24 Cr Chromium 51.996
Transition metal d-block Period 4 Group 6

Chromium

Cr · Element 24 · Chromium group

The element that puts the shine in chrome and the "stainless" in stainless steel.

STATE AT 20°C Solid
ATOMIC MASS 51.996 u
ELECTRON CONFIGURATION [Ar] 3d⁵ 4s¹
23V All 118 25Mn

Structure

The chromium atom

Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.

Orbital cloud

Measured values

Property sheet

Every bar shows where chromium sits among all 118 elements for that property.

Physical

Density 7.15 g/cm³ 44%
Melting point 2180 K 86%
Boiling point 2944 K 58%
Specific heat 0.449 J/g·K
Thermal conductivity 93.9 W/m·K 81%

Atomic

Atomic radius 128 pm 20%
Covalent radius 139 pm
Van der Waals radius 189 pm

Electronic

Electronegativity 1.66 55%
Ionisation energy 653.2 kJ/mol 44%
Electron affinity 51.1 kJ/mol 60%

Occurrence

Abundance in crust 102 mg/kg 83%

Identity

SymbolCr
Atomic number24
Atomic mass51.9961 u
CategoryTransition metal
Blockd
Crystal structurebody-centered cubic
Oxidation states-2, -1, 0, +2, +3, +4, +5, +6
Discovered1797
Discovered byLouis-Nicolas Vauquelin

Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.

Size, to scale

How big is a chromium atom?

Radius 128 pm — that is 0.128 nm, so about 3906 million of them side by side would span a millimetre.

Thermal range

Solid, liquid, gas — and when

Chromium is liquid over a 764 K window, from 2180 K to 2944 K.

Where it sits

Position in the table

Chromium sits in period 4, group 6. Everything in group 6 shares the same outer-electron count, which is why they behave so similarly.

OTHER TRANSITION METALS

21Sc 22Ti 23V 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh
All transition metals

The story

What chromium is, and how we found it

Chromium is the element that puts the "chrome" in everything shiny and makes stainless steel truly stainless. This hard, lustrous transition metal is essential for both beauty and strength in modern life. The name comes from the Greek word "chroma" meaning color, because chromium compounds create an incredible spectrum of vivid colors - from emerald green to ruby red to brilliant yellow. Discovered in 1797 by French chemist Louis-Nicolas Vauquelin while analyzing a mysterious red mineral from Siberia called crocoite, chromium revolutionized both metallurgy and art. Vauquelin noticed that this mineral produced a beautiful green oxide unlike anything known before, leading to the discovery of a completely new element. What makes chromium absolutely extraordinary is its ability to form an ultra-thin, invisible oxide layer that protects underlying metal from corrosion. When chromium is added to iron to make stainless steel, this protective layer makes the alloy virtually indestructible against rust, acids, and weather. That's why stainless steel surgical instruments can be sterilized thousands of times, and why the Chrysler Building's Art Deco crown still gleams after 90 years. Chromium is also the secret behind some of the world's most precious gemstones. The deep red color in rubies comes from trace amounts of chromium replacing aluminum in corundum crystals. Similarly, emeralds get their stunning green color from chromium (and vanadium) impurities in beryl crystals. Without chromium, these gems would be colorless. Here's something fascinating: hexavalent chromium (Cr⁶⁺) is highly toxic and carcinogenic, while trivalent chromium (Cr³⁺) is actually an essential nutrient that helps your body process glucose and fat. This shows how the same element can be both beneficial and harmful depending on its chemical form.

The discovery of chromium

The Colorful Discovery

Louis-Nicolas Vauquelin discovered chromium in 1797 while analyzing crocoite mineral (PbCrO₄) from Siberian gold mines. The mineral's brilliant orange-red color intrigued Vauquelin, leading him to isolate a new metallic element he named "chromium" from the Greek word "chroma" meaning color, reflecting the vivid hues of chromium compounds.

Industrial Development

Commercial chromium production began with ferrochrome smelting in the 1890s, enabling stainless steel development. Harry Brearley's 1913 discovery of stainless steel in Sheffield, England, revolutionized chromium demand and established the foundation for modern chromium metallurgy.

Applications

What chromium is used for

Chromium: The Mirror Maker

Chromium transforms ordinary steel into stainless perfection while creating the brilliant mirrors and decorative finishes that define modern aesthetics. This transition metal's unique properties enable everything from surgical instruments to spacecraft heat shields.

Stainless Steel Revolution

Austenitic stainless steels containing 16-26% chromium form protective oxide layers that self-heal when scratched, providing permanent corrosion resistance. Type 316 stainless steel withstands marine environments, chemical processing, and medical applications through chromium's passivation properties. Global stainless steel production consumes 85% of chromium supply, enabling corrosion-resistant infrastructure from kitchen appliances to chemical plants.

Decorative Chrome Plating

Electroplating processes deposit ultra-thin chromium layers (0.25-2.5 μm) that create mirror-bright finishes on automotive trim, plumbing fixtures, and decorative hardware. Hard chrome plating applies thicker deposits (25-250 μm) for industrial applications requiring extreme wear resistance, including hydraulic cylinders, machine tool ways, and printing rolls.

Refractory Applications

Chromite bricks line steel furnaces and glass melting tanks, withstanding temperatures exceeding 1800°C while resisting slag corrosion. Chrome-magnesite refractories enable cement kiln operation and non-ferrous metal smelting through chromium oxide's exceptional thermal stability and chemical inertness.

Pigment Technology

Chrome oxide green (Cr₂O₃) provides permanent color for ceramics, paints, and plastics with exceptional UV stability and chemical resistance. Lead chromate pigments create brilliant yellows and oranges for industrial coatings, though environmental regulations limit their use to specialized applications.

Chromium: Everywhere You Look

  • Kitchen & Home
    • Stainless steel appliances (refrigerators, dishwashers, ovens)
    • Kitchen sinks, faucets, and cookware
    • Cutlery and utensils
    • Bathroom fixtures and hardware
  • Automotive Industry
    • Chrome-plated bumpers and trim pieces
    • Stainless steel exhaust systems
    • Engine components and fasteners
    • Decorative wheels and accessories
  • Medical Equipment
    • Surgical instruments and implants
    • Hospital equipment and fixtures
    • Dental tools and orthodontic devices
    • Laboratory equipment and surfaces
  • Construction Materials
    • Architectural panels and cladding
    • Handrails and structural elements
    • Roofing and gutter systems
    • Marine construction components
  • Industrial Applications
    • Chemical processing equipment
    • Power plant components
    • Oil refinery equipment
    • Food processing machinery

Where it comes from

Natural occurrence

102 mg/kg of Earth's crust · more abundant than 83% of elements

Chromium: From Stellar Synthesis

Chromium forms through silicon burning processes in massive stars where temperatures exceed 3 billion Kelvin, enabling nuclear fusion reactions that build elements heavier than iron. Type Ia supernovae contribute significantly to chromium abundance through explosive nucleosynthesis, dispersing this element throughout galaxies as stellar remnants seed future planetary formation.

Global Distribution

Earth's crust contains 185 parts per million chromium, concentrated primarily in chromite (FeCr₂O₄) deposits formed through magmatic processes. The Bushveld Complex in South Africa contains 70% of global chromium reserves in layered igneous intrusions, while Kazakhstan and India host significant additional resources.

Chromite Formation

Podiform chromite deposits form in ophiolite complexes where oceanic crust and mantle rocks are exposed through tectonic processes. Stratiform deposits result from magmatic differentiation in large layered intrusions, creating continuous chromite layers exploited through underground mining operations.

Mining and Processing

Chromite mining requires careful ore sorting due to varying chromium-to-iron ratios affecting metallurgical applications. Gravity separation and magnetic separation concentrate chromite minerals, while ferrochrome smelting in electric arc furnaces at 1700°C produces the chromium-iron alloys essential for stainless steel production.

Handling

Safety

Chromium Safety: Critical Awareness Required

Chromium compounds exhibit dramatically different toxicity levels depending on oxidation state. Hexavalent chromium (Cr⁶⁺) represents a serious carcinogenic hazard, while trivalent chromium (Cr³⁺) shows minimal toxicity and serves as an essential trace nutrient.

Health Hazards

Hexavalent chromium causes lung cancer, nasal septum perforation, and severe skin ulceration through its strong oxidizing properties and cellular DNA damage. Chrome plating operations generate chromic acid mists requiring sophisticated ventilation and respiratory protection. Welding stainless steels produces hexavalent chromium fumes necessitating specialized fume extraction systems.

Exposure Limits

  • OSHA PEL: 0.005 mg/m³ for hexavalent chromium (8-hour TWA)
  • NIOSH REL: 0.001 mg/m³ for carcinogenic chromium compounds
  • Trivalent chromium: 0.5 mg/m³ (metal and insoluble salts)

Safety Measures

Comprehensive medical surveillance, respiratory protection, and engineering controls are mandatory for hexavalent chromium operations. Local exhaust ventilation, supplied-air respirators, and protective clothing prevent exposure through multiple pathways.

Quick answers

Chromium: common questions

What is Chromium?

Chromium (symbol Cr) is element 24 on the periodic table, a transition metal in period 4, group 6. The element that puts the shine in chrome and the "stainless" in stainless steel. At room temperature it is a solid.

What is the electron configuration of Chromium?

Chromium's ground-state electron configuration is [Ar] 3d⁵ 4s¹, giving 4 occupied shells holding 2, 8, 13, 1 electrons respectively.

What are the melting and boiling points of Chromium?

Chromium melts at 2180 K (1906.9 °C) and boils at 2944 K (2670.9 °C). That puts it among the most refractory elements — it stays solid at temperatures that vaporise most metals.

What is the atomic mass of Chromium?

The standard atomic weight of Chromium is 51.996 u. That is a weighted average across its naturally occurring isotopes, which is why it is rarely a whole number.

How dense is Chromium?

Chromium has a density of 7.15 g/cm³. Water is 1.0 g/cm³, so a block of chromium is about 7.2× heavier.

What is the electronegativity of Chromium?

Chromium has a Pauling electronegativity of 1.66. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). Values in this middle band tend to form covalent rather than strongly ionic bonds.

Who discovered Chromium, and when?

Chromium was discovered in 1797 by Louis-Nicolas Vauquelin. It is named after greek chroma, "colour".

How common is Chromium on Earth?

Chromium makes up about 102 mg/kg of the Earth's crust — common enough to be mined at scale.

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