Chromium (Cr)

Chromium has the ground-state configuration \([Ar]3d^5\,4s^1\), not \([Ar]3d^4\,4s^2\). The half-filled \(3d^5\) subshell offers extra exchange stabilization, so one electron is promoted from 4s to 3d.

• +2 (Cr²⁺): usually blue and a strong reducing agent.
• +3 (Cr³⁺): typically green/violet depending on ligands; kinetically inert complexes like \([\mathrm{Cr(H_2O)_6}]^{3+}\).
• +6 (chromate/dichromate): yellow chromate \(\mathrm{CrO_4^{2-}}\) and orange dichromate \(\mathrm{Cr_2O_7^{2-}}\); strong oxidants.

They interconvert depending on acidity:

\(\mathrm{2\,CrO_4^{2-} + 2\,H^+ \rightleftharpoons Cr_2O_7^{2-} + H_2O}\)

Alkaline solution favors chromate (yellow), while acidic solution favors dichromate (orange).

Stainless steels contain \(\ge\) 10.5% Cr. Chromium rapidly forms a thin, adherent Cr₂O₃ film that passivates the surface and self-heals if scratched, blocking further oxidation. Common grades (e.g., 18/8) use ~18% Cr and 8% Ni for enhanced corrosion resistance and formability.

In acid, dichromate oxidizes Fe²⁺ to Fe³⁺ while Cr(VI) is reduced to Cr(III):

\(\mathrm{Cr_2O_7^{2-} + 14\,H^+ + 6\,e^- \rightarrow 2\,Cr^{3+} + 7\,H_2O}\)

\(\mathrm{Fe^{2+} \rightarrow Fe^{3+} + e^-}\)

Overall (balanced): \(\mathrm{Cr_2O_7^{2-} + 14\,H^+ + 6\,Fe^{2+} \rightarrow 2\,Cr^{3+} + 7\,H_2O + 6\,Fe^{3+}}\).

Heating a chloride with K₂Cr₂O₇ and conc. H₂SO₄ forms chromyl chloride (deep red vapors) if chloride ions are present:

\(\mathrm{K_2Cr_2O_7 + 4\,NaCl + 6\,H_2SO_4 \rightarrow 2\,CrO_2Cl_2\,(g) + 2\,KHSO_4 + 4\,NaHSO_4 + 3\,H_2O}\)

On hydrolysis, \(\mathrm{CrO_2Cl_2}\) yields chromate.

From chromite ore (FeCr₂O₄) via roasting with alkali/air to soluble chromate, leaching, and reduction. Metal is produced by aluminothermic reduction or electrolytic routes. The overall aluminothermic reaction (schematic):

\(\mathrm{Cr_2O_3 + 2\,Al \rightarrow 2\,Cr + Al_2O_3}\)

Cr(VI) species (chromates/dichromates) are strong oxidants, can cross cell membranes (as chromate), and are associated with toxicity and carcinogenicity. Cr(III) is far less mobile and less toxic; some biochemical roles have been proposed for trace Cr(III), though they remain debated.

Chrome plating deposits a thin layer of Cr onto another metal (often via Cr(VI) or Cr(III) electrolytes) to improve wear, corrosion resistance, and appearance. Stainless steel is an alloy with bulk chromium content; its corrosion resistance is intrinsic to the alloy, not a coating.

• Color/anion: Cr(III) solutions are typically green/violet; Cr(VI) chromate is yellow and dichromate orange.
• Redox tests: Reducing agents (e.g., \(\mathrm{SO_2}\), \(\mathrm{H_2O_2}\) in base) can convert Cr(VI) \(\rightarrow\) Cr(III), with corresponding color change.
• Precipitation: Cr(III) forms gelatinous \(\mathrm{Cr(OH)_3}\) with base; chromate gives characteristic precipitates with Ba²⁺ (yellow BaCrO₄).