Chromium
| General properties | |||||
|---|---|---|---|---|---|
| Name, symbol | Chromium, Cr | ||||
| Appearance | Silvery metallic | ||||
| Chromium in the periodic table | |||||
| |||||
| Atomic number | 24 | ||||
| Standard atomic weight (Ar) | 51.9961(6) | ||||
| Group, block | , d-block | ||||
| Period | period 4 | ||||
| Electron configuration | [Ar] 3d5 4s1 | ||||
per shell | 2, 8, 13, 1 | ||||
| Physical properties | |||||
| Silvery metallic | |||||
| Phase | Solid | ||||
| Melting point | 2180 K (1907 °C, 3465 °F) | ||||
| Boiling point | 2944 K (2671 °C, 4840 °F) | ||||
| Density near r.t. | 7.19 g/cm3 | ||||
| when liquid, at | 6.3 g/cm3 | ||||
| Heat of fusion | 21.0 kJ/mol | ||||
| Heat of | 347 kJ/mol | ||||
| Molar heat capacity | 23.35 J/(mol·K) | ||||
| pressure | |||||
| Atomic properties | |||||
| Oxidation states | 6, 5, 4, 3, 2, 1, −1, −2, −4 (depending on the oxidation state, an acidic, basic, or amphoteric oxide) | ||||
| Electronegativity | Pauling scale: 1.66 | ||||
| energies |
1st: 652.9 kJ/mol 2nd: 1590.6 kJ/mol 3rd: 2987 kJ/mol | ||||
| Atomic radius | empirical: 128 pm | ||||
| Covalent radius | 139±5 pm | ||||
| Miscellanea | |||||
| 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 | 125·10-9 Ω·m (at 20 °C) | ||||
| Magnetic ordering | Antiferromagnetic | ||||
| 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 Registry Number | 7440-47-3 | ||||
| Discovery and first isolation | Louis Nicolas Vauquelin (1797-1798) | ||||
Chromium is a transition metal with the symbol Cr and atomic number 24.
Contents
Properties
Chemical
Chromium metal is quite unreactive. It tends to form a thin passivation layer of chromium(III) oxide, which prevents further oxidation. Nitric acid can enhance this passivation layer, but reducing agents tend to destroy it.
Most chromium compounds are amphoteric.
Physical
Chromium is a shiny silver color and is quite hard and brittle. It is antiferromagnetic at room temperature, and is the only element that exhibits this property at such high temperatures. Chromium metal melts at 1907 °C and boils at 2671 °C. It has a density of 7.19 g/cm3
Availability
Pure chromium metal is hard to find, as it tends to be alloyed with different metals. Chromoly, which consists of iron, chromium and molybdenum, tends to have a relatively high concentration of chromium compared to other alloys. Stainless steel is another option.
Chromium makes up 10% or more of stainless steel, a very accessible metal to the home chemist. See Preparation for extraction of chromium from this.
Preparation
Aluminothermic reactions (thermites) with chromium(III) oxide may form chromium metal.
Chromium hydroxide can be produced from stainless steel by first dissolving the metal in hot hydrochloric acid. Iron can be precipitated out as iron(III) sulfate by reacting the resulting solution with sulfuric acid or sodium sulfate in a heated, oxidative environment. With the precipitate filtered out, chromium hydroxide can finally be precipitated from the solution using ammonia, sodium hydroxide, or sodium carbonate. An excess of ammonia or sodium hydroxide will dissolve the precipitate, again, so a nearly stoichiometric amount is recommended. The final product has some iron contamination as well.
Projects
- Make chromium(II) salts
- Make chromium(III) complexes
- Tan leather
- Make chromium dioxide
- Make peroxochromates
- Make dichromates and trichromates
Handling
Safety
Bulk chromium has good chemical resistivity and isn't known to cause contact dermatitis. Chromium(III) is known to be an essential trace element in the organism. Studies done so far have not shown whether Cr(III) poses any toxic or mutagenic preoperties, as the results are inconclusive.
Hexavalent chromium on the other hand is a known carcinogenic.
Storage
Bulk chromium does not require special storage, though it should be kept away from any halogens and mineral acid vapors.
Disposal
Chromium and Cr(III) pose little toxicity to the environment, though it's best to avoid dumping them in the environment.
Hexavalent chromium must be reduced to Cr(III) before disposal. This is done with a reducing agent, sulfites of metabisulfites should do the trick.
