Potassium
From Freepedia
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| General | |||||||||||||||||||||||||||||||
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| Name, Symbol, Number | potassium, K, 19 | ||||||||||||||||||||||||||||||
| Chemical series | alkali metals | ||||||||||||||||||||||||||||||
| Group, Period, Block | 1, 4, s | ||||||||||||||||||||||||||||||
| Appearance | silvery white Image:K,19.jpg | ||||||||||||||||||||||||||||||
| Atomic mass | 39.0983(1) g/mol | ||||||||||||||||||||||||||||||
| Electron configuration | [Ar] 4s1 | ||||||||||||||||||||||||||||||
| Electrons per shell | 2, 8, 8, 1 | ||||||||||||||||||||||||||||||
| Physical properties | |||||||||||||||||||||||||||||||
| Phase | solid | ||||||||||||||||||||||||||||||
| Density (near r.t.) | 0.89 g/cm³ | ||||||||||||||||||||||||||||||
| Liquid density at m.p. | 0.828 g/cm³ | ||||||||||||||||||||||||||||||
| Melting point | 336.53 K (63.38 °C, 146.08 °F) | ||||||||||||||||||||||||||||||
| Boiling point | 1032 K (759 °C, 1398 °F) | ||||||||||||||||||||||||||||||
| Heat of fusion | 2.321 kJ/mol | ||||||||||||||||||||||||||||||
| Heat of vaporization | 76.90 kJ/mol | ||||||||||||||||||||||||||||||
| Heat capacity | (25 °C) 29.600 J/(mol·K) | ||||||||||||||||||||||||||||||
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| Atomic properties | |||||||||||||||||||||||||||||||
| Crystal structure | cubic body centered | ||||||||||||||||||||||||||||||
| Oxidation states | 1 (strongly basic oxide) | ||||||||||||||||||||||||||||||
| Electronegativity | 0.82 (Pauling scale) | ||||||||||||||||||||||||||||||
| Ionization energies (more) | 1st: 418.8 kJ/mol | ||||||||||||||||||||||||||||||
| 2nd: 3052 kJ/mol | |||||||||||||||||||||||||||||||
| 3rd: 4420 kJ/mol | |||||||||||||||||||||||||||||||
| Atomic radius | 220 pm | ||||||||||||||||||||||||||||||
| Atomic radius (calc.) | 243 pm | ||||||||||||||||||||||||||||||
| Covalent radius | 196 pm | ||||||||||||||||||||||||||||||
| Van der Waals radius | 275 pm | ||||||||||||||||||||||||||||||
| Miscellaneous | |||||||||||||||||||||||||||||||
| Magnetic ordering | ? | ||||||||||||||||||||||||||||||
| Electrical resistivity | (20 °C) 72.0 nΩ·m | ||||||||||||||||||||||||||||||
| Thermal conductivity | (300 K) 102.5 W/(m·K) | ||||||||||||||||||||||||||||||
| Thermal expansion | (25 °C) 83.3 µm/(m·K) | ||||||||||||||||||||||||||||||
| Speed of sound (thin rod) | (20 °C) 2000 m/s | ||||||||||||||||||||||||||||||
| Shear modulus | 1.3 GPa | ||||||||||||||||||||||||||||||
| Bulk modulus | 3.1 GPa | ||||||||||||||||||||||||||||||
| Mohs hardness | 0.4 | ||||||||||||||||||||||||||||||
| Brinell hardness | 0.363 MPa | ||||||||||||||||||||||||||||||
| CAS registry number | 7440-09-7 | ||||||||||||||||||||||||||||||
| Notable isotopes | |||||||||||||||||||||||||||||||
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Potassium is a chemical element in the periodic table. It has the symbol K (L. kalium) and atomic number 19. The name "potassium" comes from the word "potash", as potassium was first isolated from potash. Potassium is a soft silvery-white metallic alkali metal that occurs naturally bound to other elements in seawater and many minerals. It oxidizes rapidly in air, is very reactive, especially in water, and resembles sodium chemically.
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Notable characteristics
With a density less than that of water, potassium is the second lightest metal after lithium. It is a soft solid that can easily be cut with a knife and is silvery in color on fresh surfaces. It oxidizes in air rapidly and must be stored in mineral oil or kerosene for preservation.
Similar to other alkali metals, potassium reacts violently with water producing hydrogen. When in water, it may catch fire spontaneously. Its salts emit a violet color when exposed to a flame.
Applications
- Potassium oxide is primarily used in fertilizer.
- Potassium hydroxide is an important industrial chemical used as a strong base.
- Potassium nitrate is used in gunpowder and black powder. An older term for KNO3 is saltpeter.
- Potassium carbonate, known as potash, is used in glass manufacture.
- Glass treated with liquid potassium is much stronger than regular glass.
- NaK, an alloy of sodium and potassium, is used as a heat-transfer medium.
- Potassium is an essential component needed in plant growth and is found in most soil types.
- In animal cells potassium ions are vital to keeping cells alive (see Na-K pump)
- Potassium chloride is used as a substitute for table salt and is also used to stop the heart, e.g. in cardiac surgery and in executions by lethal injection in liquid form.
- The superoxide KO2 is used as a portable source of oxygen and as a carbon dioxide absorber. It is useful in portable respiration systems.
Many potassium salts are very important, and include: potassium bromide, potassium carbonate, potassium chlorate, potassium chloride, potassium chromate, potassium cyanide, potassium dichromate, potassium iodide, potassium nitrate, potassium sulfate.
History
Potassium was discovered in 1807 by Sir Humphrey Davy, who derived it from caustic potash (KOH). Potassium was the first metal that was isolated by electrolysis.
Potassium was not known in Roman times, and its names are not Classical Latin.
The name kalium was taken from the word "alkali", which came from Arabic al qalīy = "the calcined ashes".
The name potassium was made from the word "potash", which is English, and originally meant an alkali extracted in a pot from the ash of burnt wood or tree leaves.
Occurrence
Potassium makes up about 2.40% of the weight of the Earth's crust and is the seventh most abundant element in it. As it is very electropositive, potassium metal is difficult to obtain from its minerals.
Potassium salts such as carnallite, langbeinite, polyhalite, and sylvite are found in ancient lake and sea beds. These minerals form extensive deposits in these environments, making extracting potassium and its salts more economical. The principal source of potassium, potash, is mined in California, Germany, New Mexico, Utah, and in other places around the world. 3000 ft below the surface of Saskatchewan are large deposits of potash which may become important sources of this element and its salts in the future.
The oceans are another source of potassium, but the quantity present in a given volume of seawater is relatively low compared to sodium.
Potassium can be isolated through electrolysis of its hydroxide in a process that has changed little since Davy. Thermal methods also are employed in potassium production, using potassium chloride. Potassium is almost never found unbound in nature. However, in living organisms K+ ions are important in the physiology of excitable cells.
Isotopes
There are seventeen known isotopes of potassium. Three isotopes occur naturally: K-39 (93.3%), K-40 (0.01%) and K-41 (6.7%). Naturally occurring K-40 decays to stable Ar-40 (11.2%) by electron capture and by positron emission, and decays to stable Ca-40 (88.8%) by beta decay; K-40 has a half-life of 1.250 × 109 years.
The decay of K-40 to Ar-40 enables a commonly used method for dating rocks. The conventional K-Ar dating method depends on the assumption that the rocks contained no argon at the time of formation and that all the subsequent radiogenic argon (i.e., Ar-40) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic Ar-40 that has accumulated. The minerals that are best suited for dating include biotite, muscovite, plutonic/high grade metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered.
Outside of dating, potassium isotopes have been used extensively as tracers in studies of weathering. They have also be used for nutrient cycling studies because potassium is a macro-nutrient required for life.
K-40 occurs in natural potassium (and thus in some commercial salt substitutes) in sufficient quantity that large bags of those substitutes can be used as a radioactive source for classroom demonstrations.
Precautions
Solid potassium reacts violently with water, and should therefore be kept under a mineral oil such as kerosene and handled with care. Unlike lithium and sodium however, potassium cannot be stored under oil indefinitely. If stored longer than 6 months to a year, dangerous shock-sensitive peroxides can form on the metal and under the lid of the container that can detonate upon opening. DO NOT store potassium, rubidium or cesium for longer than a year unless stored in an inert (argon) atmosphere or in a vacuum.
Potassium in diet
Potassium is a necessary mineral in daily nutrition; it assists in muscle contraction and in maintaining fluid and electrolyte balance in body cells. Potassium is also important in sending nerve impulses as well as releasing energy from protein, fat, and carbohydrates during metabolism. A shortage of potassium can cause a potentially fatal condition known as hypokalemia. Eating a variety of foods that contain potassium is the best way to get an adequate amount. Healthy individuals who eat a balanced diet rarely need supplements. Foods with high sources of potassium include bananas, avocados, celery and turnips, although many other fruits, vegetables, and meats contain potassium.
References
Potassium is found in many foods, including oranges and bananas (high quantities), potatoes, avocadoes, spinach, lima beans, tomato and vegetable juices, dried fruits, including raisins, meats, milk, cooked dried beans, mushrooms, and peanut butter.
