PHOSPHORUS SYMBOL:P PERIOD:3 GROUP:15(VA) ATOMICNO:15
PHOSPHORUS SYMBOL:P PERIOD:3 GROUP:15(VA) ATOMICNO:15
ATOMICMASS:30.97376amu. VALENCE:1,3,4,and5 OXIDATIONSTATE:+3,–3, and+5 NATURALSTATE:Solid ORIGINOFNAME:ItsnameisderivedfromtheGreekwordphosphoros,whichmeans “bringeroflight”or“lightbearing.” ISOTOPES:Therearea23isotopesofphosphorus,rangingfromP-24toP-46,withhalf- livesthatrangefromafewnanosecondstoabouttwoandhalfminutes.Theonestable isotopeisphosphorus-31,whichaccountsfor100%ofthenaturalphosphorusonEarth.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=5
s2,p3
213 Properties
Guide to the Elements |
Although phosphorus is in group 15 with some other metalloids, it is usually classed as a nonmetal since it resembles nitrogen somewhat, the element above it in group 15. Both are essential to the biochemical field as vital elements to support life. Phosphorus has 10 known allotropic forms. This is an unusually high number for any element. A system of categorizing the allotropes by three colors has made it easier to keep track of them. These three colors are white, red, and black phosphorus.
White phosphorus has a white waxy appearance that turns slightly yellow with age and impurities. There are two allotropic forms of white phosphorus. The alpha (α) form has a cubic crystal structure, and the beta (β) form has a hexagonal crystalline structure. White phosphorus is extremely reactive and will spontaneously burst into flame when exposed to air at a temperature of about 35°C. It must be kept under water. But this property of spontaneous combustion has made it useful for military applications.
White phosphorus is the most useful version of the three allotropes, and it is used in processes to manufacture the other two versions of phosphorus. White phosphorus’s melting
point 44.15°C, its boiling point is 280.5°C, and its density is 1.82 c/cm 3 .
Exposing white phosphorus to a process of heat produces red phosphorus. Red phosphorus
has a density of 2.34 g/cm 3 .
Black phosphorus also starts with heating white phosphorus. The difference is that the white phosphorus is heated in the presence of a mercury catalyst and a small amount of
already-formed black phosphorus. Its density is 2.4 g/cm 3 .
Characteristics White phosphorus occurs in nature in phosphate rock. It is insoluble in water and alcohol
and will ignite spontaneously in air. It exhibits what is known as phosphorescence; that is, it glows in the dark at room temperature. White phosphorus is poisonous and must be stored under water.
Red phosphorus is less reactive than the white variety. It is not poisonous, but large amounts can explode. It is used in fireworks and matches. Black phosphorus is the only one of the three that will conduct electricity; white and red are poor conductors. Black phosphorus has no significant commercial uses.
AbundanceandSource Phosphorus is the 12th most abundant element. It makes up about 0.1% of the Earth’s crust.
Phosphorous occurs in nature in several forms, mostly as phosphates. The most common source is phosphate rock [Ca 3 (PO 4 ) 2 ] and a mineral called “apatite.” Phosphorus is found in all animal bones and teeth and in most living tissue. Phosphorous nodules are found on the ocean floor along with manganese nodules.
Most commercial phosphorus is produced in electric furnaces where the phosphate-rich minerals are heated to drive off the phosphorus as a gas, which is then condensed under water. Another process uses sulfuric acid to remove the phosphorus.
History In 1669 a German physician, Hennig Brand (1630–1692), a proponent of ancient alche-
214 | The History and Use of Our Earth’s Chemical Elements perform some other processes on it. He ended up with a distilled residue that glowed in the
dark and would burst into flames. He had discovered white phosphorus. He named it “phos- phors” (Greek for “light bearer”) after the bright early morning star that had already been given that name. Brand’s slow, smelly, ineffective procedure led to a different process that soon supplanted the old alchemist’s practice. Today, phosphorus is removed from phosphate rock,
Ca 3 (PO 4 ) 2 . In retrospect, it is amazing that Brand was able to isolate even small amounts of white phosphorus using his simple techniques. In 1841 Jöns Jakob Berzellius (1779–1884), who introduced the term “allotropy,” trans- formed white phosphorus to red phosphorous. In 1865 Johann Wilhelm Hittorf (1824–1914) was the first to produce metallic phosphorus. Brand, however, was given credit for the discov- ery of phosphorus.
CommonUses The allotropes and compounds of phosphorus have many important uses and are an
essential commercial commodity. Phosphorus is essential to all living tissue, both plant and animal. It is the main element in the compound adenosine triphosphate (ATP), the main energy source for living things.
Red phosphors are formed either by heating white phosphorus or by exposing white phos- phorus to sunlight. It is quite different from the explosive white phosphorus. For instance, when scratched on a surface, the heads of safety matches made of red phosphorus convert back to white phosphorus and ignite due to the heat of the slight friction of the match on a rough surface. Red phosphorus is also used in fireworks, smoke bombs, and pesticides and to make phosphoric acid, electroluminescent paints, and fertilizers.
Most elemental phosphorus is used to manufacture phosphoric acid, a solid that is used to produce triple-phosphate fertilizers. Some soils require large amounts of phosphorus to produce a viable crop.
Sodium tripolyphosphate is the main phosphate found in detergents. It acts as a water softener and counteracts the elements that are responsible for “hard water” while at the same time making the detergent a more effective cleaner.
Some phosphorus compounds glow in the dark as they emit light after absorbing radia- tion. This makes them useful in special fluorescent lights and the color screens of television sets and computers.
ExamplesofCompounds Phosphorus-32, the most important radioisotope of phosphorus, has a half-life of 14 days.
It provides beta radiation (high-speed electrons) and is made by inserting phosphorus into nuclear reactor piles. P-32 is used as a “tag” to trace biochemical reactions in patients. It is also used to treat leukemia and skin and thyroid diseases.
Phosphorus pentasulfide (phosphoric sulfide, P 2 S 5 ) is an insecticide. It is also an additive to oils and a component of safety matches. Phosphorus trichloride (PCl 3 ) is used in the manufacture of other phosphorus compounds. It is also an insecticide, a gasoline additive, an ingredient in dyes, and a “finisher” for the surface of textiles.
Phosphoric acid (H 4 PO 4 ). Although phosphoric acid is derived from phosphate rocks, it is the 7th highest in terms of volume of chemical produced and one of the most widely used com-
215 pounds of phosphorus wherein the PO 3- 4 ion is required in the chemical reactions. It might be
Guide to the Elements |
best known as a “flavor” in soft drinks, to make solvents, and as a component of fertilizers. It has many uses, including the manufacture of soaps and detergents, sugar refining, water treatment, animal feeds, electroplating, gasoline additive, and binder for foods, to name a few.
Hazards Many of the compounds of phosphorus are extremely dangerous, both as fire hazards
and as deadly poisons to the nervous system of humans and animals. Some of the poisonous compounds (PCl x ) can be absorbed by the skin as well as inhaled or ingested. Flushing with water is the only way to stop the burning of white phosphorus on the skin, but water does not affect the combustion of some phosphorus compounds. Although red phosphorus is not as dangerous or poisonous as white phosphorus, merely applying some frictional heating will induce the red allotrope to change back to the explosive white allotrope (the striking of a safety match is an example).
Some of the main types of poisonous gases used in warfare have a phosphorus base. Many countries stockpile these gases, but, by agreement, the supplies are being reduced.
ARSENIC SYMBOL:As PERIOD:4 GROUP:15(VA) ATOMICNO:33
ATOMICMASS:74.92158amu VALENCE:2,3,and5 OXIDATIONSTATE:+3,–3,and +5 NATURALSTATE:Solid ORIGINOFNAME: DerivedeitherfromtheLatinwordarsenicumortheGreekwordarse- nikon,bothmeaningayellowpigment.ItispossiblethattheArabicwordazzernikhwas alsoanancientnameforarsenic.
ISOTOPES:Thereareatotalof35isotopesofarsenic,rangingfromAs-60toAs-92,with half-livesspanningfromafewnanosecondsto80days.Althoughsomereferencesclaim therearenostableisotopesofarsenic,arsenic-75isclassedasastableisotopethat makesup100%ofarsenicfoundintheEarth’scrust.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=5
s2,p3
Properties Arsenic is classed as a semimetal, meaning that it is neither a metal like aluminum or lead,
nor quite a nonmetal such as oxygen, sulfur, or chlorine. Arsenic’s main allotrope is a silvery- gray, brittle, metal-like substance. Its other two isotopes are unstable crystalline substances.
216 | The History and Use of Our Earth’s Chemical Elements
Gray arsenic exhibits an unusual property in that its boiling point (614°C) is lower than its melting point (817°C). As its temperature changes, it sublimates, which means it goes from the solid state, skipping the liquid state, into a vapor state. Cooling the vapor of sublimation, the black allotrope condenses out and in turn changes from the black to the gray allotrope. If yellow arsenic is rapidly cooled from its sublimation point, yellow arsenic will condense out and will not revert back to gray arsenic upon cooling.
The following information is for the gray semimetal form of arsenic only. Its melting point is 817°C, its sublimation point varies between 613°C and 814°C depending on the atmo-
spheric pressure, and its density is 5.776 g/cm 3 .
Characteristics Arsenic in the elemental form is a brittle, grayish crystal that becomes darker when exposed
to air. It is seldom found in the pure elemental form but rather in minerals (compounds). It has a long history of use as a poison, and many alchemists were poisoned when using it in their attempts to produce gold from base metals.
Arsenic has limited commercial use. AbundanceandSource
Arsenic is the 53rd most abundant element and is widely distributed in the Earth’s crust. It occurs naturally in several minerals, but high-grade deposits are rare. Most of the minerals and ores that contain arsenic also contain other metals. Some major sources of arsenic are the minerals orpiment, scherbenkobalt, arsenopyrite, niccolite, realgar, gersdorffite, and smaltite. In addition, most sulfide ores of other metals also contain some arsenic. The three major min- erals that produce arsenic are: realgar (arsenic monosulfide, AsS), orpiment (arsenic trisulfide,
As 2 S 2 ), and arsenopyrite (iron arsenosulfide, FeAsS). Today, most arsenic is recovered as a by-product from the smelting of nickel, copper, iron, and tin. It is also recovered from the flue dust of copper- and lead-smelting furnaces.
History In about 3600 BCE, ores containing both arsenic and copper were known and mined by
the early Greeks and Romans, as well as by Chinese alchemists. This is about the time when copper was smelted and alloyed to make bronze. Some ores of copper produced harder metals than others because of impurities. One of these impurities was arsenic. Because the workers were becoming ill when smelting these types of ore, the process was abandoned, and tin was added to copper to form bronze. “Bronze” may have been the Persian (Iranian) word for “copper.”
The honor for discovering the element arsenic in or about 1250 goes to a German alche- mist, Albertus Magnus (1193–1280). He was also the first to propose the concepts of affinitas, which explained how chemicals were held together. In addition, he learned how to separate several compounds into their constituent elements.
CommonUses Over the years a number of practical uses for arsenic developed, particularly related to
its poisonous nature. Today, it is not of great commercial value except as an insecticide and herbicide.
217 It is used in the semiconductor industry to coat solid-state devices. Some compounds are
Guide to the Elements |
used in paints and fireworks. The major uses are in medicine, where its toxic properties are important for the treatment of diseases.
At the beginning of the twentieth century, arsenic compounds were used to kill spirochete bacteria, which cause the sexually transmitted disease syphilis. After this amphetamine com- pound was used to treat syphilis in Europe, the disease rate was reduced by more than half. The antibiotic penicillin has replaced arsenic for most medical purposes
The compound arsphenamine is also called “compound 606” because it was the 606th arsenic compound that Paul Ehrlich (1854–1915) had synthesized for use in treating diseases. His assistant found this compound to be effective as a treatment for syphilis. Ehrlich also coined the word “chemotherapy.”
Historically, arsenic was one of the “poisons of choice” because it killed slowly and mim- icked many other common ailments, such as gastrointestinal diseases, that could not easily
be diagnosed and treated. Once autopsies started being generally performed and it became possible to identify the presence of arsenic as the culprit in the death of the patient, arsenic lost its “luster” as the ideal poison.
ExamplesofCompounds Arsenic disulfide (AsS) is also known as ruby arsenic because it is a reddish-orange powder.
It is used as a depilatory agent, a paint pigment, and a rat poison and to make red glass and fireworks.
Arsenic trichloride (AsCl 3 ), also known as arsenic chloride, is used in the pharmaceutical industry and to make insecticides and ceramics. Arsine (AsH 3 ), as a colorless gas, is also known as arsenic hydride. It is used to synthesize organic compounds and as the major ingredient of several military poisons, including the wartime gas lewisite.
There are two important oxides of arsenic: Arsenic trioxide (As 2 O 3 ) is found in nature as arsenolite and claudetite. It is extremely poisonous. Arsenic pentoxide (As 2 O 5 ) is manufactured by adding oxygen to the trioxide form and is used to form many arsenic compounds.
Hazards Most of the compounds of arsenic are toxic when in contact with the skin, when inhaled,
or when ingested. As with arsenic’s cousin phosphorus above it in group 15 of the periodic table, care must be taken when using arsenic. The compound arsenic trioxide (As 2 O 3 ), an excellent weed-killer, is also carcinogenic. Copper acetoarsenite, known as Paris green, is used to spray cotton for boll weevils. A poisonous dose of arsenic as small as 60 milligrams can be detected within the body by using the Marsh test.