A chemical element is a pure substance composed entirely of atoms that share the same number of protons in their nucleus. This proton count, known as the atomic number, is the fundamental property that defines an element and differentiates it from all others. Although atoms of the same element can have a varying number of neutrons, creating different isotopes, the number of protons remains constant. Scientists organize the 118 currently known elements into the Periodic Table, which arranges them by increasing atomic number and recurring chemical properties.
How Scientists Define Abundance
The definition of a “common” element is not fixed, as it depends on the environment being measured. Elemental abundance is most commonly expressed as a mass fraction, the percentage of the total mass that a specific element accounts for in a given sample. This measurement can be applied to different scales, such as the entire cosmos, the Earth’s crust, or a living organism. Because of the different physical and chemical processes at work, the most abundant elements vary dramatically across these contexts.
The Most Common Elements in the Universe and Earth
The universe’s elemental composition is dominated by the two lightest elements, Hydrogen (H) and Helium (He). Hydrogen accounts for approximately 74% of the universe’s mass, while Helium makes up about 23%. These two elements were primarily formed after the Big Bang, and all other elements represent only a small fraction of the total cosmic mass.
The composition of our planet, however, is significantly different due to gravitational sorting and planetary formation processes that allow lighter gases to escape. Within the Earth’s crust, Oxygen (O) is the most common element by mass, constituting about 46%. Oxygen’s high reactivity means it rarely exists alone, instead combining with other elements to form mineral compounds called oxides and silicates.
Silicon (Si) is the second most abundant element in the crust, making up about 28% of the mass. Silicon’s ability to bond readily with oxygen forms the silicate minerals, which are the primary building blocks of more than 90% of the Earth’s crust. Aluminum (Al) and Iron (Fe) follow in abundance, accounting for a significant portion of the remaining crustal mass. Iron is particularly concentrated in the Earth’s core, though it remains a top four element in the crust, often found in iron oxide minerals.
Elements Essential for Human Life
Moving to the biological scale, the elements considered common shift to those that form the molecules of life. Four elements, often referred to as the “Big Four,” make up approximately 96% of the mass of the human body: Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N). Oxygen is the most abundant, at around 65% of body mass, largely because the body is composed mostly of water (\(\text{H}_2\text{O}\)).
Carbon, the second most abundant element in the body, is the structural foundation for all organic molecules, including carbohydrates, lipids, proteins, and nucleic acids. Its ability to form four stable bonds allows it to create the long chains and complex ring structures necessary for biological function. Hydrogen and Oxygen also play roles in energy metabolism and are components of nearly every biological molecule.
Nitrogen is a fundamental component of amino acids, the subunits of proteins, and the nitrogenous bases that form the structure of DNA and RNA. Two other elements, Phosphorus (P) and Sulfur (S), are also considered essential for life, often grouped with the Big Four as CHNOPS. Phosphorus is a part of the backbone structure of DNA and is found in adenosine triphosphate (ATP), the primary energy currency of the cell. Sulfur is incorporated into certain amino acids, where it helps proteins maintain their complex three-dimensional shapes through the formation of disulfide bridges.
Why Elements Combine to Form Compounds
Elements combine to form compounds because atoms naturally seek a state of lower energy and greater stability. For most atoms, stability is achieved by completing their outermost electron shell, typically by acquiring eight valence electrons. Atoms without a full outer shell are chemically reactive and interact with other atoms to achieve this stable configuration.
This interaction results in the formation of chemical bonds, which hold the atoms together in a fixed ratio, creating a compound. One way this happens is through ionic bonding, where one atom fully transfers electrons to another, resulting in oppositely charged ions that are strongly attracted. Alternatively, atoms can share electrons to achieve a full shell, forming a covalent bond, which is the basis of most organic molecules. This drive for stability explains why pure, highly reactive elements are rarely found in nature and why virtually all matter around us exists as compounds.