The most abundant molecule on Earth is water, chemically represented as \(\text{H}_2\text{O}\). This molecule is composed of two hydrogen atoms and one oxygen atom. Water covers roughly 71 percent of the planet’s surface and is distributed across all of Earth’s major spheres, from the deep oceans to the atmosphere. Its sheer volume and unique chemical characteristics secure its position as the single most plentiful molecular species on the planet.
Identifying the Most Abundant Molecule
The designation of water as the most abundant molecule requires a clear distinction between a molecule and an element. An element is a pure substance made up of only one kind of atom. A molecule is an electrically neutral group of two or more atoms held together by chemical bonds, which may be atoms of the same element or different elements.
Many people assume the most abundant substance is the element oxygen, which makes up approximately 46 percent of the Earth’s crust by mass. Others might point to diatomic nitrogen (\(\text{N}_2\)), which comprises about 78 percent of the atmosphere by volume. However, these are either an element or are only the most abundant within one specific Earth sphere.
Water, a compound molecule, consists of two different elements chemically bonded together. The overall mass of \(\text{H}_2\text{O}\) distributed across the hydrosphere, cryosphere, and biosphere vastly outweighs any other single molecular species. The sheer scale of the global ocean and ice reserves solidifies water’s abundance, making it the definitive answer when quantifying the most common molecule on a planetary scale.
Global Reservoirs and Distribution
The vast majority of the Earth’s water is stored in its oceans, which constitute the largest reservoir of \(\text{H}_2\text{O}\). These oceans and marginal seas hold approximately 97.2 percent of all the water on the planet. This enormous volume is saline, containing a dissolved salt concentration of around 3.5 percent.
The remaining 2.8 percent is freshwater, which is highly unevenly distributed and largely inaccessible. The distribution of global water resources is as follows:
- Oceans and marginal seas: 97.2%
- Glaciers, ice caps, and permanent snow cover: 2.15%
- Groundwater (stored in aquifers and soil moisture): 0.61%
- Lakes and rivers (surface water features): 0.017%
- Atmosphere (primarily water vapor): 0.001%
The majority of this freshwater is locked away in ice, primarily at the poles. Groundwater is the next largest freshwater reservoir and a critical source for human use and ecosystems. This distribution highlights that while water is the most abundant molecule, the readily usable liquid freshwater portion is extremely scarce.
Fundamental Molecular Properties
Water’s abundance and stability in three phases—solid, liquid, and gas—are a direct result of its unique molecular structure. The water molecule adopts a bent shape because the oxygen atom holds electrons more strongly than the two hydrogen atoms. This uneven sharing of electrons creates a polar molecule, giving the oxygen side a slight negative charge and the hydrogen sides slight positive charges.
This polarity enables water molecules to form weak attractions with one another, known as hydrogen bonds. The need to break these numerous hydrogen bonds gives water a high specific heat capacity. This means water can absorb or release large amounts of thermal energy with only a small change in its own temperature.
This property allows the vast oceans to act as a global thermostat, regulating the Earth’s climate and preventing extreme temperature fluctuations. Furthermore, water’s polarity allows it to dissolve a wide array of other polar and ionic substances, earning it the moniker of “universal solvent.”
Hydrogen bonding also explains why solid water, or ice, is less dense than liquid water. As water freezes, the hydrogen bonds lock the molecules into a rigid, open crystalline lattice structure. This structure occupies more space than the liquid state, causing ice to float. This unusual property allows aquatic life to survive winter beneath frozen surfaces.