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Potassium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


                19               argon ← potassium → calcium
                Na
                ↑
                K
                ↓
                Rb

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                     Name, Symbol, Number potassium, K, 19
                                             Chemical series alkali metals
                                              Group, Period, Block 1, 4, s
                                                  Appearance silvery white
                                             Atomic mass 39.0983 (1) g/mol
                                          Electron configuration [Ar] 4s^1
                                            Electrons per shell 2, 8, 8, 1
                                                       Physical properties
                                                               Phase solid
                                       Density (near r.t.) 0.89 g·cm^−3
                                   Liquid density at m.p. 0.828 g·cm^−3
                                                   Melting point 336.53  K
                                                 (63.38 ° C, 146.08 ° F)
                                                      Boiling point 1032 K
                                                     (759 ° C, 1398 ° F)
                                         Critical point ( K, {{{mpa}}} MPa
                                     Crystal structure cubic body centered
                                                        Oxidation states 1
                                                    (strongly basic oxide)
                                    Electronegativity 0.82 (Pauling scale)
                                                       Ionization energies
                                           ( more) 1st: 418.8 kJ·mol^−1
                                                    2nd: 3052 kJ·mol^−1
                                                    3rd: 4420 kJ·mol^−1
                                                      Atomic radius 220 pm
                                              Atomic radius (calc.) 243 pm
                                                    Covalent radius 196 pm
                                               Van der Waals radius 275 pm
                                                             Miscellaneous
                                                      Magnetic ordering  ?
                      Thermal conductivity (300 K) 102.5 W·m^−1·K^−1
                       Thermal expansion (25 °C) 83.3 µm·m^−1·K^−1
                               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
                                                         Selected isotopes

   CAPTION: Main article: Isotopes of potassium

                              iso    NA    half-life   DM  DE ( MeV)  DP
                              ^39K 93.26% K is stable with 20 neutrons
                              ^40K 0.012% 1.277×10^9 y β^- 1.311     ^40Ca
                                                       ε   1.505     ^40Ar
                                                       β^+ 1.505     ^40Ar
                              ^41K 6.73%  K is stable with 22 neutrons

                                                                References

   Potassium ( IPA: /pə(ʊ)ˈtasiəm/) is a chemical element. It has the
   symbol K (Arabic: al qalja‎ → Latin: 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 and is very reactive,
   especially towards water. In many respects, potassium and sodium are
   chemically similar, although organisms in general, and animal cells in
   particular, treat them very differently.

Notable characteristics

   The flame test
   Enlarge
   The flame test

   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 in air it begins to tarnish toward gray visibly and
   immediately. Potassium must be protected from air for storage to
   prevent disintegration of the metal from oxide and hydroxide corrosion.
   Often samples are maintained under an inert medium such as kerosene.

   Like the other alkali metals, potassium reacts violently with water
   producing hydrogen. The reaction is notably more violent than that of
   lithium or sodium with water, and is sufficiently exothermic that the
   evolved hydrogen gas ignites.

          2K(s) + 2H[2]O(l) → H[2](g) + 2KOH(aq)

   Because potassium reacts quickly with even traces of water, and its
   reaction products are nonvolatile, it is sometimes used alone, or as
   NaK (an alloy with sodium which is liquid at room temperature) to dry
   solvents prior to distillation. In this role, it serves as a potent
   desiccant.

   Potassium is an essential element for all living organisms .

   Potassium and its compounds emit a violet colour in a flame. This fact
   is the basis of the flame test for the presence of potassium in a
   sample.

   Potassium compounds generally have excellent water solubility, due to
   the high hydration energy of the K^+ ion. The potassium ion is
   colorless in water.

   Potassium may be detected by taste because it triggers all the types of
   tastebuds, according to concentration. Dilute solutions of potassium
   ion taste sweet (allowing moderate concentrations in milk and juices),
   while higher concentrations become increasingly bitter/alkaline, and
   finally also salty to the taste. The combined bitterness and saltiness
   of high potassium content solutions makes high-dose potassium
   supplementation by liquid drinks a palatability challenge.

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 KNO[3] is saltpeter.
     * Potassium carbonate, known as potash, is used in glass
       manufacturing.
     * Glass treated with liquid potassium is much stronger than regular
       glass.
     * NaK, pronounced "nack", an alloy of sodium and potassium which is
       liquid at room temperature, is used as a heat-transfer medium. It
       can also be used as a desiccant for producing dry and air-free
       solvents.
     * 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 KO[2] 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 Humphry Davy, who derived it
   from caustic potash (K OH). Potassium was the first metal that was
   isolated by electrolysis.

   Potassium was not known in Roman times, and its names are not Classical
   Latin but rather neo-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 in feldspar
   Enlarge
   Potassium in feldspar

   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 metal is difficult to obtain from its minerals. It is never
   found free in nature. 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 Saskatchewan, California,
   Germany, New Mexico, Utah, and in other places around the world. 3000
   feet below the surface of Saskatchewan are large deposits of potash
   which are important sources of this element and its salts, with several
   large mines in operation since the 1960's. Saskatchewan pioneered the
   use of freezing of wet sands (the Blairmore formation) in order to
   drive mine shafts through them. See Potash Corporation of Saskatchewan.
   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.

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 1.250×10^9 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 than ^14C. In a human
   body of 70 kg mass, about 4,400 nuclei of ^40K decay per second.

Precautions

   Peroxides(Yellow) and Ozonides (Red) on surface of potassium metal.
   Enlarge
   Peroxides(Yellow) and Ozonides (Red) on surface of potassium metal.

   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, which can detonate upon opening. DO NOT store potassium,
   rubidium or caesium for longer than a year unless stored in an inert
   (argon) atmosphere or in a vacuum.

   The extremely alkaline potassium hydroxide (KOH) residue on the surface
   of potassium which has been exposed to moisture, is a caustic hazard.
   As with sodium metal, the "soapy" feel of potassium metal on skin is
   due to caustic breakdown of the fats in skin into crude soft potassium
   soap, and represents the beginning of an alkali burn. Potassium should
   obviously be handled only with careful skin and eye protection.

   Potassium fires are exacerbated by water, and only a few dry chemicals
   are effective for them. For a fire discussion which applies to alkali
   metals in general, see the precaution section for sodium.

Potassium in nutrition and medicine

   Potassium is an essential mineral micronutrient in human nutrition; it
   is the major cation (positive ion) inside animal cells, and it is thus
   important in maintaining fluid and electrolyte balance in the body.

   Potassium is also important in allowing muscle contraction and the
   sending of all nerve impulses in animals. See action potential for an
   explanation of the interplay of sodium and potassium in all excitable
   animal cells. Because of the interaction of the charge on a potassium
   ion and its surrounding water molecules, K^+ ions are actually a little
   smaller than Na^+ ions, and ion channels and pumps in cell membranes
   can easily distinguish between the two types of ions, actively pumping
   or passively allowing one of the two ions to pass, while blocking the
   other.

   A shortage of potassium in body fluids may cause a potentially fatal
   condition known as hypokalemia (see article for detail), typically
   resulting from diarrhea, increased diuresis and vomiting. Deficiency
   symptoms include muscle weakness, paralytic ileus, ECG abnormalities,
   decreased reflex response and (in severe cases) respiratory paralysis,
   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, potatoes, bananas, avocados, apricots, 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 ( ). Similarly, in the European
   Union, particularly in Germany and Italy, insufficient potassium intake
   is widespread ( ).

   Supplements of potassium in medicine are most widely used in
   conjunction with the most powerful classes of diuretics, which rid the
   body of sodium and water, but have the side effect of also causing
   potassium loss in urine. A variety of medical supplements are
   available.

   Some people with kidney disease are advised to avoid large quantities
   of dietary potassium. End stage renal failure patients undergoing
   therapy by renal dialysis must observe strict dietary limits on
   potassium intake, since the kidneys control potassium excretion, and
   buildup of blood concentrations of potassium may trigger fatal heart
   dysrhythmias.

   Retrieved from " http://en.wikipedia.org/wiki/Potassium"
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   with only minor checks and changes (see www.wikipedia.org for details
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