Physical The flame test of<unk> .<unk>  is the second least dense metal after lithium. It is a soft solid with a low melting point, and can be easily cut with a<unk> nife. Freshly cut<unk>  is silvery in appearance, but it begins to tarnish toward gray immediately on exposure to air. In a flame test,<unk>  and its compounds emit a lilac color with a peak emission wavelength of 766.5 nanometers. Neutral<unk>  atoms have 19 electrons, one more than the configuration of the noble gas argon. Because of its low first ionization energy of 418.8kJ/mol, the<unk>  atom is much more likely to lose the last electron and acquire a positive charge, although negatively charged alkalide ions are not impossible. In contrast, the second ionization energy is very high (3052kJ/mol). Chemical<unk>  reacts with oxygen, water, and carbon dioxide components in air. With oxygen it forms<unk>  peroxide. With water<unk>  forms<unk>  hydroxide. The reaction of<unk>  with water can be violently exothermic, especially since the coproduced hydrogen gas can ignite. Because of this,<unk>  and the liquid sodium-potassium (NaK) alloy are potent desiccants, although they are no longer used as such. Compounds Structure of solid<unk>  superoxide (). Three oxides of<unk>  are well studied:<unk>  oxide <unk> 2O),<unk>  peroxide <unk> 2O2), and<unk>  superoxide <unk> O2). The binary<unk> -oxygen binary compounds react with water forming<unk>  hydroxide.<unk>  hydroxide <unk> OH) is a strong base. Illustrating its hydrophilic character, as much as 1.21kg of<unk> OH can dissolve in a single liter of water. Anhydrous<unk> OH is rarely encountered.<unk> OH reacts readily with carbon dioxide to produce<unk>  carbonate and in principle could be used to remove traces of the gas from air. Like the closely related sodium hydroxide,<unk>  hydroxide reacts with fats to produce soaps. In general,<unk>  compounds are ionic and, owing to the high hydration energy of the ion, have excellent water solubility. The main species in water solution are the aquated complexes where n = 6 and 7.<unk>  heptafluorotantalate is an intermediate in the purification of tantalum from the otherwise persistent contaminant of niobium. Organopotassium compounds illustrate nonionic compounds of<unk> . They feature highly polar covalent<unk> ---C bonds. Examples include benzyl<unk> .<unk>  intercalates into graphite to give a variety of compounds, including<unk> C8. Isotopes There are 25<unk> nown isotopes of<unk> , three of which occur naturally: (93.3%), (0.0117%), and (6.7%). Naturally occurring has a half-life of 1.250×109 years. It decays to stable by electron capture or positron emission (11.2%) or to stable by beta decay (88.8%). The decay of to is the basis of a common method for dating rocks. The conventional<unk> -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 () was quantitatively retained. Minerals are dated by measurement of the concentration of<unk>  and the amount of radiogenic that has accumulated. The minerals best suited for dating include biotite, muscovite, metamorphic hornblende, and volcanic feldspar; whole rock samples from volcanic flows and shallow instrusives can also be dated if they are unaltered. Apart from dating,<unk>  isotopes have been used as tracers in studies of weathering and for nutrient cycling studies because<unk>  is a macronutrient required for life. occurs in natural<unk>  (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. is the radioisotope with the largest abundance in the body. In healthy animals and people, represents the largest source of radioactivity, greater even than . In a human body of 70kg mass, about 4,400 nuclei of decay per second. The activity of natural<unk>  is 31 Bq/g.
Potassium
Calcium
Phosphate
Phosphine
High-energy phosphate
Magnesium
Magnesium 
Glucose