Ozone is not an element. To understand this, it is helpful to first define what a chemical element is and then examine the nature of ozone itself.
What an Element Is
A chemical element is a fundamental substance that cannot be broken down into simpler substances by ordinary chemical processes. Each element is uniquely defined by the number of protons in the nucleus of its atoms, known as its atomic number. All atoms of a specific element contain the same number of protons. For example, every atom of oxygen contains eight protons. Common examples of elements include hydrogen, carbon, and iron.
What Ozone Is
Ozone is a molecule composed entirely of oxygen atoms. Its chemical formula is O₃, indicating it contains three oxygen atoms bonded together. Ozone forms naturally in the atmosphere when diatomic oxygen (O₂) is broken apart by energy sources like ultraviolet (UV) radiation or electrical discharges from lightning. These individual oxygen atoms then combine with other O₂ molecules to form O₃.
Ozone as an Allotrope
Ozone is not an element because it is a specific molecular arrangement of a single element, oxygen. This concept is explained by allotropy, which describes different structural forms an element can exist in while in the same physical state. Oxygen, the element, can exist as common diatomic oxygen (O₂) or as triatomic ozone (O₃).
The distinct arrangement of oxygen atoms gives O₂ and O₃ different chemical and physical properties. While O₂ is essential for respiration, O₃ is a pale-blue gas with a pungent odor and is highly reactive. This difference in molecular structure, despite being composed of the same element, is why ozone is considered an allotrope of oxygen, rather than a separate element or a compound.
Where Ozone is Found
Ozone occurs naturally in two regions of Earth’s atmosphere. Approximately 90% of atmospheric ozone resides in the stratosphere, located roughly 10 to 50 kilometers (6 to 31 miles) above the Earth’s surface. This stratospheric ozone absorbs most of the sun’s harmful ultraviolet (UV) radiation, protecting life on Earth.
The remaining 10% of atmospheric ozone is found in the troposphere, the lowest layer of the atmosphere, closer to the Earth’s surface. Ground-level ozone primarily forms through chemical reactions involving pollutants emitted by human activities, such as vehicle exhaust and industrial processes, in the presence of sunlight.