Chromium
Atomic Data
| Atomic Number | 24 |
| Symbol | Cr |
| Atomic Weight | 51.996 u |
| Density (STP) | 7.15 g/cm³ |
| Melting Point | 1906.85 °C (2180 K) |
| Boiling Point | 2670.85 °C (2944 K) |
| Electronegativity | 1.66 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d5 4s1 |
| Oxidation States | +2, +3, +6 |
| Phase at STP | Solid |
| Category | Transition Metal |
| Period / Group | 4 / 6 |
| CAS Number | 7440-47-3 |
Electron Configuration
[Ar] 3d5 4s1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 5 | 23 |
| N | 4 | 4s | 1 | 24 |
| Total | 24 | 24 | ||
Isotopes of Chromium
Chromium has four naturally occurring stable isotopes. The most abundant is ⁵²Cr, comprising 83.789% of all naturally occurring Chromium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Chromium-50 | ⁵⁰Cr | 24 | 26 | 4.345 | Stable |
| Chromium-52 | ⁵²Cr | 24 | 28 | 83.789 | Stable |
| Chromium-53 | ⁵³Cr | 24 | 29 | 9.501 | Stable |
| Chromium-54 | ⁵⁴Cr | 24 | 30 | 2.365 | Stable |
Abundance & Occurrence
Chromium is present in Earth's crust at approximately 102 ppm by mass and at approximately 15 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Hexavalent chromium: carcinogen: Cr(VI) compounds (chromates, dichromates) are highly toxic and classified as Group 1 human carcinogens (IARC); occupational inhalation is associated with lung cancer, and they also cause kidney damage and allergic sensitisation.
- Skin sensitisation: Chromium(VI) compounds cause allergic contact dermatitis (chrome allergy); once sensitised, even trace exposures cause severe skin reactions: sensitisation is permanent.
- Trivalent chromium: lower risk: Cr(III) is far less toxic than Cr(VI) and is an essential trace nutrient; most common uses (stainless steel, leather tanning) involve Cr(III), though dust and fumes still require respiratory controls.
- Chromic acid and dichromates: Chromic acid and dichromate solutions are strongly oxidising and corrosive; contact causes severe chemical burns and ulceration; inhalation of mists causes perforated nasal septum (a classic sign of chronic exposure).
- Regulatory controls: Many Cr(VI) applications are restricted or banned under REACH and RoHS regulations; verify compliance before using any chromate-containing coating, plating, or pigment.
Chromium in the Real World
Real-World Uses
- Stainless steel production: Chromium (minimum 10.5%) forms a passive oxide layer on steel that prevents corrosion, making stainless steel essential for kitchen equipment, surgical instruments, food processing, and chemical plant construction.
- Hard chrome plating: Electrodeposited chromium coatings on industrial components such as hydraulic cylinder rods, piston rings, and moulds provide exceptional hardness, wear resistance, and a low coefficient of friction.
- Chromate and chrome tanning: Chromium(III) sulfate is the dominant tanning agent in global leather production, reacting with collagen fibres to produce soft, durable, water-resistant hides used in shoes, handbags, and upholstery.
- Pigments: Chrome green (Cr₂O₃) and chrome yellow (lead chromate) have historically been used in artists' paints, road markings, and industrial coatings; chrome green is still used where chemical stability is needed.
- Refractory materials: Chromite (FeCr₂O₄) and chromium oxide bricks line high-temperature metallurgical furnaces, glass tanks, and cement kilns because chromia withstands slag attack and temperatures above 1800 °C.
- Superalloys: Chromium is a major component of nickel-based superalloys (Inconel, Waspaloy) used in jet turbine blades and combustion chambers, where it provides oxidation resistance at extreme temperatures.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is chromium used for?
Chromium is best known as a key component of stainless steel, where it provides corrosion resistance by forming a protective oxide layer. It is also used in hard chromium plating for industrial tools, in chromium pigments for paints and inks, and in the tanning of leather. Chromium compounds are important catalysts in the chemical industry.
Is chromium toxic?
Chromium exists in several oxidation states with very different toxicities. Trivalent chromium (Cr3+) is an essential trace nutrient in humans and is relatively non-toxic. Hexavalent chromium (Cr6+), however, is a known carcinogen and potent toxin, associated with lung cancer and skin ulcers upon prolonged exposure. Industrial use of Cr6+ compounds is now heavily regulated.
How was chromium discovered?
Chromium was discovered in 1798 by French chemist Louis-Nicolas Vauquelin, who isolated it from the mineral crocoite (lead chromate). He named it chromium from the Greek word 'chroma' meaning colour, because of the vivid reds, greens, and yellows formed by its compounds. The distinctive green of emeralds and red of rubies both come from trace chromium.
Why does stainless steel not rust?
Stainless steel contains at least 10.5% chromium. When exposed to oxygen, the chromium forms an invisible, tightly adherent layer of chromium oxide on the steel surface. This passive film is self-repairing: even after scratching, it re-forms in the presence of oxygen: physically blocking iron from reacting with moisture and oxygen that would cause rust.