abstract Elements = Merge ** {

  --  Silvery radioactive metallic element, belongs to 
  -- group 3 of the periodic table. The most stable isotope, Ac_227, has a 
  -- half_life of 217 years. Ac_228 (half_life of 6.13 hours) also occurs in 
  -- nature. There are 22 other artificial isotopes, all radioactive and 
  -- having very short half_lives. Chemistry similar to lanthanum. Used as a 
  -- source of alpha particles. Discovered by A. Debierne in 1899.
  fun Actinium : Class ;
  fun Actinium_Class : SubClass Actinium ElementalSubstance ;

  -- Silvery_white lustrous metallic element of group 
  -- 3 of the periodic table. Highly reactive but protected by a thin 
  -- transparent layer of the oxide which quickly forms in air. There are many 
  -- alloys of aluminum, as well as a good number of industrial uses. Makes up 
  -- 8.1 percent of the Earth's crust, by weight. Isolated in 1825 by H.C. 
  -- Oersted.
  fun Aluminum : Class ;
  fun Aluminum_Class : SubClass Aluminum ElementalSubstance ;

  -- Radioactive metallic transuranic element, 
  -- belongs to the actinoids. Ten known isotopes. Am_243 is the most stable 
  -- isotope, with a half_life of 7.95*10^3 years. Discovered by Glenn T. 
  -- Seaborg and associates in 1945, it was obtained by bombarding 
  -- {uranium}_238 with alpha particles.
  fun Americium : Class ;
  fun Americium_Class : SubClass Americium ElementalSubstance ;

  -- Element of group 15. Multiple allotropic forms. 
  -- The stable form of antimony is a blue_white metal. Yellow and black 
  -- antimony are unstable non_metals. Used in flame_proofing, paints, 
  -- ceramics, enamels, and rubber. Attacked by oxidizing acids and halogens. 
  -- First reported by Tholden in 1450.
  fun Antimony : Class ;
  fun Antimony_Class : SubClass Antimony ElementalSubstance ;

  -- Monatomic noble gas. Makes up 0.93 percent of the 
  -- air. Colourless, odorless. Is inert and has no true compounds. Lord 
  -- Rayleigh and Sir william Ramsey identified argon in 1894.
  fun Argon : Class ;
  fun Argon_Class : SubClass Argon ElementalSubstance ;

  -- Metalloid element of group 15. There are three 
  -- allotropes, yellow, black, and grey. Reacts with halogens, concentrated 
  -- oxidizing acids and hot alkalis. Albertus Magnus is believed to have been 
  -- the first to isolate the element in 1250.
  fun Arsenic : Class ;
  fun Arsenic_Class : SubClass Arsenic ElementalSubstance ;

  -- Radioactive halogen element. Occurs naturally 
  -- from uranium and thorium decay. At least 20 known isotopes. At_210, the 
  -- most stable, has a half_life of 8.3 hours. Synthesized by nuclear 
  -- bombardment in 1940 by D.R. Corson, K.R. MacKenzie and E. Segre at the 
  -- University of California.
  fun Astatine : Class ;
  fun Astatine_Class : SubClass Astatine ElementalSubstance ;

  -- Silvery_white reactive element, belonging to group 
  -- 2 of the periodic table. Soluble barium compounds are extremely 
  -- poisonous. Identified in 1774 by Karl Scheele and extracted in 1808 by 
  -- Humphry Davy.
  fun Barium : Class ;
  fun Barium_Class : SubClass Barium ElementalSubstance ;

  -- Radioactive metallic transuranic element. 
  -- Belongs to actinoid series. Eight known isotopes, the most common Bk_247, 
  -- has a half_life of 1.4*10^3 years. First produced by Glenn T. Seaborg 
  -- and associates in 1949 by bombarding americium_241 with alpha 
  -- particles.
  fun Berkelium : Class ;
  fun Berkelium_Class : SubClass Berkelium ElementalSubstance ;

  -- Grey metallic element of group 2 of the periodic 
  -- table. Is toxic and can cause severe lung diseases and dermatitis. Shows 
  -- high covalent character. It was isolated independently by F. Wohler and 
  -- A.A. Bussy in 1828.
  fun Beryllium : Class ;
  fun Beryllium_Class : SubClass Beryllium ElementalSubstance ;

  -- White crystalline metal with a pink tinge, belongs 
  -- to group 15. Most diamagnetic of all metals and has the lowest thermal 
  -- conductivity of all the elements except mercury. Lead_free bismuth 
  -- compounds are used in cosmetics and medical procedures. Burns in the air 
  -- and produces a blue flame. In 1753, C.G. Junine first demonstrated that 
  -- it was different from lead.
  fun Bismuth : Class ;
  fun Bismuth_Class : SubClass Bismuth ElementalSubstance ;

  -- An element of group 13 of the periodic table. There 
  -- are two allotropes, amorphous boron is a brown power, but metallic boron 
  -- is black. The metallic form is hard (9.3 on Mohs' scale) and a bad 
  -- conductor in room temperatures. It is never found free in nature. 
  -- Boron_10 is used in nuclear reactor control rods and shields. It was 
  -- discovered in 1808 by Sir Humphry Davy and by J.L. Gay_Lussac and L.J. 
  -- Thenard.
  fun Boron : Class ;
  fun Boron_Class : SubClass Boron ElementalSubstance ;

  -- Halogen element. Red volatile liquid at room 
  -- temperature. Its reactivity is somewhere between chlorine and iodine. 
  -- Harmful to human tissue in a liquid state, the vapour irritates eyes and 
  -- throat. Discovered in 1826 by Antoine Balard.
  fun Bromine : Class ;
  fun Bromine_Class : SubClass Bromine ElementalSubstance ;

  -- Soft bluish metal belonging to group 12 of the 
  -- periodic table. Extremely toxic even in low concentrations. Chemically 
  -- similar to zinc, but lends itself to more complex compounds. Discovered 
  -- in 1817 by F. Stromeyer.
  fun Cadmium : Class ;
  fun Cadmium_Class : SubClass Cadmium ElementalSubstance ;

  -- Soft silvery_white metallic element belonging to 
  -- group 1 of the periodic table. One of the three metals which are liquid 
  -- at room temperature. Cs_133 is the natural, and only stable, isotope. 
  -- Fifteen other radioisotopes exist. Caesium reacts explosively with cold 
  -- water, and ice at temperatures above 157K. Caesium hydroxide is the 
  -- strongest base known. Caesium is the most electropositive, most alkaline 
  -- and has the least ionization potential of all the elements. Known uses 
  -- include the basis of atomic clocks, catalyst for the hydrogenation of some 
  -- organic compounds, and in photoelectric cells. Caesium was discovered by 
  -- Gustav Kirchoff and Robert Bunsen in Germany in 1860 spectroscopically. 
  -- Its identification was based upon the bright blue lines in its spectrum. 
  -- The name comes from the latin word caesius, which means sky blue. Caesium 
  -- should be considered highly toxic. Some of the radioisotopes are even 
  -- more toxic.
  fun Caesium : Class ;
  fun Caesium_Class : SubClass Caesium ElementalSubstance ;

  -- Soft grey metallic element belonging to group 2 of 
  -- the periodic table. Used a reducing agent in the extraction of thorium, 
  -- zirconium and uranium. Essential element for living organisms.
  fun Calcium : Class ;
  fun Calcium_Class : SubClass Calcium ElementalSubstance ;

  -- Radioactive metallic transuranic element. 
  -- Belongs to actinoid series. Cf_251 has a half life of about 700 years. 
  -- Nine isotopes are known. Cf_252 is an intense {neutron} source, which 
  -- makes it an intense {neutron} source and gives it a use in {neutron} 
  -- activation analysis and a possible use as a radiation source in medicine. 
  -- First produced by Glenn T. Seaborg and associates in 1950.
  fun Californium : Class ;
  fun Californium_Class : SubClass Californium ElementalSubstance ;

  -- Carbon is a member of group 14 of the periodic 
  -- table. It has three allotropic forms of it, diamonds, graphite and 
  -- fullerite. Carbon_14 is commonly used in radioactive dating. Carbon 
  -- occurs in all organic life and is the basis of organic chemistry. Carbon 
  -- has the interesting chemical property of being able to bond with itself, 
  -- and a wide variety of other elements.
  fun Carbon : Class ;
  fun Carbon_Class : SubClass Carbon ElementalSubstance ;

  -- Silvery metallic element, belongs to the 
  -- lanthanoids. Four natural isotopes exist, and fifteen radioactive 
  -- isotopes have been identified. Used in some rare_earth alloys. The 
  -- oxidized form is used in the glass industry. Discovered by Martin .H. 
  -- Klaproth in 1803.
  fun Cerium : Class ;
  fun Cerium_Class : SubClass Cerium ElementalSubstance ;

  -- Halogen element. Poisonous greenish_yellow gas. 
  -- Occurs widely in nature as sodium chloride in seawater. Reacts directly 
  -- with many elements and compounds, strong oxidizing agent. Discovered by 
  -- Karl Scheele in 1774. Humphrey David confirmed it as an element in 
  -- 1810.
  fun Chlorine : Class ;
  fun Chlorine_Class : SubClass Chlorine ElementalSubstance ;

  -- Hard silvery transition element. Used in 
  -- decorative electroplating. Discovered in 1797 by Vauquelin.
  fun Chromium : Class ;
  fun Chromium_Class : SubClass Chromium ElementalSubstance ;

  -- Light grey transition element. Some meteorites 
  -- contain small amounts of metallic cobalt. Generally alloyed for use. 
  -- Mammals require small amounts of cobalt salts. Cobalt_60, an artificially 
  -- produced radioactive isotope of Cobalt is an important radioactive tracer 
  -- and cancer_treatment agent. Discovered by G. Brandt in 1737.
  fun Cobalt : Class ;
  fun Cobalt_Class : SubClass Cobalt ElementalSubstance ;

  -- Red_brown transition element. Known by the Romans 
  -- as 'cuprum.' Extracted and used for thousands of years. Malleable, 
  -- ductile and an excellent conductor of heat and electricity. When in moist 
  -- conditions, a greenish layer forms on the outside.
  fun Copper : Class ;
  fun Copper_Class : SubClass Copper ElementalSubstance ;

  -- Radioactive metallic transuranic element. Belongs 
  -- to actinoid series. Nine known isotopes, Cm_247 has a half_life of 
  -- 1.64*10^7 years. First identified by Glenn T. Seaborg and associates in 
  -- 1944, first produced by L.B. Werner and I. Perlman in 1947 by bombarding 
  -- americium_241 with {neutron}s. Named for Marie Curie.
  fun Curium : Class ;
  fun Curium_Class : SubClass Curium ElementalSubstance ;

  -- Metallic with a bright silvery_white lustre. 
  -- Dysprosium belongs to the lanthanoids. It is relatively stable in air at 
  -- room temperatures, it will however dissolve in mineral acids, evolving 
  -- hydrogen. It is found in from rare_earth minerals. There are seven 
  -- natural isotopes of dysprosium, and eight radioisotopes, Dy_154 being the 
  -- most stable with a half_life of 3*10^6 years. Dysprosium is used as a 
  -- neutron absorber in nuclear fission reactions, and in compact disks. It 
  -- was discovered by Paul Emile Lecoq de Boisbaudran in 1886 in France. Its 
  -- name comes from the Greek word dysprositos, which means hard to obtain.
  fun Dysprosium : Class ;
  fun Dysprosium_Class : SubClass Dysprosium ElementalSubstance ;

  -- Appearance is unknown, however it is most 
  -- probably metallic and silver or gray in color. Radioactive metallic 
  -- transuranic element belonging to the actinoids. Es_254 has the longest 
  -- half_life of the eleven known isotopes at 270 days. First identified by 
  -- Albert Ghiorso and associates in the debris of the 1952 hydrogen bomb 
  -- explosion. In 1961 the first microgram quantities of Es_232 were 
  -- separated. While einsteinium never exists naturally, if a sufficient 
  -- amount was assembled, it would pose a radiation hazard.
  fun Einsteinium : Class ;
  fun Einsteinium_Class : SubClass Einsteinium ElementalSubstance ;

  -- Soft silvery metallic element which belongs to the 
  -- lanthanoids. Six natural isotopes that are stable. Twelve artificial 
  -- isotopes are known. Used in nuclear technology as a neutron absorber. It 
  -- is being investigated for other possible uses. Discovered by Carl G. 
  -- Mosander in 1843.
  fun Erbium : Class ;
  fun Erbium_Class : SubClass Erbium ElementalSubstance ;

  -- Soft silvery metallic element belonging to the 
  -- lanthanoids. Eu_151 and Eu_153 are the only two stable isotopes, both of 
  -- which are {neutron} absorbers. Discovered in 1889 by Sir William 
  -- Crookes.
  fun Europium : Class ;
  fun Europium_Class : SubClass Europium ElementalSubstance ;

  -- Radioactive metallic transuranic element, belongs 
  -- to the actinoids. Ten known isotopes, most stable is Fm_257 with a 
  -- half_life of 10 days. First identified by Albert Ghiorso and associates 
  -- in the debris of the first hydrogen_bomb explosion in 1952.
  fun Fermium : Class ;
  fun Fermium_Class : SubClass Fermium ElementalSubstance ;

  -- A poisonous pale yellow gaseous element belonging 
  -- to group 17 of the periodic table (The halogens). It is the most 
  -- chemically reactive and electronegative element. It is highly dangerous, 
  -- causing severe chemical burns on contact with flesh. Fluorine was 
  -- identified by Scheele in 1771 and first isolated by Moissan in 1886.
  fun Fluorine : Class ;
  fun Fluorine_Class : SubClass Fluorine ElementalSubstance ;

  -- Radioactive element, belongs to group 1 of the 
  -- periodic table. Found in uranium and thorium ores. The 22 known isotopes 
  -- are all radioactive, with the most stable being Fr_223. Its existence was 
  -- confirmed in 1939 by Marguerite Perey.
  fun Francium : Class ;
  fun Francium_Class : SubClass Francium ElementalSubstance ;

  -- Soft silvery metallic element belonging to the 
  -- lanthanoids. Seven natural, stable isotopes are known in addition to 
  -- eleven artificial isotopes. Gd_155 and Gd_157 and the best neutron 
  -- absorbers of all elements. Gadolinium compounds are used in electronics. 
  -- Discovered by J.C.G Marignac in 1880.
  fun Gadolinium : Class ;
  fun Gadolinium_Class : SubClass Gadolinium ElementalSubstance ;

  -- Soft silvery metallic element, belongs to group 13 
  -- of the periodic table. The two stable isotopes are Ga_69 and Ga_71. 
  -- Eight radioactive isotopes are known, all having short half_lives. 
  -- Gallium Arsenide is used as a semiconductor. Corrodes most other metals 
  -- by diffusing into their lattice. First identified by Francois Lecoq de 
  -- Boisbaudran in 1875.
  fun Gallium : Class ;
  fun Gallium_Class : SubClass Gallium ElementalSubstance ;

  -- Lustrous hard metalloid element, belongs to 
  -- group 14 of the periodic table. Forms a large number of organometallic 
  -- compounds. Predicted by Mendeleev in 1871, it was actually found in 1886 
  -- by Winkler.
  fun Germanium : Class ;
  fun Germanium_Class : SubClass Germanium ElementalSubstance ;

  -- Gold is gold colored. It is the most malleable and 
  -- ductile metal known. There is only one stable isotope of gold, and five 
  -- radioisotopes of gold, Au_195 being the most stable with a half_life of 
  -- 186 days. Gold is used as a monetary standard, in jewelry, dentistry, 
  -- electronics. Au_198 is used in treating cancer and some other medical 
  -- conditions. Gold has been known to exist as far back as 2600 BC. Gold 
  -- comes from the Anglo_Saxon word gold. Its symbol, Au, comes from the 
  -- Latin word aurum, which means gold. Gold is not particularly toxic, 
  -- however it is known to cause damage to the liver and kidneys in some.
  fun Gold : Class ;
  fun Gold_Class : SubClass Gold ElementalSubstance ;

  -- Silvery lustrous metallic transition element. 
  -- Used in tungsten alloys in filaments and electrodes, also acts as a 
  -- neutron absorber. First reported by Urbain in 1911, existence was finally 
  -- established in 1923 by D. Coster, G.C. de Hevesy in 1923.
  fun Hafnium : Class ;
  fun Hafnium_Class : SubClass Hafnium ElementalSubstance ;

  -- Colourless, odourless gaseous nonmetallic element. 
  -- Belongs to group 18 of the periodic table. Lowest boiling point of all 
  -- elements and can only be solidified under pressure. Chemically inert, no 
  -- known compounds. Discovered in the solar spectrum in 1868 by Lockyer.
  fun Helium : Class ;
  fun Helium_Class : SubClass Helium ElementalSubstance ;

  -- Relatively soft and malleable silvery_white 
  -- metallic element, which is stable in dry air at room temperature. It 
  -- oxidizes in moist air and at high temperatures. It belongs to the 
  -- lanthanoids. A rare_earth metal, it is found in the minerals monazite and 
  -- gadolinite. It possesses unusual magnetic properties. One natural 
  -- isotope, Ho_165 exists, six radioisotopes exist, the most stable being 
  -- Ho_163 with a half_life of 4570 years. Holmium is used in some metal 
  -- alloys, it is also said to stimulate the metabolism. Discovered by Per 
  -- Theodor Cleve and J.L. Soret in Switzerland in 1879. The name homium 
  -- comes from the Greek word Holmia which means Sweden. While all holmium 
  -- compounds should be considered highly toxic, initial evidence seems to 
  -- indicate that they do not pose much danger. The metal's dust however, is 
  -- a fire hazard.
  fun Holmium : Class ;
  fun Holmium_Class : SubClass Holmium ElementalSubstance ;

  -- Colourless, odourless gaseous chemical element. 
  -- Lightest and most abundant element in the universe. Present in water and 
  -- in all organic compounds. Chemically reacts with most elements. 
  -- Discovered by Henry Cavendish in 1776.
  fun Hydrogen : Class ;
  fun Hydrogen_Class : SubClass Hydrogen ElementalSubstance ;

  -- Soft silvery element belonging to group 13 of the 
  -- periodic table. The most common natural isotope is In_115, which has a 
  -- half_life of 6*10^4 years. Five other radioisotopes exist. Discovered in 
  -- 1863 by Reich and Richter.
  fun Indium : Class ;
  fun Indium_Class : SubClass Indium ElementalSubstance ;

  -- Dark violet nonmetallic element, belongs to group 
  -- 17 of the periodic table. Insoluble in water. Required as a trace 
  -- element for living organisms. One stable isotope, I_127 exists, in 
  -- addition to fourteen radioactive isotopes. Chemically the least reactive 
  -- of the halogens, and the most electropositive metallic halogen. 
  -- Discovered in 1812 by Courtois.
  fun Iodine : Class ;
  fun Iodine_Class : SubClass Iodine ElementalSubstance ;

  -- Very hard and brittle, silvery metallic transition 
  -- element. It has a yellowish cast to it. Salts of iridium are highly 
  -- colored. It is the most corrosion resistant metal known, not attacked by 
  -- any acid, but is attacked by molten salts. There are two natural isotopes 
  -- of iridium, and 4 radioisotopes, the most stable being Ir_192 with a 
  -- half_life of 73.83 days. Ir_192 decays into {platinum}, while the other 
  -- radioisotopes decay into {osmium}. Iridium is used in high temperature 
  -- apparatus, electrical contacts, and as a hardening agent for platinum. 
  -- Discovered in 1803 by Smithson Tennant in England. The name comes from 
  -- the Greek word iris, which means rainbow. Iridium metal is generally 
  -- non_toxic due to its relative unreactivity, but iridium compounds should 
  -- be considered highly toxic.
  fun Iridium : Class ;
  fun Iridium_Class : SubClass Iridium ElementalSubstance ;

  -- Silvery malleable and ductile metallic transition 
  -- element. Has nine isotopes and is the fourth most abundant element in the 
  -- earth's crust. Required by living organisms as a trace element (used in 
  -- hemoglobin in humans.) Quite reactive, oxidizes in moist air, displaces 
  -- hydrogen from dilute acids and combines with nonmetallic elements.
  fun Iron : Class ;
  fun Iron_Class : SubClass Iron ElementalSubstance ;

  -- Colorless gaseous element, belongs to the noble 
  -- gases. Occurs in the air, 0.0001 percent by volume. It can be extracted 
  -- from liquid air by fractional distillation. Generally not isolated, but 
  -- used with other inert gases in fluorescent lamps. Five natural isotopes, 
  -- and five radioactive isotopes. Kr_85, the most stable radioactive 
  -- isotope, has a half_life of 10.76 years and is produced in fission 
  -- reactors. Practically inert, though known to form compounds with 
  -- {fluorine}.
  fun Krypton : Class ;
  fun Krypton_Class : SubClass Krypton ElementalSubstance ;

  -- (From the Greek word lanthanein, to line hidden) 
  -- Silvery metallic element belonging to group 3 of the periodic table and 
  -- oft considered to be one of the lanthanoids. Found in some rare_earth 
  -- minerals. Twenty_five natural isotopes exist. La_139 which is stable, 
  -- and La_138 which has a half_life of 10^10 to 10^15 years. The other 
  -- twenty_three isotopes are radioactive. It resembles the lanthanoids 
  -- chemically. Lanthanum has a low to moderate level of toxicity, and should 
  -- be handled with care. Discovered in 1839 by C.G. Mosander.
  fun Lanthanum : Class ;
  fun Lanthanum_Class : SubClass Lanthanum ElementalSubstance ;

  -- Appearance unknown, however it is most likely 
  -- silvery_white or grey and metallic. Lawrencium is a synthetic rare_earth 
  -- metal. There are eight known radioisotopes, the most stable being Lr_262 
  -- with a half_life of 3.6 hours. Due to the short half_life of lawrencium, 
  -- and its radioactivity, there are no known uses for it. Identified by 
  -- Albert Ghiorso in 1961 at Berkeley. It was produced by bombarding 
  -- californium with boron ions. The name is temporary {IUPAC} nomenclature, 
  -- the origin of the name comes from Ernest O. Lawrence, the inventor of the 
  -- cyclotron. If sufficient amounts of lawrencium were produced, it would 
  -- pose a radiation hazard.
  fun Lawrencium : Class ;
  fun Lawrencium_Class : SubClass Lawrencium ElementalSubstance ;

  -- Heavy dull grey ductile metallic element, belongs to 
  -- group 14. Used in building construction, lead_place accumulators, bullets 
  -- and shot, and is part of solder, pewter, bearing metals, type metals and 
  -- fusible alloys.
  fun Lead : Class ;
  fun Lead_Class : SubClass Lead ElementalSubstance ;

  -- Socket silvery metal. First member of group 1 of 
  -- the periodic table. Lithium salts are used in psychomedicine.
  fun Lithium : Class ;
  fun Lithium_Class : SubClass Lithium ElementalSubstance ;

  -- Silvery_white rare_earth metal which is 
  -- relatively stable in air. It happens to be the most expensive rare_earth 
  -- metal. Its found with almost all rare_earth metals, but is very difficult 
  -- to separate from other elements. Least abundant of all natural elements. 
  -- Used in metal alloys, and as a catalyst in various processes. There are 
  -- two natural, stable isotopes, and seven radioisotopes, the most stable 
  -- being Lu_174 with a half_life of 3.3 years. The separation of lutetium 
  -- from {ytterbium} was described by Georges Urbain in 1907. It was 
  -- discovered at approximately the same time by Carl Auer von Welsbach. The 
  -- name comes from the Greek word lutetia which means Paris.
  fun Lutetium : Class ;
  fun Lutetium_Class : SubClass Lutetium ElementalSubstance ;

  -- Silvery metallic element belonging to group 2 of 
  -- the periodic table (alkaline_earth metals). It is essential for living 
  -- organisms, and is used in a number of light alloys. Chemically very 
  -- reactive, it forms a protective oxide coating when exposed to air and 
  -- burns with an intense white flame. It also reacts with sulphur, nitrogen 
  -- and the halogens. First isolated by Bussy in 1828.
  fun Magnesium : Class ;
  fun Magnesium_Class : SubClass Magnesium ElementalSubstance ;

  -- Grey brittle metallic transition element. 
  -- Rather electropositive, combines with some non_metals when heated. 
  -- Discovered in 1774 by Scheele.
  fun Manganese : Class ;
  fun Manganese_Class : SubClass Manganese ElementalSubstance ;

  -- Half_life of approximately 5ms. The creation 
  -- of this element demonstrated that fusion techniques could indeed be used 
  -- to make new, heavy nuclei. Made and identified by physicists of the Heavy 
  -- Ion Research Laboratory, Darmstadt, West Germany in 1982. Named in honor 
  -- of Lise Meitner the Austrian physicist.
  fun Meitnerium : Class ;
  fun Meitnerium_Class : SubClass Meitnerium ElementalSubstance ;

  -- Radioactive metallic transuranic element. 
  -- Belongs to the actinoid series. Only known isotope, Md_256 has a 
  -- half_life of 1.3 hours. First identified by Glenn T. Seaborg, Albert 
  -- Ghiorso and associates in 1955. Alternative name {unnilunium} has been 
  -- proposed. Named after the 'inventor' of the periodic table, Dmitri 
  -- Mendeleev.
  fun Mendelevium : Class ;
  fun Mendelevium_Class : SubClass Mendelevium ElementalSubstance ;

  -- Heavy silvery liquid metallic element, belongs to 
  -- the zinc group. Used in thermometers, barometers and other scientific 
  -- apparatus. Less reactive than zinc and cadmium, does not displace 
  -- hydrogen from acids. Forms a number of complexes and organomercury 
  -- compounds.
  fun Mercury : Class ;
  fun Mercury_Class : SubClass Mercury ElementalSubstance ;

  -- Silvery_white, hard metallic transition 
  -- element. It is chemically unreactive and is not affected by most acids. 
  -- It oxidizes at high temperatures. There are seven natural isotopes, and 
  -- four radioisotopes, Mo_93 being the most stable with a half_life of 3500 
  -- years. Molybdenum is used in almost all high_strength steels, it has 
  -- nuclear applications, and is a catalyst in petroleum refining. Discovered 
  -- in 1778 by Carl Welhelm Scheele of Sweden. Impure metal was prepared in 
  -- 1782 by Peter Jacob Hjelm. The name comes from the Greek word molybdos 
  -- which means lead. Trace amounts of molybdenum are required for all known 
  -- forms of life. All molybdenum compounds should be considered highly 
  -- toxic, and will also cause severe birth defects.
  fun Molybdenum : Class ;
  fun Molybdenum_Class : SubClass Molybdenum ElementalSubstance ;

  -- Soft bright silvery metallic element, belongs to 
  -- the lanthanoids. Seven natural isotopes, Nd_144 being the only 
  -- radioactive one with a half_life of 10^10 to 10^15 years. Six artificial 
  -- radioisotopes have been produced. The metal is used in glass works to 
  -- color class a shade of violet_purple and make it dichroic. One of the 
  -- more reactive rare_earth metals, quickly reacts with air. Used in some 
  -- rare_earth alloys. Neodymium is used to color the glass used in welder's 
  -- glasses. Neodymium is also used in very powerful, permanent magnets 
  -- (Nd2Fe14B). Discovered by Carl F. Auer von Welsbach in Austria in 1885 
  -- by separating didymium into its elemental components {praseodymium} and 
  -- neodymium. The name comes from the Greek words 'neos didymos' which means 
  -- 'new twin'. Neodymium should be considered highly toxic, however evidence 
  -- would seem to show that it acts as little more than a skin and eye 
  -- irritant. The dust however, presents a fire and explosion hazard.
  fun Neodymium : Class ;
  fun Neodymium_Class : SubClass Neodymium ElementalSubstance ;

  -- Colourless gaseous element of group 18 on the 
  -- periodic table (noble gases). Neon occurs in the atmosphere, and 
  -- comprises 0.0018 percent of the volume of the atmosphere. It has a 
  -- distinct reddish glow when used in discharge tubes and neon based lamps. 
  -- It forms almost no chemical compounds. Neon was discovered in 1898 by Sir 
  -- William Ramsey and M.W. Travers.
  fun Neon : Class ;
  fun Neon_Class : SubClass Neon ElementalSubstance ;

  -- Radioactive metallic transuranic element, 
  -- belongs to the actinoids. Np_237, the most stable isotope, has a 
  -- half_life of 2.2*10^6 years and is a by product of nuclear reactors. The 
  -- other known isotopes have mass numbers 229 through 236, and 238 through 
  -- 241. Np_236 has a half_life of 5*10^3 years. First produced by Edwin M. 
  -- McMillan and P.H. Abelson in 1940.
  fun Neptunium : Class ;
  fun Neptunium_Class : SubClass Neptunium ElementalSubstance ;

  -- Malleable ductile silvery metallic transition 
  -- element. Discovered by A.F. Cronstedt in 1751.
  fun Nickel : Class ;
  fun Nickel_Class : SubClass Nickel ElementalSubstance ;

  -- Soft, ductile grey_blue metallic transition 
  -- element. Used in special steels and in welded joints to increase 
  -- strength. Combines with halogens and oxidizes in air at 200 degrees 
  -- celsius. Discovered by Charles Hatchett in 1801 and isolated by 
  -- Blomstrand in 1864. Called {columbium} originally.
  fun Niobium : Class ;
  fun Niobium_Class : SubClass Niobium ElementalSubstance ;

  -- Colourless, gaseous element which belongs to 
  -- group 15 of the periodic table. Constitutes ~78 percent of the atmosphere 
  -- and is an essential part of the ecosystem. Nitrogen for industrial 
  -- purposes is acquired by the fractional distillation of liquid air. 
  -- Chemically inactive, reactive generally only at high temperatures or in 
  -- electrical discharges. It was discovered in 1772 by D. Rutherford.
  fun Nitrogen : Class ;
  fun Nitrogen_Class : SubClass Nitrogen ElementalSubstance ;

  -- Radioactive metallic transuranic element, belongs 
  -- to the actinoids. Seven known isotopes exist, the most stable being 
  -- No_254 with a half_life of 255 seconds. First identified with certainty 
  -- by Albert Ghiorso and Glenn T. Seaborg in 1966. {Unnilbium} has been 
  -- proposed as an alternative name.
  fun Nobelium : Class ;
  fun Nobelium_Class : SubClass Nobelium ElementalSubstance ;

  -- Hard blue_white metallic transition element. Found 
  -- with platinum and used in some alloys with platinum and iridium.
  fun Osmium : Class ;
  fun Osmium_Class : SubClass Osmium ElementalSubstance ;

  -- A colourless, odourless gaseous element belonging 
  -- to group 16 of the periodic table. It is the most abundant element 
  -- present in the earth's crust. It also makes up 20.8 percent of the 
  -- Earth's atmosphere. For industrial purposes, it is separated from liquid 
  -- air by fractional distillation. It is used in high temperature welding, 
  -- and in breathing. It commonly comes in the form of Oxygen, but is found 
  -- as Ozone in the upper atmosphere. It was discovered by Priestley in 
  -- 1774.
  fun Oxygen : Class ;
  fun Oxygen_Class : SubClass Oxygen ElementalSubstance ;

  -- Soft white ductile transition element. Found 
  -- with some copper and nickel ores. Does not react with oxygen at normal 
  -- temperatures. Dissolves slowly in hydrochloric acid. Discovered in 1803 
  -- by W.H. Wollaston.
  fun Palladium : Class ;
  fun Palladium_Class : SubClass Palladium ElementalSubstance ;

  -- Non_metallic element belonging to group 15 of 
  -- the periodic table. Has a multiple allotropic forms. Essential element 
  -- for living organisms. It was discovered by Brandt in 1669.
  fun Phosphorus : Class ;
  fun Phosphorus_Class : SubClass Phosphorus ElementalSubstance ;

  -- Attractive greyish_white metal. When pure, it is 
  -- malleable and ductile. Does not oxidize in air, insoluble in hydrochloric 
  -- and nitric acid. Corroded by halogens, cyandies, sulphur and alkalis. 
  -- {Hydrogen} and {oxygen} react explosively in the presence of platinum. 
  -- There are six stable isotopes and three radioisotopes, the most stable 
  -- being Pt_193 with a half_life of 60 years. Platinum is used in jewelry, 
  -- laboratory equipment, electrical contacts, dentistry, and anti_pollution 
  -- devices in cars. PtCl2(NH3)2 is used to treat some forms of cancer. 
  -- Platinum_{cobalt} alloys have magnetic properties. It is also used in the 
  -- definition of the Standard Hydrogen Electrode. Discovered by Antonio de 
  -- Ulloa in South America in 1735. The name comes from the Spanish word 
  -- platina which means silver. Platinum metal is generally not a health 
  -- concern due to its unreactivity, however platinum compounds should be 
  -- considered highly toxic.
  fun Platinum : Class ;
  fun Platinum_Class : SubClass Platinum ElementalSubstance ;

  -- Dense silvery radioactive metallic transuranic 
  -- element, belongs to the actinoids. Pu_244 is the most stable isotope with 
  -- a half_life of 7.6*10^7 years. Thirteen isotopes are known. Pu_239 is 
  -- the most important, it undergoes nuclear fission with slow neutrons and is 
  -- hence important to nuclear weapons and reactors. Plutonium production is 
  -- monitored down to the gram to prevent military misuse. First produced by 
  -- Gleen T. Seaborg, Edwin M. McMillan, J.W. Kennedy and A.C. Wahl in 
  -- 1940.
  fun Plutonium : Class ;
  fun Plutonium_Class : SubClass Plutonium ElementalSubstance ;

  -- Rare radioactive metallic element, belongs to 
  -- group 16 of the periodic table. Over 30 known isotopes exist, the most of 
  -- all elements. Po_209 has a half_life of 103 years. Possible uses in 
  -- heating spacecraft. Discovered by Marie Curie in 1898 in a sample of 
  -- pitchblende.
  fun Polonium : Class ;
  fun Polonium_Class : SubClass Polonium ElementalSubstance ;

  -- Soft silvery metallic element belonging to group 
  -- 1 of the periodic table (alkali metals). Occurs naturally in seawater and 
  -- a many minerals. Highly reactive, chemically, it resembles sodium in its 
  -- behavior and compounds. Discovered by Sir Humphry Davy in 1807.
  fun Potassium : Class ;
  fun Potassium_Class : SubClass Potassium ElementalSubstance ;

  -- Soft silvery metallic element, belongs to the 
  -- lanthanoids. Only natural isotope is Pr_141 which is not radioactive. 
  -- Fourteen radioactive isotopes have been artificially produced. Used in 
  -- rare_earth alloys. Discovered in 1885 by C.A. von Welsbach.
  fun Praseodymium : Class ;
  fun Praseodymium_Class : SubClass Praseodymium ElementalSubstance ;

  -- Soft silvery metallic element, belongs to the 
  -- lanthanoids. Pm_147, the only natural isotope, is radioactive and has a 
  -- half_life of 252 years. Eighteen radioisotopes have been produced, but 
  -- all have very short half_lives. Found only in nuclear decay waste. 
  -- Pm_147 is of interest as a beta_decay source, however Pm_146 and Pm_148 
  -- have to be removed from it first, as they generate gamma radiation. 
  -- Discovered by J.A. Marinsky, L.E. Glendenin and C.D. Coryell in 
  -- 1947.
  fun Promethium : Class ;
  fun Promethium_Class : SubClass Promethium ElementalSubstance ;

  -- Radioactive metallic element, belongs to the 
  -- actinoids. The most stable isotope, Pa_231 has a half_life of 2.43*10^4 
  -- years. At least 10 other radioactive isotopes are known. No practical 
  -- applications are known. Discovered in 1917 by Lise Meitner and Otto 
  -- Hahn.
  fun Protactinium : Class ;
  fun Protactinium_Class : SubClass Protactinium ElementalSubstance ;

  -- Radioactive metallic transuranic element, belongs 
  -- to group 2 of the periodic table. Most stable isotope, Ra_226 has a 
  -- half_life of 1602 years, which decays into radon. Isolated from 
  -- pitchblende in 1898 Marie and Pierre Curie.
  fun Radium : Class ;
  fun Radium_Class : SubClass Radium ElementalSubstance ;

  -- Colorless radioactive gaseous element, belongs to 
  -- the noble gases. Of the twenty known isotopes, the most stable is Rn_222 
  -- with a half_life of 3.8 days. Formed by the radioactive decay of 
  -- {Radium}_226. Radon itself decays into {polonium}. Used in radiotherapy. 
  -- As a noble gas, it is effectively inert, though radon fluoride has been 
  -- synthesized. First isolated in 1908 by Ramsey and Gray.
  fun Radon : Class ;
  fun Radon_Class : SubClass Radon ElementalSubstance ;

  -- Silvery_white metallic transition element. 
  -- Obtained as a by_product of molybdenum refinement. Rhenium_molybdenum 
  -- alloys are superconducting.
  fun Rhenium : Class ;
  fun Rhenium_Class : SubClass Rhenium ElementalSubstance ;

  -- Silvery white metallic transition element. Found 
  -- with platinum and used in some platinum alloys. Not attacked by acids, 
  -- dissolves only in aqua regia. Discovered in 1803 by W.H. Wollaston.
  fun Rhodium : Class ;
  fun Rhodium_Class : SubClass Rhodium ElementalSubstance ;

  -- Soft silvery metallic element, belongs to group 1 
  -- of the periodic table. Rb_97, the naturally occurring isotope, is 
  -- radioactive. It is highly reactive, with properties similar to other 
  -- elements in group 1, like igniting spontaneously in air. Discovered 
  -- spectroscopically in 1861 by W. Bunsen and G.R. Kirchoff.
  fun Rubidium : Class ;
  fun Rubidium_Class : SubClass Rubidium ElementalSubstance ;

  -- Hard white metallic transition element. Found 
  -- with platinum, used as a catalyst in some platinum alloys. Dissolves in 
  -- fused alkalis, and is not attacked by acids. Reacts with halogens and 
  -- oxygen at high temperatures. Isolated in 1844 by K.K. Klaus.
  fun Ruthenium : Class ;
  fun Ruthenium_Class : SubClass Ruthenium ElementalSubstance ;

  -- Soft silvery metallic element, belongs to the 
  -- lanthanoids. Seven natural isotopes, Sm_147 is the only radioisotope, and 
  -- has a half_life of 2.5*10^11 years. Used for making special alloys needed 
  -- in the production of nuclear reactors. Also used as a neutron absorber. 
  -- Small quantities of samarium oxide is used in special optical glasses. 
  -- The largest use of the element is its ferromagnetic alloy which produces 
  -- permanent magnets that are five times stronger than magnets produced by 
  -- any other material. Discovered by Francois Lecoq de Boisbaudran in 
  -- 1879.
  fun Samarium : Class ;
  fun Samarium_Class : SubClass Samarium ElementalSubstance ;

  -- Rare soft silvery metallic element belonging to 
  -- group 3 of the periodic table. There are ten isotopes, nine of which are 
  -- radioactive and have short half_lives. Predicted in 1869 by Mendeleev, 
  -- isolated by Nilson in 1879.
  fun Scandium : Class ;
  fun Scandium_Class : SubClass Scandium ElementalSubstance ;

  -- Metalloid element, belongs to group 16 of the 
  -- periodic table. Multiple allotropic forms exist. Chemically resembles 
  -- sulphur. Discovered in 1817 by Jons J. Berzelius.
  fun Selenium : Class ;
  fun Selenium_Class : SubClass Selenium ElementalSubstance ;

  -- Metalloid element belonging to group 14 of the 
  -- periodic table. It is the second most abundant element in the Earth's 
  -- crust, making up 25.7 percent of it by weight. Chemically less reactive 
  -- than carbon. First identified by Lavoisier in 1787 and first isolated in 
  -- 1823 by Berzelius.
  fun Silicon : Class ;
  fun Silicon_Class : SubClass Silicon ElementalSubstance ;

  -- White lustrous soft metallic transition element. 
  -- Found in both its elemental form and in minerals. Used in jewellery, 
  -- tableware and so on. Less reactive than silver, chemically.
  fun Silver : Class ;
  fun Silver_Class : SubClass Silver ElementalSubstance ;

  -- Soft silvery reactive element belonging to group 1 
  -- of the periodic table (alkali metals). It is highly reactive, oxidizing 
  -- in air and reacting violently with water, forcing it to be kept under oil. 
  -- It was first isolated by Humphrey Davy in 1807.
  fun Sodium : Class ;
  fun Sodium_Class : SubClass Sodium ElementalSubstance ;

  -- Soft yellowish metallic element, belongs to 
  -- group 2 of the periodic table. Highly reactive chemically. Sr_90 is 
  -- present in radioactive fallout and has a half_life of 28 years. 
  -- Discovered in 1798 by Klaproth and Hope, isolated in 1808 by Humphry 
  -- Davy.
  fun Strontium : Class ;
  fun Strontium_Class : SubClass Strontium ElementalSubstance ;

  -- Yellow, nonmetallic element belonging to group 16 
  -- of the periodic table. It is an essential element in living organisms, 
  -- needed in the amino acids cysteine and methionine, and hence in many 
  -- proteins. Absorbed by plants from the soil as sulphate ion.
  fun Sulphur : Class ;
  fun Sulphur_Class : SubClass Sulphur ElementalSubstance ;

  -- Heavy blue_grey metallic transition element. 
  -- Ta_181 is a stable isotope, and Ta_180 is a radioactive isotope, with a 
  -- half_life in excess of 10^7 years. Used in surgery as it is unreactive. 
  -- Forms a passive oxide layer in air. Identified in 1802 by Ekeberg and 
  -- isolated in 1820 by Jons J. Berzelius.
  fun Tantalum : Class ;
  fun Tantalum_Class : SubClass Tantalum ElementalSubstance ;

  -- Radioactive metallic transition element. Can 
  -- be detected in some stars and the fission products of uranium. First made 
  -- by Perrier and Segre by bombarding molybdenum with deutrons, giving them 
  -- Tc_97. Tc_99 is the most stable isotope with a half_life of 2.6*10^6 
  -- years. Sixteen isotopes are known. Organic technetium compounds are used 
  -- in bone imaging. Chemical properties are intermediate between rhenium and 
  -- manganese.
  fun Technetium : Class ;
  fun Technetium_Class : SubClass Technetium ElementalSubstance ;

  -- Silvery metalloid element of group 16. Eight 
  -- natural isotopes, nine radioactive isotopes. Used in semiconductors and 
  -- to a degree in some steels. Chemistry is similar to {sulphur}. 
  -- Discovered in 1782 by Franz Miller.
  fun Tellurium : Class ;
  fun Tellurium_Class : SubClass Tellurium ElementalSubstance ;

  -- Silvery metallic element belonging to the 
  -- lanthanoids. Tb_159 is the only stable isotope, there are seventeen 
  -- artificial isotopes. Discovered by G.G. Mosander in 1843.
  fun Terbium : Class ;
  fun Terbium_Class : SubClass Terbium ElementalSubstance ;

  -- Pure, unreacted thallium appears silvery_white 
  -- and exhibits a metallic lustre. Upon reacting with air, it begins to turn 
  -- bluish_grey and looks like lead. It is very malleable, and can be cut 
  -- with a knife. There are two stable isotopes, and four radioisotopes, 
  -- Tl_204 being the most stable with a half_life of 3.78 years. Thallium 
  -- sulphate was used as a rodenticide. Thallium sulphine's conductivity 
  -- changes with exposure to infrared light, this gives it a use in infrared 
  -- detectors. Discovered by Sir William Crookes via spectroscopy. Its name 
  -- comes from the Greek word thallos, which means green twig. Thallium and 
  -- its compounds are toxic and can cause cancer.
  fun Thallium : Class ;
  fun Thallium_Class : SubClass Thallium ElementalSubstance ;

  -- Grey radioactive metallic element. Belongs to 
  -- actinoids. Found in monazite sand in Brazil, India and the US. 
  -- Thorium_232 has a half_life of 1.39x10^10 years. Can be used as a nuclear 
  -- fuel for breeder reactors. Thorium_232 captures slow {neutron}s and 
  -- breeds uranium_233. Discovered by Jons J. Berzelius in 1829.
  fun Thorium : Class ;
  fun Thorium_Class : SubClass Thorium ElementalSubstance ;

  -- Soft grey metallic element that belongs to the 
  -- lanthanoids. One natural isotope exists, Tm_169, and seventeen artificial 
  -- isotopes have been produced. No known uses for the element. Discovered 
  -- in 1879 by Per Theodor Cleve.
  fun Thulium : Class ;
  fun Thulium_Class : SubClass Thulium ElementalSubstance ;

  -- Silvery malleable metallic element belonging to group 
  -- 14 of the periodic table. Twenty_six isotopes are known, five of which 
  -- are radioactive. Chemically reactive. Combines directly with chlorine 
  -- and oxygen and displaces hydrogen from dilute acids.
  fun Tin : Class ;
  fun Tin_Class : SubClass Tin ElementalSubstance ;

  -- White metallic transition element. Occurs in 
  -- numerous minerals. Used in strong, light corrosion_resistant alloys. 
  -- Forms a passive oxide coating when exposed to air. First discovered by 
  -- Gregor in 1789.
  fun Titanium : Class ;
  fun Titanium_Class : SubClass Titanium ElementalSubstance ;

  -- White or grey metallic transition element, 
  -- formerly called {wolfram}. Forms a protective oxide in air and can be 
  -- oxidized at high temperature. First isolated by Jose and Fausto de 
  -- Elhuyer in 1783.
  fun Tungsten : Class ;
  fun Tungsten_Class : SubClass Tungsten ElementalSubstance ;

  -- Half_life of approximately 10ms. Reported in 
  -- 1994 by German researchers at Darmstadt, Germany.
  fun Unnildecium : Class ;
  fun Unnildecium_Class : SubClass Unnildecium ElementalSubstance ;

  -- Half_life of 0.9 +/_ 0.2 s. Discovered by the 
  -- Joint Institute for Nuclear Research at Dubna (U.S.S.R.) in June of 1974. 
  -- Its existence was confirmed by the Lawrence Berkeley Laboratory and 
  -- Livermore National Laboratory in September of 1974.
  fun Unnilhexium : Class ;
  fun Unnilhexium_Class : SubClass Unnilhexium ElementalSubstance ;

  -- Radioactive transition metal.
  fun Unniloctium : Class ;
  fun Unniloctium_Class : SubClass Unniloctium ElementalSubstance ;

  -- Radioactive transactinide element. Half_life 
  -- of 1.6s. Discovered in 1970 by Berkeley researchers. So far, seven 
  -- isotopes have been discovered.
  fun Unnilpentium : Class ;
  fun Unnilpentium_Class : SubClass Unnilpentium ElementalSubstance ;

  -- Radioactive transactinide element. Expected 
  -- to have similar chemical properties to those displayed by hafnium. Rf_260 
  -- was discovered by the Joint Nuclear Research Institute at Dubna (U.S.S.R.) 
  -- in 1964. Researchers at Berkeley discovered Unq_257 and Unq_258 in 
  -- 1964.
  fun Unnilquadium : Class ;
  fun Unnilquadium_Class : SubClass Unnilquadium ElementalSubstance ;

  -- Radioactive transition metal. Half_life of 
  -- approximately 1/500 s. Discovered by the Joint Institute for Nuclear 
  -- Research at Dubna (U.S.S.R.) in 1976. Confirmed by West German physicists 
  -- at the Heavy Ion Research Laboratory at Darmstadt.
  fun Unnilseptium : Class ;
  fun Unnilseptium_Class : SubClass Unnilseptium ElementalSubstance ;

  -- White radioactive metallic element belonging to 
  -- the actinoids. Three natural isotopes, U_238, U_235 and U_234. 
  -- Uranium_235 is used as the fuel for nuclear reactors and weapons. 
  -- Discovered by Martin H. Klaproth in 1789.
  fun Uranium : Class ;
  fun Uranium_Class : SubClass Uranium ElementalSubstance ;

  -- Soft and ductile, bright white metal. Good 
  -- resistance to corrosion by alkalis, sulphuric and hydrochloric acid. It 
  -- oxidizes readily about 933K. There are two naturally occurring isotopes 
  -- of vanadium, and 5 radioisotopes, V_49 having the longest half_life at 337 
  -- days. Vanadium has nuclear applications, the foil is used in cladding 
  -- titanium to steel, and vanadium_gallium tape is used to produce a 
  -- superconductive magnet. Originally discovered by Andres Manuel del Rio of 
  -- Mexico City in 1801. His discovery went unheeded, however, and in 1820, 
  -- Nils Gabriel Sefstron of Sweden rediscovered it. Metallic vanadium was 
  -- isolated by Henry Enfield Roscoe in 1867. The name vanadium comes from 
  -- {Vanadis}, a goddess of Scandinavian mythology. Silvery_white metallic 
  -- transition element. Vanadium is essential to {ascidian}s. Rats and 
  -- chickens are also known to require it. Metal powder is a fire hazard, and 
  -- vanadium compounds should be considered highly toxic. May cause lung 
  -- cancer if inhaled.
  fun Vanadium : Class ;
  fun Vanadium_Class : SubClass Vanadium ElementalSubstance ;

  -- Colourless, odourless gas belonging to group 18 on 
  -- the periodic table (the noble gases.) Nine natural isotopes and seven 
  -- radioactive isotopes are known. Xenon was part of the first noble_gas 
  -- compound synthesized. Several others involving Xenon have been found 
  -- since then. Xenon was discovered by Ramsey and Travers in 1898.
  fun Xenon : Class ;
  fun Xenon_Class : SubClass Xenon ElementalSubstance ;

  -- Silvery metallic element of the lanthanoids. 
  -- Seven natural isotopes and ten artificial isotopes are known. Used in 
  -- certain steels. Discovered by J.D.G. Marignac in 1878.
  fun Ytterbium : Class ;
  fun Ytterbium_Class : SubClass Ytterbium ElementalSubstance ;

  -- Silvery_grey metallic element of group 3 on the 
  -- periodic table. Found in uranium ores. The only natural isotope is Y_89, 
  -- there are 14 other artificial isotopes. Chemically resembles the 
  -- lanthanoids. Stable in the air below 400 degrees, celsius. Discovered in 
  -- 1828 by Friedrich Wohler.
  fun Yttrium : Class ;
  fun Yttrium_Class : SubClass Yttrium ElementalSubstance ;

  -- Blue_white metallic element. Occurs in multiple 
  -- compounds naturally. Five stable isotopes are six radioactive isotopes 
  -- have been found. Chemically a reactive metal, combines with oxygen and 
  -- other non_metals, reacts with dilute acids to release hydrogen.
  fun Zinc : Class ;
  fun Zinc_Class : SubClass Zinc ElementalSubstance ;

  -- Grey_white metallic transition element. Five 
  -- natural isotopes and six radioactive isotopes are known. Used in nuclear 
  -- reactors for a {neutron} absorber. Discovered in 1789 by Martin Klaproth, 
  -- isolated in 1824 by Berzelius.
  fun Zirconium : Class ;
  fun Zirconium_Class : SubClass Zirconium ElementalSubstance ;

}
