Cadmium
| General properties | |||||
|---|---|---|---|---|---|
| Name, symbol | Cadmium, Cd | ||||
| Appearance | Silvery-gray | ||||
| Cadmium in the periodic table | |||||
| |||||
| Atomic number | 48 | ||||
| Standard atomic weight (Ar) | 112.414(4) | ||||
| Group, block | , d-block | ||||
| Period | period 5 | ||||
| Electron configuration | [Kr] 4d10 5s2 | ||||
per shell | 2, 8, 18, 18, 2 | ||||
| Physical properties | |||||
| Silvery-grey | |||||
| Phase | Solid | ||||
| Melting point | 594.22 K (321.07 °C, 609.93 °F) | ||||
| Boiling point | 1040 K (767 °C, 1413 °F) | ||||
| Density near r.t. | 8.65 g/cm3 | ||||
| when liquid, at | 7.996 g/cm3 | ||||
| Heat of fusion | 6.21 kJ/mol | ||||
| Heat of | 99.87 kJ/mol | ||||
| Molar heat capacity | 26.020 J/(mol·K) | ||||
| pressure | |||||
| Atomic properties | |||||
| Oxidation states | 2, 1, −2 (a mildly basic oxide) | ||||
| Electronegativity | Pauling scale: 1.69 | ||||
| energies |
1st: 867.8 kJ/mol 2nd: 1631.4 kJ/mol 3rd: 3616 kJ/mol | ||||
| Atomic radius | empirical: 151 pm | ||||
| Covalent radius | 144±9 pm | ||||
| Van der Waals radius | 158 pm | ||||
| Miscellanea | |||||
| Crystal structure | | ||||
| Speed of sound thin rod | 2310 m/s (at 20 °C) | ||||
| Thermal expansion | 30.8 µm/(m·K) (at 25 °C) | ||||
| Thermal conductivity | 96.6 W/(m·K) | ||||
| Electrical resistivity | 72.7 Ω·m at 22 °C | ||||
| Magnetic ordering | Diamagnetic | ||||
| Young's modulus | 50 GPa | ||||
| Shear modulus | 19 GPa | ||||
| Bulk modulus | 42 GPa | ||||
| Poisson ratio | 0.30 | ||||
| Mohs hardness | 2.0 | ||||
| Brinell hardness | 203–220 MPa | ||||
| CAS Registry Number | 7440-43-9 | ||||
| History | |||||
| Discovery | Karl Samuel Leberecht Hermann and Friedrich Stromeyer (1817) | ||||
| Named by | Friedrich Stromeyer (1817) | ||||
Cadmium is a chemical element with symbol Cd and atomic number 48. It is a toxic metal with a few applications as pure metal. Although it's classified as a transition metal, cadmium's properties are closer to that of post-transition metals and some sources consider group 12 to be post-transition metals.
Contents
Properties
Chemical
Cadmium metal resists corrosion in open air by forming a protective oxide layer on its surface, a property similar to that of the metal above, zinc.
Cadmium burns in air to form brown amorphous cadmium oxide (CdO). The crystalline form of CdO is a dark red solid which changes color when heated, similar to zinc oxide. Cadmium oxide is a slightly basic oxide.
Cadmium metal dissolves in hydrochloric acid, nitric acid and sulfuric acid to yield their respective salts.
Similar to mercury, cadmium also displays the +1 oxidation state, which can be obtained by dissolving cadmium metal in a mixture of cadmium chloride and aluminium chloride, forming the Cd22+ cation, which is similar to the Hg22+ cation in mercury(I) chloride.
Cadmium salts form a characteristic yellow precipitate of CdS with hydrogen sulfide that even precipitates in weakly acidic solution. The presence of other metals can obscure the color with their own. CdS is thermochromic (red when hot, yellow when cold).[1]
Physical
Cadmium is a soft, malleable, ductile silvery-gray or grayish-white metal. Like the metal above it, zinc, cadmium has a low melting point, of only 321 °C and a boiling point of 767 °C, much lower than that of lead (1749 °C), another metal with a similar melting point (327 °C). It has a density of 7.996 g/cm3.
Availability
Cadmium metal can be found in the anode of Ni-Cd batteries, where it's usually plated as a sponge on a metal frame. It can be extracted by dissolving it with an acid and reducing the salt to metal.
Certain metal platings on aircraft parts contain cadmium metal.
An interesting source of cadmium are certain Made in China jewelries, such as pendants, necklaces, bracelets, often sold at a very cheap price, and produced by various "shady" Chinese companies. While most Chinese secondhand stores are very likely to have such products, cadmium jewelry is also sold by various big companies, even in malls. Some jewelries can have as much as 99.6 % cadmium by weight.[2]
Cadmium compounds such as cadmium sulfide (Cadmium yellow), cadmium selenide (Cadmium red), used as pigments, can also be used to make cadmium metal.
In the European Union, the Restriction of Hazardous Substances Directive limits the use of cadmium in products and restricts its use in the manufacture of various types of electronic and electrical equipment.
Isolation
Cadmium metal can be obtained by reducing cadmium sulfide by roasting it.
Electrolysis of cadmium salt will give very pure cadmium metal.
Alternatively, iron or aluminium can be used to displace cadmium metal from its salts.
Projects
- Make cadmium selenide
- Make Ni-Cd battery
- Make CdS quantum dots
Handling
Safety
Cadmium and its salts are extremely toxic to organisms. Organocadmium compounds are deadly in minute doses. Cadmium is classified as a potent carcinogen. Certain insoluble compounds such as cadmium sulfide however, have lower toxicity.
Storage
Cadmium should be stored in closed containers or ampouled.
Disposal
Cadmium must be taken to speacial disposal centers and NEVER RELEASED IN THE ENVIRONMENT.
References
- ↑ G. Jander, E. Blasius, Lehrbuch der analytischen und präparativen anorganischen Chemie, 16th edition, 2006, S. Hirzel Verlag Stuttgart, p. 483
- ↑ https://www.youtube.com/watch?v=208-irQ1Yus
