Chromium

Physical Properties and Atomic Configuration
– Chromium is a chemical element with symbol Cr and atomic number 24.
– It is a steely-grey, lustrous, hard, and brittle transition metal.
– Chromium metal is valued for its high corrosion resistance and hardness.
– Polished chromium reflects almost 70% of the visible spectrum and almost 90% of infrared light.
– The name ‘chromium’ is derived from the Greek word χρῶμα, meaning color.
– Chromium is the first element in the periodic table whose ground-state electron configuration violates the Aufbau principle.
– The energy gap between the 3d and 4s subshells in chromium is small, making electron promotion energetically feasible.
– The 3d electrons in chromium contribute less to metallic bonding compared to the preceding element vanadium.
– Chromium(VI) is a strong oxidizing agent.
– Promotions of electrons also occur in other transition metal atoms, except for palladium.

Bulk Properties and Passivation
– Chromium is extremely hard, with a Mohs hardness of 8.5.
– It is highly resistant to tarnishing, unlike metals such as copper, magnesium, and aluminum.
– Chromium has a melting point of 1907°C and a boiling point of 2671°C.
– The electrical resistivity of chromium at 20°C is 125 nanoohm-meters.
– Chromium has a high specular reflection, especially in the infrared range.
– Chromium forms a thin, protective surface layer of oxide when exposed to air, inhibiting the diffusion of oxygen into the metal.
– Passivation can be enhanced by short contact with oxidizing acids like nitric acid.
– Passivated chromium is stable against acids but can be dissolved in weak acids.
– Chromium does not suffer from hydrogen embrittlement but can suffer from nitrogen embrittlement.
– Nitrogen embrittlement occurs when chromium reacts with nitrogen from air at high temperatures.

Isotopes and Decay
– Chromium is composed of four stable isotopes: Cr, Cr, Cr, and Cr.
– The most abundant isotope of chromium is Cr, with a natural abundance of 83.789%.
– Chromium can theoretically decay to Ti through double electron capture with a minimum half-life of 1.3×10 years.
– There are 25 radioisotopes of chromium, with the most stable being Cr with a half-life of 27.7 days.
– The primary decay mode before Cr is electron capture, while the primary mode after is beta decay.

Chemistry and Compounds
– Chromium is a member of group 6 transition metals.
– The most common oxidation states for chromium compounds are +3 and +6, followed by +2.
– Many Cr(0) complexes are known, including bis(benzene)chromium and chromium hexacarbonyl.
– Chromium(II) compounds are uncommon and readily oxidize to chromium(III) derivatives in air.
– Chromium(III) compounds are numerous and include chromium(III) nitrate, chromium(III) acetate, and chromium(III) oxide.
– Chromium(VI) compounds are oxidants at low or neutral pH.
– Chromate and dichromate ions are the principal ions at this oxidation state.
– Chromate and dichromate ions exist in equilibrium, which is pH-dependent.
– Chromium(VI) oxyhalides, such as chromyl fluoride and chromyl chloride, are also known.
– Chromium(VI) oxide is produced industrially by the oxidative roasting of chromite ore with sodium carbonate.
– Compounds of chromium(V) are rare, with the only binary compound being chromium(V) fluoride.
– Chromium(V) compounds are intermediates in many oxidation reactions involving chromate.
– Compounds of chromium(IV) are slightly more common than chromium(V) compounds.
– Chromium(IV) tetrahalides (CrF, CrCl, CrBr) can be produced by treating the corresponding trihalides.
– Chromium(I) compounds are obtained by the oxidation of electron-rich, octahedral chromium(0) complexes.

Occurrence, History, Production, Applications, and Metallurgy
– Chromium is the 21st most abundant element in Earth’s crust, with an average concentration of 100ppm.
– Chromium compounds are found in the environment due to the erosion of chromium-containing minerals.
– Crocoite (PbCrO) is a mineral that contains chromium.
– Chromite ore is a major source of chromium.
– Chromium is typically found in association with manganese isotopes, providing evidence for nucleosynthetic processes in the early Solar System.
– Chromium minerals were used as pigments in the 18th century, with crocoite being discovered as a paint pigment.
– Traces of chromium were found in precious gemstones.
– Increased demand for chromium occurred with the development of metallurgy and chemical industries.
– In 2013, 28.8 million metric tons of chromite ore were produced.
– Ferrochromium is the leading end use of chromite ore.
– South Africa, Kazakhstan, Turkey, and India are the largest producers of chromium ore.
– Different processes are used for producing ferrochromium and metallic chromium.
– Chromium is added to steel for increased strength and corrosion resistance.
– Nickel-based alloys also contain chromium for increased strength.
– Chromium is used in chemical, refractory, and foundry industries.
– Chromium is used in condenser and heat-exchanger tubes, as well as castings with high strength at elevated temperatures.
– Chromium is used in stainless steel for corrosion resistance.
– Ferrochromium is added to molten iron for stainless steel formation.
– Nickel superalloys contain chromium for high-temperature properties.
– Chromium is used in ASTM B163 and ASTM A567 standards.
– Nichrome, a chromium-containing alloy, is used for heating elements.
– Chromium is used for surface coating due to hardness and corrosion resistance.
– Chromium is popular for sheet coating and decorative chrome plating on various objects Source:  https://en.wikipedia.org/wiki/Chromium

Chromium (Wikipedia)

This article is about the chemical element. For the web browser, see Chromium (web browser).

Chromium is a chemical element; it has symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.

Chromium, ₂₄Cr

Chromium
Appearance	silvery metallic
Standard atomic weightAr°(Cr)
	51.9961±0.0006 51.996±0.001 (abridged)
Chromium in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson – ↑ Cr ↓ Mo vanadium ← chromium → manganese
Atomic number (Z)	24
Group	group 6
Period	period 4
Block	d-block
Electron configuration	[Ar] 3d5 4s1
Electrons per shell	2, 8, 13, 1
Physical properties
Phaseat STP	solid
Melting point	2180 K ​(1907 °C, ​3465 °F)
Boiling point	2944 K ​(2671 °C, ​4840 °F)
Density (near r.t.)	7.15 g/cm3
when liquid (at m.p.)	6.3 g/cm3
Heat of fusion	21.0 kJ/mol
Heat of vaporization	347 kJ/mol
Molar heat capacity	23.35 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1656 1807 1991 2223 2530 2942
Atomic properties
Oxidation states	−4, −2, −1, 0, +1, +2, +3, +4, +5, +6 (depending on the oxidation state, an acidic, basic, or amphoteric oxide)
Electronegativity	Pauling scale: 1.66
Ionization energies	1st: 652.9 kJ/mol 2nd: 1590.6 kJ/mol 3rd: 2987 kJ/mol (more)
Atomic radius	empirical: 128 pm
Covalent radius	139±5 pm
Spectral lines of chromium
Other properties
Natural occurrence	primordial
Crystal structure	​body-centered cubic (bcc)
Speed of sound thin rod	5940 m/s (at 20 °C)
Thermal expansion	4.9 µm/(m⋅K) (at 25 °C)
Thermal conductivity	93.9 W/(m⋅K)
Electrical resistivity	125 nΩ⋅m (at 20 °C)
Magnetic ordering	antiferromagnetic (rather: SDW)
Molar magnetic susceptibility	+280.0×10−6 cm3/mol (273 K)
Young's modulus	279 GPa
Shear modulus	115 GPa
Bulk modulus	160 GPa
Poisson ratio	0.21
Mohs hardness	8.5
Vickers hardness	1060 MPa
Brinell hardness	687–6500 MPa
CAS Number	7440-47-3
History
Discovery and first isolation	Louis Nicolas Vauquelin (1794, 1797)
Isotopes of chromium v e
Main isotopes Decay abun­dance half-life(t1/2) mode pro­duct 50Cr 4.34% stable 51Cr synth 27.7025 d ε 51V γ – 52Cr 83.8% stable 53Cr 9.50% stable 54Cr 2.37% stable
Category: Chromium view talk edit | references

Chromium metal is valued for its high corrosion resistance and hardness. A major development in steel production was the discovery that steel could be made highly resistant to corrosion and discoloration by adding metallic chromium to form stainless steel. Stainless steel and chrome plating (electroplating with chromium) together comprise 85% of the commercial use. Chromium is also greatly valued as a metal that is able to be highly polished while resisting tarnishing. Polished chromium reflects almost 70% of the visible spectrum, and almost 90% of infrared light. The name of the element is derived from the Greek word χρῶμα, chrōma, meaning color, because many chromium compounds are intensely colored.

Industrial production of chromium proceeds from chromite ore (mostly FeCr₂O₄) to produce ferrochromium, an iron-chromium alloy, by means of aluminothermic or silicothermic reactions. Ferrochromium is then used to produce alloys such as stainless steel. Pure chromium metal is produced by a different process: roasting and leaching of chromite to separate it from iron, followed by reduction with carbon and then aluminium.

In the United States, trivalent chromium (Cr(III)) ion is considered an essential nutrient in humans for insulin, sugar, and lipid metabolism. However, in 2014, the European Food Safety Authority, acting for the European Union, concluded that there was insufficient evidence for chromium to be recognized as essential.

While chromium metal and Cr(III) ions are considered non-toxic, hexavalent chromium, Cr(VI), is toxic and carcinogenic. According to the European Chemicals Agency (ECHA), chromium trioxide that is used in industrial electroplating processes is a "substance of very high concern" (SVHC).

Abandoned chromium production sites often require environmental cleanup.