Chromium, a transition metal with atomic number 24, is highly valued in industrial applications because it is hard and shows excellent resistance to corrosion. The element’s name is derived from the Greek word chrōma, meaning “color,” which hints at its chemical diversity.
The Appearance of Pure Elemental Chromium
Pure elemental chromium is a silvery-white or steely-gray metal with a strong metallic luster. When polished, it is highly reflective, producing a mirror-like finish. This inherent brightness makes it a popular choice for decorative applications requiring a brilliant shine.
In its consolidated form, the metal is notably hard and brittle. It cannot be easily bent or shaped like more malleable metals such as gold or copper.
Physical Properties That Define Its Look
Chromium maintains its reflective, untarnished appearance due to a natural process called passivation. When exposed to air, the metal immediately reacts with oxygen to form an extremely thin layer of transparent chromium oxide on the surface. This allows the bright metallic color underneath to show through.
This oxide layer is dense, non-porous, and self-healing. If the surface is scratched, the exposed metal quickly reacts with oxygen again to repair the layer, preventing further oxidation or rusting. The metal also possesses a high melting point, which contributes to its stability.
The Vibrant Colors of Chromium Compounds
While the pure metal is silvery, its compounds are highly colorful, which is how the element earned its name. Chromium has several different oxidation states, and each state produces a distinct, vibrant color. The most common state, chromium(III), is often seen in green compounds, such as the widely used pigment chromium(III) oxide.
Chromium in the +6 oxidation state yields bright yellow and orange colors. Examples include the yellow chromate ion and the vivid orange dichromate ion. Historically, lead chromate was a popular pigment known as chrome yellow. Other oxidation states can produce blue, red, or even purple hues, showcasing the element’s chromatic range.
Everyday Surfaces That Feature Chromium
The bright, corrosion-resistant nature of chromium makes it a common feature on many everyday objects. Chrome plating involves electroplating a thin layer of the metal onto a base material for decoration and protection. This technique creates the mirror-like finish seen on automotive trims, bathroom fixtures, and household appliances.
Chromium is also a major component in stainless steel, an alloy containing a minimum of 10% chromium. The chromium content forms the same self-healing, invisible oxide layer, preventing the iron from rusting. This allows items like cutlery, kitchen sinks, and medical instruments to remain bright and untarnished. The element can also be used to color glass, giving it a characteristic emerald-green hue.