elements in the Cosmos
The universe is dominated by the two lightest elements, Hydrogen and Helium, which together make up approximately 98% of all baryonic matter. These elements originated during the Big Bang nucleosynthesis, when the cosmos was hot and dense enough to allow the fusion of subatomic particles. Hydrogen is the simplest and most abundant element, serving as the fuel for stars across the galaxies.
Heavier elements were not present at the beginning of the universe but were forged later within stars. This process, known as stellar nucleosynthesis, occurs when the extreme pressure and temperature in a star’s core force lighter atomic nuclei to fuse into heavier ones. Stars convert Helium into Carbon, and then into progressively heavier elements like Oxygen and Neon.
Elements up to Iron are created through fusion within massive stars, but elements heavier than Iron require an even more violent event. These heaviest elements, such as Gold and Uranium, are primarily synthesized during the explosive death of massive stars in a supernova. The remnants of these explosions are scattered into the interstellar medium, providing the raw material for new stars, planets, and life.
Earth’s Elemental Composition
The elements found on Earth result from the cosmic recycling of matter, but their distribution is highly segregated. During formation, differentiation caused heavier elements to sink toward the center, while lighter elements rose to the surface. Consequently, the Earth’s core is predominantly an alloy of Iron (estimated at 85.5% by weight) and Nickel (about 5.2%), with the remainder consisting of lighter elements like Silicon, Sulfur, and Oxygen.
The mantle, the layer between the core and the crust, is composed mainly of dense, iron- and magnesium-rich silicate minerals. The Earth’s crust, the thin outer layer, is dominated by Oxygen and Silicon, which together account for roughly 75% of its mass. Oxygen is the most abundant element in the crust (about 46.6% by weight), followed by Silicon (27.7%).
The planet’s atmosphere is a separate reservoir, composed primarily of gases outgassed from the Earth’s interior and modified by biological processes. By volume, the atmosphere is overwhelmingly Nitrogen (about 78%), with Oxygen making up nearly 21%. This composition reflects the unique history of volatile elements and the profound impact of life.
Elements in Life
A select group of elements forms the structural and functional basis of all living organisms within the Earth’s biosphere. The six most abundant elements in biological matter are Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, and Sulfur (CHNOPS). These elements possess the chemical properties necessary for forming complex, stable, and reactive organic molecules.
Carbon is the foundational element, capable of forming four stable covalent bonds, which allows it to create the long chains and rings that serve as the backbones of all organic molecules, including carbohydrates, lipids, and proteins. Hydrogen and Oxygen form water, the solvent of life, and play roles in energy storage and transfer. Nitrogen is an essential constituent of amino acids and is also found in the nitrogenous bases of DNA and RNA.
Phosphorus is incorporated into the phosphate groups of the DNA backbone and is necessary for adenosine triphosphate (ATP), the primary energy currency of the cell. Sulfur is a component of certain amino acids, such as cysteine and methionine, and its ability to form disulfide bridges helps stabilize the complex three-dimensional shapes of proteins. These six elements are inextricably linked, forming a system that powers all known biological processes.
Elements That Don’t Exist Naturally
While 94 elements are found naturally on Earth, scientists have artificially created others that do not occur in nature. These synthetic elements, known as transuranic elements, have an atomic number greater than 92 (Uranium). They are all highly radioactive and possess half-lives much shorter than the age of the Earth.
These elements are created in specialized facilities, such as particle accelerators or nuclear reactors, by bombarding heavy target elements with high-speed nuclei. A heavy element target is struck with an accelerated beam of lighter ions, forcing their nuclei to fuse to form a heavier nucleus. This method is primarily used for scientific research, allowing physicists to explore the limits of the periodic table and the fundamental forces that hold matter together.