Potassium
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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 
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| 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) | ||||||||||||||||||||||||||||||
| Critical point | (extrapolated) 2223 K, 16 MPa | ||||||||||||||||||||||||||||||
| 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 (chemical symbol K, atomic number 19) is a silvery white metal.
- is the chemical element in the periodic table.
- 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 and is very reactive, especially toward water. In many respects, potassium and sodium are chemically similar.
Discovery and etymology
Potassium was discovered in 1807 by Sir Humphry Davy, who derived it from caustic potash (potassium hydroxide, KOH). It was the first metal isolated by electrolysis.
Potassium was not known to the ancient Romans, and its names are neo-Latin, not Classical Latin. The Latin name kalium (from which potassium gets its symbol, K) was taken from the word "alkali", which came from Arabic al qalīy, meaning "the calcined ashes." The name potassium was made from the English word "potash," which originally meant an alkali extracted in water in a pot of ash of burnt wood or tree leaves.
Occurrence
Potassium makes up about 2.4% of the weight of the Earth's crust and is the seventh most abundant element in it. As it is very electropositive, potassium is never found as the free element in nature. Rather, it forms compounds from which it is difficult to separate.
Potassium minerals such as carnallite, langbeinite, polyhalite, and sylvite form extensive deposits in the beds of ancient lakes and seas. The principal source of potassium, potash (impure form of potassium carbonate), is mined in California, New Mexico, Utah, Germany, and in other places around the world. Large deposits of potash have been found about 3,000 feet below the surface of Saskatchewan, and several large mines have been in operation since the 1960s. Saskatchewan pioneered the method of freezing wet sands (the Blairmore formation) as a way to drive mine shafts through them. The oceans are another source of potassium, but the quantity present in a given volume of seawater is relatively low, compared to the concentration of sodium.
Potassium can be isolated through the electrolysis of its hydroxide, by a process that has changed little since the time of Davy. Thermal methods are also employed in potassium production from potassium chloride.
Notable characteristics
With a density less than that of water, potassium is the second least dense metal after lithium. It is a soft, low-melting solid that can easily be cut with a knife. Freshly cut potassium is silvery in appearance but it quickly tarnishes. It oxidizes in air rapidly and thus should be protected for storage. Often samples are maintained under an inert medium such as kerosene.
Similar to other alkali metals but more so than Li and Na, potassium reacts violently with water producing hydrogen. The reaction is sufficiently exothermic that the evolved hydrogen gas ignites. Potassium and its compounds emit a violet color in a flame. This fact is the basis of the flame test for the presence of potassium in a sample.
Applications
- It is primarily used in fertilizer as either the chloride, sulfate or carbonate - not as the oxide.
- Potassium hydroxide is an important industrial chemical used as a strong base.
- Potassium nitrate is used in gunpowder (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 solution.
- 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.
Isotopes
There are seventeen known isotopes of potassium. Three isotopes occur naturally: 39K (93.3%), 40K (0.012%) and 41K (6.7%). Naturally occurring 40K decays to stable 40Ar (11.2%) by electron capture and by positron emission, and decays to stable 40Ca (88.8%) by beta decay; 40K has a half-life of 1.250×109 years.
The decay of 40K to 40Ar 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., 40Ar) was quantitatively retained. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40Ar 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.
40K 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. In healthy animals and people, 40K represents the largest source of radioactivity, greater even then 14C. In a human body of 70 kg mass, about 4,000 nuclei of 40K decay per second. [1]
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 an essential mineral micronutrient in human 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, typically resulting from diarrhea, increased diuresis and vomiting. Deficiency symptoms include, muscle weakness, paralytic ileus, ECG abnormalities, decreased reflex response, and in severe form respiratory paralysis and alkalosis and arrhythmia.
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 orange juice, avocados, celery, parsnips and turnips, although many other fruits, vegetables, and meats contain potassium. Research has indicated that diets high in potassium can reduce the risk of hypertension.
The 2004 guidelines of the Institute of Medicine specify an RDA of 4,700 mg of potassium. However, it is thought that most Americans consume only half that amount per day ([2]). Similarly, in the European Union, particularly in Germany and Italy, insufficient potassium intake is widespread ([3]).
Some people with kidney disease are advised to avoid large quantities of dietary potassium.
See also
- Potassium compounds
- Potassium in biology
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