Chromium

About Chromium

Chromium is the canonical exception to Aufbau filling — its [Ar] 3d⁵ 4s¹ configuration shows up in the first lecture on transition metals because the half-filled d shell beats the expected d⁴ s² by enough to flip an electron. That same d-orbital arrangement is why Cr³⁺ is responsible for both the red of ruby and the green of emerald: identical ion, different crystal field, different splitting energy, different absorbed wavelength. In practical chemistry, you meet chromium two ways. As an alloying addition above about 11% by mass it forms the self-healing Cr₂O₃ passivation layer that turns iron into stainless steel — scratch the surface and it reoxidizes within milliseconds. As Cr(VI), in chromate or dichromate, it is a vicious oxidizer and a class-1 carcinogen; the orange-to-green color change as dichromate accepts electrons from a primary alcohol used to be the standard breathalyzer test before electrochemical sensors took over. Tanneries still consume hundreds of thousands of tonnes of Cr(III) sulfate every year.