The element oxygen, represented by the symbol O and atomic number 8, is fundamental to life as we know it. We breathe it every second, yet its presence extends far beyond the air that is currently sustaining us. This highly reactive nonmetal forms compounds with nearly all other elements, making its story a fascinating blend of chemistry, planetary history, and surprising physical properties.
An Element of Abundance
Oxygen is not only prevalent in the atmosphere but is one of the most abundant elements on Earth. By mass, oxygen accounts for approximately 46.6% of the Earth’s crust, primarily locked up in silicate minerals and other compounds. This means that nearly half of the solid rock beneath our feet is composed of oxygen atoms bonded to other elements like silicon and aluminum.
The element is also the most abundant in the human body by mass, making up about 65% of an average person’s weight. Most of this immense proportion is contained within water molecules, which compose the majority of the human body. This surprising statistic shows that oxygen’s presence is structural, extending far beyond the 21% of the gas that we inhale with every breath.
The Race to Discover Oxygen
Several scientists isolated the gas independently in the 18th century. Swedish pharmacist Carl Wilhelm Scheele first isolated the gas in 1772 or earlier, calling it “fire air” because of its ability to promote combustion. However, Scheele’s work was not published until 1777, several years after his independent discovery.
English chemist Joseph Priestley isolated the same gas in 1774 by heating mercuric oxide and referred to it as “dephlogisticated air”. Priestley was the first to publish his findings, securing him priority for the discovery in many historical accounts.
It was French chemist Antoine Lavoisier who ultimately recognized the gas as a distinct chemical element and correctly identified its role in combustion. Lavoisier is credited with naming the element “oxygen” in 1777, combining Greek words that meant “acid-former,” based on his incorrect belief that it was a component of all acids.
The Poison That Changed the Planet
The atmosphere contained almost no free oxygen, and early life forms were anaerobic, thriving in its absence. Approximately 2.4 billion years ago, photosynthetic cyanobacteria started generating oxygen as a waste product of photosynthesis, releasing it first into the oceans and eventually into the atmosphere.
This buildup of atmospheric oxygen is known as the Great Oxygenation Event (GOE), and it triggered the planet’s first major extinction event. For the dominant anaerobic life forms, the accumulating oxygen was a powerful toxin, leading to a biological catastrophe. The GOE fundamentally altered the planet’s geochemistry, paving the way for the evolution of organisms that could eventually utilize oxygen for respiration.
The Magnetic Nature of Liquid O2
Oxygen gas (O2) possesses a unique physical property called paramagnetism. Unlike most gases, which are diamagnetic and slightly repelled by magnetic fields, molecular oxygen has two unpaired electrons in its outer orbitals. These unpaired electrons give the molecule a net magnetic moment.
While gaseous oxygen is too diffuse to demonstrate this property easily, cooling it to a liquid state at temperatures below -183 °C concentrates the molecules. Liquid oxygen is strongly attracted to a magnet, and in a classic demonstration, it can be visibly suspended between the poles of a strong horseshoe magnet.
The Surprising Blue State
Its liquid and solid forms exhibit a distinct pale blue hue. When oxygen is cooled below its boiling point of -182.96 °C, it condenses into a clear liquid that is visibly cyan-colored. This coloration is not caused by light scattering, as is the case with the blue sky.
The blue tint results from the absorption of light by highly concentrated oxygen molecules. When two oxygen molecules collide in the liquid state, they can momentarily absorb a photon in the red part of the visible spectrum. Since red light is absorbed, the light transmitted and reflected back to the observer appears to be its complementary color.