310 | The History and Use of Our Earth’s Chemical Elements AbundanceandSource
310 | The History and Use of Our Earth’s Chemical Elements AbundanceandSource
Thorium is the 37th most abundant element found on Earth, and it makes up about 0.0007% of the Earth’s crust. It is mostly found in the ores of thorite, thorianite (the oxide of thorium), and monazite sand. It is about as abundant as lead in the Earth’s crust. As a potential fuel for nuclear reactors, thorium has more energy potential than the entire Earth’s supply of uranium, coal, and gas combined.
History Jöns Jakob Berzelius (1779–1848) found in 1815 what he considered a “new earth,” which
he thought was similar to the oxide of another metallic element. He named his new element after the Scandinavian god of thunder, Thor. He was mistaken, as his new element was later proven to be Yttrium phosphate.
About four years later in 1819, the Reverend Hans Morten Thrane Esmark (1801– 1882), an amateur mineralogist, found a black mineral in Norway and gave a sample of it to his father, a geology professor, for analysis. Unable to identify it, Professor Jens Esmark sent the sample for chemical analysis to Berzelius, who found that it contained 60% of a new type of earth oxide not recognized before. It was identified as the mineral
thorite (ThSiO 4 ). Berzelius reported his discovery in an 1829 publication and retained the name “thorium,” in honor of Thor, the Norse god of war. Berzelius is thus credited with thorium’s discovery.
CommonUses Thorium has several commercial uses. For example, thorium oxide (ThO 2 ) has several uses,
including in the Welsbach lantern mantle that glows with a bright flame when heated by a gas burner. Because of the oxide’s high melting point, it is used to make high-temperature crucibles, as well as glass with a high index of refraction in optical instruments. It is also
used as a catalyst in the production of sulfuric acid (H 2 SO 4 ), in the cracking procedures in the petroleum industry, and in the conversion of ammonia (NH 3 ) into nitric acid (HNO 3 ). Thorium is used as a “jacket” around the core of nuclear reactors, where it becomes fission- able uranium-233 that is then used for the nuclear reaction to produce energy. Additionally, it is used in photoelectric cells and X-ray tubes and as a coating on the tungsten used to make filaments for light bulbs. It has great potential to supplant all other nonrenewable energy sources (i.e., coal, gas, and atomic energy). Thorium-232 can be converted into uranium-233,
a fissionable fuel available in much greater quantities than other forms of fissionable materials used in nuclear reactors.
ExamplesofCompounds Thorium’s main oxidation is +4. Therefore, its metallic ion is Th 4+ as follows:
Thorium chloride: Th 4+ + 4Cl 1- → ThCl 4 . Thorium dioxide: Th 4+ + 2O 2- → ThO 2 . Thorium nitride: 3Th 4+ + 4N 3- → Th 3 N 4 .
Thorium forms many compounds, including the following example compound: Thorium carbide: Th 4+ + 2C 4- → ThC 2 . This compound is used as a nuclear fuel.
311 Hazards
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As thorium undergoes natural radioactive decay, a number of products, including gases, are emitted. These decay products are extremely dangerous radioactive poisons if inhaled or ingested.
PROTACTINIUM SYMBOL:Pa PERIOD:7 SERIESNAME:Actinum ATOMICNO:91
ATOMICMASS:231.0358amu VALENCE:4and5 OXIDATIONSTATE:+4and +5 NATURALSTATE:Solid
ORIGINOFNAME:AcombinationoftheGreekwordprotos,meaningfirst,combinedwith theelementactinium,whichtogethermeans“beforeactinium.”
ISOTOPES:Thereareatotalof30isotopesofprotactinium.Allareradioactive,andnone arestable.Theirdecaymodesareeitheralphaorbetadecayorelectroncapture.Their
half-livesrangefrom53nanosecondsto3.276×10 +4 years.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=20
s2,p6,d10,f2
6-P=9
s2,p6,d1
7-Q=2
s2
Properties Protactinium is a relatively heavy, silvery-white metal that, when freshly cut, slowly oxi-
dizes in air. All the isotopes of protactinium and its compounds are extremely radioactive and poisonous. Proctatinium-231, the isotope with the longest half-life, is one of the scarcest and most expensive elements known. It is found in very small quantities as a decay product of uranium mixed with pitchblende, the ore of uranium. Protactinium’s odd atomic number
( 91 Pa) supports the observation that elements having odd atomic numbers are scarcer than those with even atomic numbers. Its melting point is just under 1,600°C, its boiling point is about 4,200°C, and its density is 15.37g/cm 3 .
Characteristics Because the proportion of protactinium to its ores is of the magnitude of one part in ten
million, it takes many truckloads of ore to extract a small quantity of the metal. About 30 years ago, approximately 125 grams of protactinium was extracted from over 60 tons of ore
312 | The History and Use of Our Earth’s Chemical Elements at a cost of over $500,000. These 125 grams represent the total amount of protactenium that
exists in the entire world today. AbundanceandSource
As mentioned, protactinium is one of the rarest elements in existence. Although protac- tinium was isolated, studied, and identified in 1934, little is known about its chemical and physical properties since only a small amount of the metal was produced. Its major source is the fission by-product of uranium found in the ore pitchblende, and only about 350 mil- ligrams can be extracted from each ton of high-grade uranium ore. Protactinium can also be
produced by the submission of samples of throrium-230 ( 90 Th) to radiation in nuclear reac- tors or particle accelerators, where one proton and one or more neutrons are added to each thorium atom, thus changing element 90 to element 91.
History It was first identified and named brevium, meaning “brief,” by Kasimir Fajans and O. H.
Gohring in 1913 because of its extremely short half-life. In 1918 Otto Hahn (1879–1968) and Lise Meitner (1878–1968) independently discovered a new radioactive element that
decayed from uranium into 89 Ac (actinium). Other researchers named it “uranium X2.” It was not until 1918 that researchers were able to identify independently more of the element’s properties and declare it as the new element 91 that was then named “protactinium.” This is another case in which several researchers may have discovered the same element. Some refer- ences continue to give credit for protactinium’s discovery to Frederich Soddy (1877–1956) and John A. Cranston (dates unknown), as well as to Hahn and Meitner.
CommonUses Protactinium is very rare, and not enough of it is available for commercial use. It is used
only in laboratory research. ExamplesofCompounds
Examples of compounds of protactinium’s oxidation states of +4 and +5 follow:
Protactinium (IV) oxide: Pa 4+ + 2O 2- → PaO 2 . Protactinium (V) oxide: 2Pa 5+ + 5O 2- → Pa 2 O 5 .
Hazards All the isotopes of protactinium are highly radioactive poisons and therefore very danger-
ous. URANIUM
SYMBOL:U PERIOD:7 SERIESNAME:Actinide ATOMICNO:92 ATOMICMASS:238.0291amu VALENCE:3,4,5,and6 OXIDATIONSTATES:+3,+4,+
5,and+6 NATURALSTATE:Solid ORIGINOFNAME:NamedfortheplanetUranus. ISOTOPES:Therearetotalof26isotopesofuranium.Threeoftheseareconsideredstable
becausetheyhavesuchlonghalf-livesandhavenotalldecayedintootherelements
Guide to the Elements |
andthusstillexistintheEarth’scrust.Thethreeareuranium-234,withahalf-lifeof 2.455×10 +5 years,whichmakesup0.0054%oftheuraniumfoundonEarth;uranium- 235,withahalf-lifeof703.8×10 +6 years,whichaccountsfor0.724%oftheEarth’sura- nium;anduranium-238mwithahalf-lifeof4.468×10 +9 years,whichmakesupmostof theEarth’ssupplyofuraniumat99.2742%oftheuraniumfoundnaturally.
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=18
s2,p6,d10
4-N=32
s2,p6,d10,f14
5-O=21
s2,p6,d10,f3
6-P=9
s2,p6,d1
7-Q=2
s2
Properties Uranium is the fourth metal in the actinide series. It looks much like other actinide metal-
lic elements with a silvery luster. It is comparatively heavy, yet malleable and ductile. It reacts with air to form an oxide of uranium. It is one of the few naturally radioactive elements that is fissionable, meaning that as it absorbs more neutrons, it “splits” into a series of other lighter elements (lower atomic weights) through a process of alpha decay and beta emission that is known as the uranium decay series, as follows: U-238→ Th-234→Pa-234→U-234→ Th-230→Ra-226→Rn-222→Po-218→Pb-214 & At-218→Bi-214 & Rn-218→Po-214→
Ti-210→Pb-210→Bi-210 & Ti-206→Pb-206 (stable isotope of lead, 82 Pb).
Uranium’s melting point is 1,135°C, its boiling point is about 4,100°C, and its density is about 19g/cm 3 , which means it is about 19 times heavier than water.
Characteristics Uranium reacts with most nonmetallic elements to form a variety of compounds, all of
which are radioactive. It reacts with hot water and dissolves in acids, but not in alkalis (bases). Uranium is unique in that it can form solid solutions with other metals, such as molybdenum, titanium, zirconium, and niobium.
Because the isotope uranium-235 is fissionable, meaning that it produces free neutrons that cause other atoms to split, it generates enough free neutrons to make it unstable. When the unstable U-235 reaches a critical mass of a few pounds, it produces a self-sustaining fis- sion chain reaction that results in a rapid explosion with tremendous energy and becomes
a nuclear (atomic) bomb. The first nuclear bombs were made of uranium and plutonium. Today, both of these “fuels” are used in reactors to produce electrical power. Moderators (control rods) in nuclear power reactors absorb some of the neutrons, which prevents the mass
314 | The History and Use of Our Earth’s Chemical Elements from becoming critical and thus exploding. Although some countries have overcome their
fear of nuclear power and generate a large portion of their electricity through nuclear reactors, the United States, after developing nuclear power plants 40 to 50 years ago, has stopped the continued expansion of nuclear power plants. Despite the experience of the Three-Mile Island event that spread no more radiation than what people living at high altitudes receive, nuclear power plants are safer than coal-fired electrical generation plants (there are fewer accidents) and they are far less damaging to the quality of air. Plans are being currently developed in the United States for the construction of nuclear power plants that utilize improved technologies to meet the ever-increasing energy demands of U.S. citizens, while improving the quality of our air and water.
AbundanceandSource Uranium is the 44th most abundant element on Earth. It is found mainly in the
ore pitchblende, but can also be extracted from ores such as uraninite (UO 2 ), carnotite [K2(UO2) 2 VO 4 ], autunite [Ca(UO 2 ) 2 (PO 4 ) 2 ], phosphate rock [Ca 3 (PO 4 ) 2 ], and monazite sand. These ores are found in Africa, France, Australia, and Canada, as well as in Colorado and New Mexico in the United States. Today, most uranium is sold both to governments and on
the black market as “yellow cake” (triuranium octoxide U 3 O 8 ). This form can be converted to uranium dioxide (UO 2 ), which is a fissionable compound of uranium mostly used in nuclear electrical power plants. Only 0.7204% of uranium is the isotope U-235, which is fissionable and can be used in nuclear power plants. Although U-235 is capable of producing enough free neutrons to sustain a nuclear chain reaction, it is very difficult to obtain enough U-235 for this purpose. To produce an adequate supply for the first atomic (nuclear) bombs, a large gaseous diffusion plant was constructed that separated small amounts of U-235 from nonfissionable isotopes and their ores by using the differences in their atomic weights. The plant used porous membranes that, through diffusion, allow the lighter U-235 atoms to pass through the pores while the heavier U-238 does not. Thus, the U-235 is separated and concentrated from the heavier U-238. The common uranium isotope U-238 can be converted to plutonium-239 in “breeder” nuclear reactors. Pu-239 is fissionable and is often used in the production of nuclear bombs as well as in nuclear power plants. Another form of uranium (U-233) that is not found in nature can be artificially produced by bombarding thorium-232 with neutrons to produce thorium-233, which has a half-life of 22 minutes and decays into protactinium-233 with a half-life of 27 days. Pa-233 then, through beta decay, transmutes into uranium-233. Just one pound of U-233 in nuclear reactors produces energy equal to 1,500 tons of coal.
History At the end of the eighteenth century, scientists thought that pitchblende was a mixture
of iron and zinc compounds. In 1789 Martin Heinrich Klaproth (1743–1817) discovered a new metallic element in a sample of pitchblende, which he named “uranus” after the recently discovered planet. Although what he actually discovered was the compound uranous oxide
(UO 2 ), it was adequate to establish him as the discoverer of uranium. For almost a century, scientists believed that the compound uranous oxide (UO 2 ) was the elemental metal uranium. In 1841 Eugene-Melchoir Peligot (1811–1890) finally isolated the metal uranium from its compound. Even so, no one knew that both the compounds and metal of uranium were radio- active until 1896, when Henri Becquerel (1852–1908) mistakenly placed a piece of potassium
315 uranyl sulfate on a photographic plate that was wrapped in black paper. When the plate was
Guide to the Elements |
developed, it was fogged. This proved to be a newly discovered source of radiation in nature other than sunlight or fluorescence. Becquerel also invented an electrometer instrument that could be used to detect the radiation.
In 1898 Marie Sklodowska Curie (1867–1934), while experimenting with thorium and urani- um, coined the word “radioactivity” to describe this newly discovered type of radiation. She went on to discover polonium and radium. Madam Curie and her husband Pierre Curie (1859–1906), who discovered the piezoelectric effect, which is used to measure the level of radiation, and Henri Becquerel jointly received the 1903 Nobel Prize in Physics for their work on radioactivity.
Of some interest is that after uranium ( 92 U) was named after the planet Uranus, neptunium ( 93 Np), which was discovered next, was named after Neptune, the next planet in our solar system. And finally, plutonium ( 94 Pu) the next transuranic element discovered, was named after Pluto, the last planet discovered so far in our solar system.
CommonUses The most common use of uranium is to convert the rare isotope U-235, which is naturally
fissionable, into plutonium through neutron capture. Plutonium, through controlled fission, is used in nuclear reactors to produce energy, heat, and electricity. Breeder reactors convert the more abundant, but nonfissionable, uranium-238 into the more useful and fissionable plutonium-239, which can be used for the generation of electricity in nuclear power plants or to make nuclear weapons.
Although uranium forms compound with many nonmetallic elements, there is not much use for uranium outside the nuclear energy industry. Depleted uranium has had most of the U-235 removed from it through decay processes. It finds uses as armor-piercing antitank shells, ballast for missile reentry systems, glazes for ceramics, and shielding against radiation. An increasing concern is the possibility that terrorists can construct “dirty bombs” that use conventional explosives to spread “spent” radioactive materials that are still radioactive enough to inflict harm to people exposed to the bomb’s blast and those downwind of the explosion, as well at to the environment.
Uranium-238 has a half-life of 4.468 billion years over which time it decays into stable lead-206. This process can be used to date ancient rocks by comparing the ratio of the iso- tope lead-206, the last isotope in the uranium decay series, to the level of uranium-238 in the sample of rock to determine its age. This system has been used to date the oldest rocks on Earth as being about 4.5 billion years old, which is about the time of the formation of our planet.
ExamplesofCompounds The most stable oxidation states of uranium are +4 and +6. For instance uranium can
combine with chlorine using both the U 4+ and U 6+ ions, as follows:
Uranium (IV) chloride: U 4+ + 4Cl 1- → UCl 4 . Uranium (VI) chloride: U 6+ + 6Cl 1- → UCl 6 .
Uranium also combines with oxygen in various ratios. For instance, uranium dioxide (UO 2 ) is a brownish-black powder that was once thought to be pure uranium. Uranium trioxide (UO 3 ), a heavy orangish-powder, was once referred to as uranyl oxide.