Oxidation is a natural chemical process where a metal loses electrons, typically by reacting with oxygen in the air or water, resulting in a metal oxide compound. This compound often has a different color than the original shiny metal. While many common metals change color upon oxidation, the specific metal that forms a distinctly red compound upon initial exposure is copper.
Copper The Metal That Turns Red
The reddish-brown color observed on a newly tarnished copper surface is due to the formation of copper(I) oxide, also known as cuprous oxide (Cu2O). This is the initial protective layer that forms on copper when it reacts with oxygen in the atmosphere. The Cu2O compound appears brick red and is relatively stable under dry conditions.
This red oxide layer is frequently seen on older electrical wiring, the inside of copper pipes, or on copper artifacts. Over time, particularly in moist air, this red layer will continue to react with oxygen and moisture. The continued oxidation process eventually transforms the red copper(I) oxide into copper(II) oxide (CuO), which is a black compound.
The presence of moisture and carbon dioxide in the air then causes the black oxide to react further, ultimately leading to the familiar blue-green patina seen on structures like the Statue of Liberty. The initial stage of oxidation, which gives the metal its red color, is the formation of the Cu2O layer. This first oxide layer provides a degree of protection, slowing down the corrosion of the copper beneath it.
The Chemical Reason for Red Coloration
Copper(I) oxide (Cu2O) appears red due to its specific atomic structure and how that structure interacts with visible light. The color we see in any object is a result of which wavelengths of light are absorbed and which are reflected back to our eyes. In the case of Cu2O, the crystal structure creates a specific energy difference between where electrons sit and where they can move to, often simplified as a band gap.
This energy gap in cuprous oxide is precisely tuned to absorb light with higher energy, specifically the green, blue, and violet wavelengths. When white light hits the red oxide, those higher-energy colors are absorbed to excite the electrons within the material. The light that is not absorbed—primarily the lower-energy red and yellow-orange wavelengths—is reflected.
The reflection of the red and orange light is what makes the copper(I) oxide appear red-brown to our perception. This is distinct from the metallic luster of the copper itself, which reflects nearly all wavelengths of light. The specific 2.1 electronvolt energy gap of Cu2O ensures that only the red end of the spectrum is largely reflected, giving the compound its characteristic hue.
Oxidation Colors of Other Common Metals
The red color of oxidized copper contrasts sharply with the oxidation products of other common metals. When iron oxidizes, it forms hydrated iron(III) oxide (Fe2O3), commonly known as rust. This compound is typically a flaky, reddish-brown or orange-brown color, which is similar to the color of oxidized copper. Unlike the oxidation on copper, the rust layer on iron is not protective and flakes away, continuously exposing the underlying metal to further corrosion.
Aluminum, a highly reactive metal, also oxidizes immediately upon exposure to air. The resulting aluminum oxide (Al2O3) is nearly invisible or appears as a dull, grayish-white film. This oxide layer is remarkably dense and tenacious, forming a protective barrier that seals the aluminum underneath from any further reaction. While aluminum corrodes rapidly, its oxidation product does not produce a visible color change like the red seen on copper.