The fundamental inquiry into the nature of existence begins with the chemical elements, the universal building blocks of matter. These elements comprise everything from interstellar dust to the complex machinery of living cells. While ancient philosophy proposed that all matter consisted of Earth, Air, Fire, and Water, modern science defines a set of unique elements that assemble to create the physical world. Understanding life requires focusing on a select group of elements that form the intricate structures of biology.
Defining the Scientific Element
A chemical element is defined by the number of protons contained within the nucleus of its atoms. This count, known as the atomic number, is a unique identifier; changing the number of protons instantly changes the element itself. For example, every atom of carbon possesses six protons, setting it apart from the seven protons found in nitrogen or the eight in oxygen. All known elements are systematically organized on the Periodic Table based on their atomic number and recurring chemical properties.
The number of neutrons within an atom can vary without altering the element’s identity, resulting in different forms called isotopes. Isotopes of the same element have nearly identical chemical behavior because the number of protons and electrons remains unchanged. However, their masses differ due to the varying neutron count. For instance, carbon-12 has six neutrons, while the less common carbon-14 isotope contains eight neutrons.
The Four Pillars of Biological Life
Of the many elements in the universe, only a small subset serves as the structural foundation for all known life on Earth. These four elements—Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N)—are collectively known as CHON. They account for approximately 96% of the mass of all living organisms due to their unique chemical suitability for forming complex, stable molecules.
Carbon is the primary structural element of organic life, owing to its ability to form four stable chemical bonds with other atoms, including itself. This tetravalency allows carbon to link into long, diverse chains and rings. This creates the structural framework for large molecules like carbohydrates, lipids, and proteins.
Hydrogen and Oxygen are omnipresent, primarily because life is composed largely of water ($\text{H}_2\text{O}$). Oxygen also plays a role in energy production as the final electron acceptor in cellular respiration. Nitrogen is integrated into the structure of amino acids, the building blocks of proteins. It is also a fundamental part of the nitrogenous bases that form the structure of nucleic acids, DNA and RNA.
Abundance in Earth’s Environment
While the CHON elements dominate living matter, a different set of elements defines the non-living environment of our planet. The Earth’s crust is predominantly composed of Oxygen and Silicon, which together constitute about 74% of the crust’s mass. Oxygen is the most abundant element in the crust, making up about 46% by mass, typically found bound in minerals like silicates and oxides. Silicon is the second most common element, readily combining with oxygen to form the silicate minerals that make up more than 90% of the crust.
Elements such as Aluminum (about 8%) and Iron (about 5%) are also highly abundant in the crust, illustrating the difference between geological and biological composition. Conversely, the Earth’s atmosphere is dominated by Nitrogen (approximately 78% of the gaseous volume), followed by Oxygen (around 21%). This atmospheric nitrogen exists mostly as an inert gas ($\text{N}_2$), contrasting sharply with the chemically reactive forms required for biological processes.
How Elements Form Complex Structures
The complexity of life and the environment arises from the way elements bond together to form molecules and compounds. Chemical bonds represent the mechanisms by which atoms achieve a more stable electron configuration. The two primary types of bonds that govern all chemical structures are covalent and ionic bonds.
Covalent bonds are formed when atoms share electrons, a mechanism prevalent between nonmetals, such as the CHON elements. The sharing of electrons allows carbon and its partners to create stable, complex organic molecules. For example, in water ($\text{H}_2\text{O}$), oxygen shares electrons with two hydrogen atoms. This sharing is the foundation of all biological macromolecules.
Ionic bonds involve the transfer of electrons from one atom to another, typically between a metal and a nonmetal. This transfer creates oppositely charged ions—a positively charged cation and a negatively charged anion—held together by strong electrostatic attraction. Common table salt, sodium chloride (NaCl), is a simple example of an ionic compound.