Metals are generally known for their monochromatic appearance, typically shades of silver or gray. This uniformity is due to free-moving electrons reflecting nearly all wavelengths of visible light. Exceptions like gold and copper display inherent color because their electronic structure absorbs certain light frequencies. A metal’s ability to turn pink is rooted in changes to its chemical composition or physical structure, resulting in a color shift based on how the material interacts with light.
The Most Common Pink Metal: Copper
The metal most closely associated with a reddish-pink color is copper, a transition metal. A freshly polished surface of pure copper exhibits a distinct pinkish-orange metallic luster. This natural coloration is due to the metal’s unique electron band structure, which causes it to absorb green and blue light, reflecting red and orange wavelengths.
The vibrant pink or red color commonly observed on aged copper objects is often a surface compound formed by a chemical reaction. When copper is exposed to air, it reacts with oxygen in a process called oxidation, resulting in a thin layer of cuprous oxide (\(\text{Cu}_2\text{O}\)). This oxide presents as a red or rose-red powder or film.
Cuprous oxide, also known as copper(I) oxide, is a semiconductor whose color can range from yellow to bright red depending on particle size. The chemical product of corrosion, not the metal itself, turns pink. This reddish coating acts as a protective layer, unlike the rust that forms on iron, preventing further decay.
Mechanisms of Metallic Color Change
The fundamental science behind color change in metals is connected to oxidation states. The oxidation state refers to the number of electrons an atom loses or gains when forming a compound, which alters how the material interacts with light. Transition metals like copper are prone to this because their \(d\)-orbitals, which house their outer electrons, are not completely full.
When a metal forms a compound, surrounding atoms or molecules, known as ligands, cause the \(d\)-orbitals to split into different energy levels. Color is produced when an electron absorbs a specific wavelength of visible light, gaining energy to jump from a lower \(d\)-orbital level to a higher one. The light that is not absorbed—the complementary color—is what the human eye perceives.
A change in the metal’s oxidation state modifies the electron configuration and the energy gap between the split \(d\)-orbitals. This shift dictates which wavelengths of light are absorbed and reflected, resulting in a color shift. For instance, while cuprous oxide (\(\text{Cu}_2\text{O}\)) is red, copper(II) compounds often appear blue or green because they absorb light from the opposite end of the spectrum.
Pink Hues Through Alloying
The most recognizable pink metal in consumer goods is rose gold, an alloy. Rose gold is produced by combining pure yellow gold with copper and often a small amount of silver.
The concentration of copper in the mixture determines the final pink hue and its intensity. For example, 18-karat rose gold contains 75% pure gold and approximately 20% to 22.5% copper. The reddish color of the copper is physically blended with the yellow of the gold, shifting the overall color towards the warm, pinkish-red spectrum.
This process is a physical manipulation of color through composition, contrasting with chemical reactions like oxidation. The higher the copper content in the alloy, the deeper the red-pink color becomes. Adding silver can lighten the tone to a softer blush or pink gold.