Air is classified as matter because it possesses mass and occupies space. All substances considered to be matter, from the largest star to the smallest dust particle, are ultimately composed of atoms. Therefore, the air we breathe is indeed built from these microscopic building blocks.
Air as Molecular Matter
While air is made of atoms, the vast majority of air particles exist as molecules, formed when two or more atoms chemically bond together. For instance, oxygen is typically found as a diatomic molecule, where two oxygen atoms are linked to form O2.
Air exists in the gaseous state, meaning the molecules are separated by distances significantly larger than the size of the molecules themselves. These gaseous molecules are in continuous, rapid motion, constantly colliding with one another and the surfaces they encounter. This spacing and movement allows air to be easily compressed and to expand freely to fill any container.
The Main Chemical Components of Air
The atmosphere is a homogeneous mixture of various gases, dominated by two components. Nitrogen gas (N2) is the most abundant, making up approximately 78% of the air by volume, and Oxygen gas (O2) accounts for nearly 21%.
The remaining 1% of the atmosphere is a mix of various trace gases. Argon (Ar), a noble gas, is the third most common component at close to 0.93%, existing as single, unbonded atoms. Carbon dioxide (CO2) is also present, though in a much smaller fraction, hovering around 0.04% of the volume. Despite its small quantity, this triatomic molecule plays a significant role in atmospheric processes.
Why We Cannot See Air
The primary reason we cannot see air is related to the extremely small size of its molecules relative to the wavelength of visible light. The molecules of nitrogen and oxygen are far smaller than the range of visible light wavelengths, which is approximately 400 to 700 nanometers. Most light waves simply pass straight through the widely spaced molecules without being absorbed or reflected back to the eye.
The small molecules in the air do interact with light through a process called Rayleigh scattering, which is size-dependent. This scattering is responsible for the blue color of the sky, as it preferentially redirects the shorter, blue wavelengths of light. However, the amount of light scattered by any small volume of air is negligible, which is why a small sample of air remains transparent. It takes the depth of the entire atmosphere for the cumulative effect of this minimal scattering to become visible to the human eye.