Chromium

Chromium, a chemical element with the atomic number 24 and symbol Cr, is part of Group 6 (VIB), period 4, and the d-block of the periodic table. It is a complex, malleable, shiny, greyish-white transition metal commonly used in alloy production and stainless steel manufacturing.

1.0Introduction

Chromium is a metal with a shiny, silvery-grey appearance that can be polished to a brilliant shine. When exposed to air, it resists tarnishing, quickly forming a thin oxide layer that shields the metal from further oxidation. When heated, it produces green chromic oxide. This oxide layer boosts chromium's stability, preventing further oxidation and contributing to its high resistance to corrosion.

Chromium exists in several forms, each with distinct effects on living organisms. Both natural and human activities release chromium(III) and chromium(VI) into the air, water, and soil. Trivalent chromium (chromium(III)) is an essential trace element involved in the human body's metabolism of lipids, insulin, and sugar.

2.0Occurrence and Isotopes

Chromium is relatively abundant in the Earth's crust but has never been found as a free metal. Most chromium ores comprise the mineral chromite, which has the ideal chemical formula FeCr₂O₄. Chromite is widely distributed in natural deposits, though these deposits are typically mixed with other elements such as oxygen, magnesium, aluminium, and silica.

Natural chromium is composed of a mixture of four stable isotopes: chromium-52, which accounts for 83.76%; chromium-53 at 9.55%; chromium-50 at 4.31%; and chromium-54, making up 2.38% of the total chromium composition.

3.0Physical Properties of Chromium

4.0Chemical Properties of Chromium

The chemical properties of chromium are described below:

• Oxidation States: Chromium commonly exhibits oxidation states of +6, +3, and +2. However, only a few stable compounds exist in the +5, +4, and +1 oxidation states.
• Coordination Compounds: Chromium forms stable coordination compounds, such as the hexaaquachromium(III) ion, [Cr(H₂O)₆]³⁺.
• Precipitation Reaction: When chromium(III) ions are mixed with aqueous ammonia (in alkaline conditions) or sodium hydroxide, a green gelatinous precipitate of chromium(III) hydroxide [Cr(OH)₃(H₂O)₃] forms.

[Cr(H₂O)₆]³⁺+ 3OH⁻→ [Cr(OH)₃(H₂O)₃] + 3H₂O

• Chromate and Dichromate Ions: In the +6 oxidation state, chromium produces chromate (CrO₄²⁻​) and dichromate (Cr₂O₇²⁻​) ions, which are important in many industrial applications. These ions are found in compounds like sodium chromate (Na₂CrO₄) and sodium dichromate (Na₂Cr₂O₇) used in leather tanning and metal surface treatment.
• Potassium Dichromate: Potassium dichromate (K₂Cr₂O₇​) is widely oxidising in organic chemistry. It oxidises primary alcohols to aldehydes and secondary alcohols to ketones.

Cr₂O₇²⁻+8H⁺+3CH₃CH₂OH → 2Cr³⁺+7H₂O+3CH₃CHO

• Electron Configuration: Chromium's electron configuration deviates from the Aufbau principle for the 3d subshell to achieve a more stable electronic configuration through energy exchange.
• Organometallic Compounds: Chromium can form organometallic compounds in the +1, +2, +4, and +5 oxidation states.
• Reactivity with Acids: Chromium slowly dissolves in dilute hydrochloric and sulfuric acids, releasing hydrogen gas. However, concentrated nitric acid forms a protective oxide layer, preventing further reaction.
• Reactivity with Other Elements: Chromium does not react with seawater or air (moist or dry) at room temperature. Although, at elevated temperatures, it reacts directly with halogens, sulfur, silicon, boron, nitrogen, carbon, or oxygen.

5.0Uses of Chromium

• Chromium plays a significant role in the stainless steel industry and is essential for manufacturing various alloys. A layer of chromium applied to stainless steel gives it a mirror-like finish. Many stainless steel components contain chromium, including car and truck license plates.
• In gemstones, chromium is responsible for the vibrant red colour of rubies, while it also contributes to the emerald green hue in treated glass.
• In the leather industry, chromium is used to tanning leather products. As, this process has become highly toxic over time, prompting the search for safer alternatives.
• When polished, chromium exhibits a mirror-like shine that reflects its distinct metallic outline. Various automotive decorations incorporate chromium or its alloys.