The most common isotope of hydrogen, atomic number 1, has only one proton in the nucleus. It is the only atom that does not contain any neutrons, which are necessary to keep the protons packed together in the nucleus. There are two isotopes of hydrogen: deuterium, which contains one neutron and tritium, which contains two neutrons. Deuterium is part of the fusion cycle that occurs in the sun (and all stars) to produce huge amounts of energy when hydrogen atoms fuse to form helium atoms.



Helium is a noble gas, so it does not react with other elements. Its atomic weight is 4, so it is much lighter than air. It is commonly used to fill party balloons so they float up high. Helium is also used to fill blimps: it is not only buoyant, but it will not react or burn. Originally blimps were filled with hydrogen gas, also lighter than air, but hydrogen will burn, which led to the famous disaster of the Hindenburg.



Lithium is the lightest of the alkali metals. atomic number 3, so it has three protons in its nucleus. Silver in color, it is so soft that it can be cut with a butter knife. Lithium is now used to make strong batteries, used in electronic devices.


Beryllium and Boron

Beryllium is on the left side of the periodic table, and is clearly a metal. It is light, silver-gray in color, and has a high melting point. Boron straddles the line on the periodic table that separates the metals from the non-metals, and is thus classified as a metalloid (or semi-metal.) Borax is a compound of boron that is used as a cleanser, and boric acid, H3BO3, is a mild antiseptic.



Carbon is a non-metal with 4 valence electrons. It can thus form covalent bonds with many other elements, but most importantly and uniquely among all elements, carbon atoms bond together to form long chains and rings. Organic chemistry is the branch of chemistry that deals with carbon compounds, and there are more organic compounds than all other compounds combined. These include the essential molecules of all living organisms: proteins, fats, starches, enzymes, DNA, and the compounds that make up cells and tissues. Carbon in its elemental form occurs in four forms: coal, graphite, diamond, and fullerene (Bucky balls).



Nitrogen is a colorless, odorless gas that comprises 78% of our atmosphere in the form of N2 molecules. It is also an essential component of proteins which are necessary for life. In the nitrogen cycle, atmospheric nitrogen is “fixed” by bacteria into nitrates which are absorbed by plants and used to make proteins and other useful compounds. Animals eat these plants to obtain necessary nutrients. When plants and animals decay, the nitrogen compounds are decomposed and turned back into atmospheric nitrogen, so the cycle can start all over again.



Oxygen is essential for life: in its elemental form as O2 gas and as one of the two elements that make up water, H2O.



Fluorine is the smallest element in the halogen family and has the greatest electronegativity of any atom, meaning it has a strong attraction for electrons and will pull them away from other atoms in order to form ionic bonds. This is because its electron configuration is 1s2 2s2 2p5, only one electron short of a full octet.



Neon is one of the noble gases, the family that occupies the last column of the periodic table. These elements are called noble or inert because they are highly unreactive due to the fact that they have a full complement of 8 electrons in their outermost s and p orbitals. (Although the larger noble gases do form compounds under the right conditions.) Neon is commonly used to make “neon lights,” which glow a red color when a current of electricity passes through the gas.



Sodium is a member of the alkali metal family, and is a silvery colored, soft metal that is very reactive. It is part of the compound NaCl, where one atom of sodium combines with one atom of chlorine to make table salt.



Magnesium is a silver colored metal in the family of the alkaline earth metals. When ignited, it burns very brightly with a flame too intense to look at directly. The result of the combustion reaction is magnesium oxide, a white powder.



Soda cans are made from aluminum because it is light, easy to work with and does not rust. (Airplanes are also made from aluminum for the same reason.) In fact, aluminum is reactive, but when the can is exposed to air, it forms a thin layer of aluminum oxide on the surface, which resists further corrosion and maintains the shiny metallic appearance of the aluminum. Because aluminum does not corrode, it is also an environmental problem, which is addressed by recycling programs.



Silicon is in the carbon family, and is classified as a metalloid. It is the most abundant element in the earth’s crust and is found in sand and many rocks. Pure silicon is produced by heating sand (silicon dioxide). As a semiconductor, silicon is used to make integrated circuits that power computers. Because of its critical role in the computer industry, the Sana Clara Valley in California where the semiconductor industry is located is nicknamed Silicon Valley.



Phosphorus occurs in two forms: red and white. White phosphorus is very flammable and will ignite spontaneously in air. When exposed to sunlight, or when heated in its own vapour to 250°C, it is converted to the red variety. This form does not ignite spontaneously and it is a little less dangerous than white phosphorus.



Sulfur was originally found in its elemental form and could be excavated for commercial use. Today it is primarily produced as a by-product of oil refining. The largest commercial use for sulfur is to first convert it to sulfuric acid (H2SO4) which is used to produce fertilizers and other compounds. Pure sulfur does not smell, but some of its compounds do, most notably hydrogen sulfide, which smells like rotten eggs.



Chlorine is a halogen and is a greenish yellow gas in its elemental form. It is highly reactive, and is used to disinfect the water in swimming pools. The gas is extremely irritating to the lungs and can be fatal after a few deep breathes. It was used as a chemical weapon in World War I.


Argon is an odorless and colorless noble gas. It is inert, and does not react to form compounds with other elements.



Potassium is an alkali metal with the symbol K. This symbol refers to its Latin name, Kalium. Potassium ions play a critical role in the sodium/potassium pump that moves sodium and potassium ions across cell membranes to maintain an electrical cell potential. Enzymes pump sodium ions out of cells and potassium ions in. This cell potential is what enables nerve cells to transmit messages and muscles to contract. It is thus essential in maintaining your heartbeat.



Calcium is an essential constituent of leaves, bones, teeth, and shells and is the most abundant of the metallic elements in the human body. The average adult body contains about 1 kg, 99% of which is in the bones and teeth. Dairy products such as milk are excellent sources of calcium and are an essential part of a healthy diet. Other elements in the same family, alkaline earth metals, can be dangerous. Strontium-90, for example, is a product of nuclear fallout and, because it is chemically similar to calcium, can be taken up by the body and incorporated into bones, where it emits harmful radiation.



Scandium was discovered by Lars F. Nilson in 1879, in Uppsala, Sweden. He named the element scandium after the Latin word ‘Scanda’ meaning Scandinavia. It is a silvery white metal, the first in the group of transition metals, so it is the first element with an electron in a d orbital.



Titanium is a light, silvery-white, hard, lustrous metal. It has excellent strength and corrosion resistance and also has a high strength to weight ratio: it is as strong as steel, but it is 45% lighter. Alloys of titanium are mainly used in aerospace, aircraft and engines where strong, lightweight, temperature-resistant materials are needed. It is used to make high performance bicycles. Titanium is also used in joint replacement implants, such as the ball-and-socket hip joint.



Vanadium is a bright white, soft, ductile transition metal with good structural strength. The main use of vanadium is in alloys, especially with steel. A small amount of vanadium adds strength, toughness, heat resistance, and an increased strength-to-weight ratio.



Chromium is a silvery, lustrous, very hard metal that can take a high mirror polish. It is also odorless, tasteless, and malleable. Chromium is used in stainless steel, and other alloys. Chromium plating, for example on cars and bicycles, produces a smooth, silver finish that is highly resistant to corrosion. The emerald is a regular crystal of beryl (beryllium aluminum silicate) that contains chromium as an impurity. It is the chromium that gives the emerald its green color.



Manganese is a gray-white, hard, brittle, metal that can take a high polish. Like other transition metals, manganese has more than one common oxidation state: all oxidation states from -3 to +7 have been observed. Manganese dioxide and the intensely purple permanganate anion MnO4, are strong oxidizing agents, meaning they take electrons away from other atoms.



The metal iron has been known since ancient times. It is a strong metal and corrodes easily, but the addition of a small amount of carbon produces steel, which is stronger and resists corrosion. The earth’s magnetic field, which protects us from the solar wind, is generated by the motion of iron alloys in the earth’s liquid core. Iron also plays an important role in biology: it is part of the hemoglobin molecule, and bonds with oxygen so that oxygen can be carried by the blood throughout the body.



Cobalt is a bluish-white, lustrous, hard, brittle metal. Cobalt blue is a well-known blue pigment made with cobalt (II) oxide. Cobalt blue is extremely stable and has historically been used as a coloring agent in ceramics, especially Chinese porcelain, jewelry, paint and glass.



Nickel is a silvery white metal that takes on a high polish. The nickel coin is made from 25% nickel and 75% copper.



Copper is a reddish orange, soft metal that takes on a bright metallic luster. Copper has been used by humans for as many as ten thousand years. It is malleable, ductile, and an excellent conductor of heat and electricity – only silver has a higher electrical conductivity than copper. Its high conductivity makes it an excellent metal for cooking, although pots should be lined with a non-reactive metal like stainless steel because copper reacts with acidic foods and can be toxic.



Zinc is a bluish-silver, lustrous metal that tarnishes in moist air. Zinc is used to galvanize iron to prevent corrosion. Liquid zinc metal is applied as a coating over the iron, and it protects the iron from rusting.



Gallium is a silvery, glass-like, soft metal. It is in the same family as aluminum and is positioned close to the non-metals in the periodic table, so its metallic properties are not as obvious as other metals. Solid gallium is brittle and is a poor electrical conductor. Gallium has a very low melting point (29.8 oC, 85.6 oF ), and therefore it will melt if held in your hand, since body temperature is higher.



Germanium is a lustrous, hard, gray-white semi-metallic element with a crystalline and brittle structure. Germanium was one of the elements whose existence was predicted in 1869 by Russian chemist Dmitri Mendeleev, who created the first periodic table. The elements were organized by atomic mass and chemical/physical properties. When he constructed the table, he had to leave a gap between silicon and tin because the next known element (arsenic) did not belong in the same family as silicon and tin. Mendeleev called the predicted element eka-silicon. Because Mendel had predicted the properties of this unknown element, it was more easily discovered by Clemens A. Winkler in 1886, in Germany. Mendeleev had predicted the new element’s density would be 5.5 g/cm3 and its atomic weight would be 70, which were very close to the actual properties. Winkler’s name for the element comes from the Latin ‘Germania’ meaning Germany.



Arsenic is a metalloid that occurs in several forms: yellow, metallic grey or black. Arsenic and its compounds are poisonous, and are commonly used in murder mysteries as the cause of death. The most famous example is the play “Arsenic and Old Lace” by the American playwright Joseph Kesselring,



Selenium is a non-metal with properties similar to other members of the chalcogen family, sulfur and tellurium. It was discovered in 1817 by Berzelius, who first thought his sample contained the element tellurium. Since tellurium had been named for the earth (tellus means earth goddess in Latin), Berzelius decided to name the new element for the moon (selene means moon goddess in Latin.)

Selenium is a semiconductor and is used in solar cells and photocells.

Selenium sulfide is used in anti-dandruff shampoos. Hydrogen selenide gas (SeH2) is very toxic and it has a very irritating smell (much like the related compound hydrogen sulfide).



Bromine is a halogen and is one of only two elements that are liquid at room temperature. (The other is mercury.) The element is a red, poisonous, noxious smelling liquid that causes skin burns. The name comes from an ancient Greek word meaning “stench.” It is very volatile, releasing fumes at room temperature. The bromide ion is very soluble so the element accumulates in the ocean and is extracted from brine and seaweed.



Krypton is a colorless, odorless, tasteless noble gas. It has an important use in the krypton fluoride laser, which has a high beam uniformity and emits light in the ultraviolet range.



Rubidium is an alkali metal, and therefore, like all alkali metals, reacts strongly with water. The larger the alkali atom, the more violent the reaction, so if you drop a piece of rubidium into water, there will be an explosive reaction, producing hydrogen gas (which combusts due to the heat released) and rubidium hydroxide.



Strontium is a soft, silvery alkaline earth metal, with properties similar to barium. One of its isotopes, strontium-90, is a radioactive product of nuclear fission with a half-life of about 29 years. Because strontium is chemically similar to calcium, this isotope can be taken up by the body and incorporated into bones, where it emits harmful radiation.


Carl Arrhenius first discovered the ore containing yttrium in 1787 in a mine near Ytterby Sweden. He named the mineral ytterbite, after the town Ytterby. In 1794 Johan Gadolin analyzed the mineral and identified a new element, which was eventually named ytterbium. It is a soft, silvery rare earth element in group three on the periodic table. The most important uses of yttrium are in LEDs and phosphors, especially the red phosphors in TV cathode ray tubes.


Zirconium is a strong, malleable, ductile, grayish-white metal. Zircon (zirconium silicate, ZrSiO4) is a natural gemstone, and synthetic cubic zirconia (zirconium dioxide, ZrO2) is produced as a low-cost substitute for diamond.


Niobium is a shiny, white, ductile metal. It is very resistant to heat and wear, so it is used to make alloys with other metals. Niobium was discovered independently by two different chemists. In 1801, the English scientist Charles Hatchett identified a new element in an ore that had been sent to England from Connecticut. He named it columbium in honor of its American origins. In 1846 the German chemist Henrich Rose announced the discovery of a new element that he named niobium, not realizing this was the same element discovered by Hatchett.  He named it niobium after Niobe, the daughter of Tantalus in Greek mythology, because the properties of the new element were very similar to the properties of tantalum. For a century both names were used interchangeably. Niobium was officially adopted as the name of the element in 1949.


Molybdenum is a silvery-white metal with a high melting point. Molybdenum is a crucial component of the nitrogenase enzyme, which is found in nitrogen fixing bacteria. The nitrogenase molecule allows conversion of atmospheric nitrogen gas (N2) into nitrates, which are vital for plant and animal growth. Organisms require “fixed” nitrogen to produce the organic molecules (proteins, nucleotides) that are essential for life.


Mendeleev predicted the discovery of several new elements, including one that would fill a gap underneath the element manganese. He gave it the temporary name ekamanganese. In 1937 technetium-97 became the first artificial element to be produced, and its name is derived from the Greek word for artificial. Technetium is produced by spontaneous fission of uranium or as the product of neutron capture by molybdenum. It is the lightest element of which all isotopes are radioactive.
The isotope Tc-99, a short-lived gamma emitter, is used in nuclear medicine for a variety of diagnostic tests.


Ruthenium is a hard, lustrous, brittle, silvery-white metal that does not tarnish at room temperature. It is a member of the platinum group of metals. It is not affected by air, water or acids. Ruthenium is used is some pen nibs (tips), such as the Parker 51, because of its hardness and durability.


Ruthenium is a platinum group metal that is hard, silvery white, lustrous, and does not tarnish at room temperature. The name comes from the Latin word Ruthenia, meaning Russia, since the ore from which it was isolated came from the Ural Mountains in Russia. It is used to make alloys that are resistant to corrosion, and can be used for electrical contacts. It is also used in pen nibs, specifically the Parker 51.


Rhodium is a member of the platinum group and one of the “noble metals,” so called because of their high resistance to corrosion and oxidation. The other noble metals include ruthenium, palladium, silver, osmium, iridium, platinum and gold. Rhodium was discovered by William Hyde Wollaston in a platinum-containing ore, and was named for the rose color of one of its chlorine compounds. Its major use (like platinum) is in catalytic converters for cars. It is also used to make white gold for jewelry.


Palladium is a rare, lustrous, expensive, silvery white metal, and like rhodium, it is part of the platinum family and one of the noble metals. It is malleable and ductile like gold, and is highly resistant to corrosion. The largest use for palladium is in catalytic converters, as is platinum. It is an excellent catalyst (as is platinum) and is used for hydrogenation reactions. It is also used in dentistry, surgical instruments, and electrical contacts.


Silver is a soft, white, lustrous transition metal. It possesses the highest electrical conductivity, thermal conductivity and reflectivity of any metal. It reflects almost all the light that shines upon it. In the 16th century, glass mirrors were constructed by coating the glass with silver. Silver has long been valued as a precious metal, and has been used for many monetary systems. Dimes and quarters were made of 90% silver up until 1965, after which time the silver was too expensive, so modern coins are made from copper and nickel.


Cadmium is a transition metal, discovered in 1817 by Friedrich Stromeyer. It is a soft bluish-white metal that can be cut with a knife. It is an excellent electrical conductor, and is resistant to corrosion. It is in the same family as zinc, and is usually found in zinc ores and is a by-product of zinc production. In the past it was used in nickel-cadmium batteries, but lithium ion batteries are replacing them today. Cadmium and its ions are toxic to humans. Cadmium compounds are used to create bright pigments for paints: cadmium orange, cadmium red and cadmium yellow are all familiar artist’s colors. Cadmium yellow is CdS.


Indium is a silvery-white, ductile metal, soft enough to be cut with a knife. It was discovered in Germany by Ferdinand Reich and Hieronymus Richter. They were able to identify this new element because they observed a bright indigo blue line in its emission spectrum, which had not been seen in any other element. It was named for this indigo blue line. Indium oxide is transparent to light but is a good conductor of electricity, so it is used in LCD displays. In addition, the compound indium tin oxide is used to coat touch screens on cells phones and tablets.


Tin is a malleable, ductile silvery-white metal. It resists corrosion, so it was used for many purposes: cooking pans and utensils, as well as roofing. It is most commonly associated with “tin” cans, which were introduced around 1810. The original cans were made from steel that was coated with tin to prevent rusting. Once widely used, tin cans have largely been replaced with aluminum. The first metal alloy used on a wide scale since 3000BC was bronze, which is a mixture of tin and copper. The invention of bronze was so important that the era was named the Bronze Age (following the stone age). Bronze allowed people to create more durable tools and artifacts, including weapons, pottery and artwork.


Antimony is a silvery, brittle, gray metalloid that has been known since ancient times. Antimony sulfide (Sb2S3) is mentioned in an Egyptian papyrus of the 16th century BC. The black form of this pigment, which occurs naturally as the mineral stibnite, was used as black eyeliner and known as kohl. The symbol for antimony, Sb, comes from the ore stibnite, and the name antimony is derived from two Greek words: anti and monos, meaning “not alone.” The name was given because antimony is rarely, if ever, found in its elemental form; it is usually combined with sulfur or heavier metals such as copper, lead and silver.


Tellurium is a rare, silvery-white, brittle, lustrous metalloid, in the same family as sulfur and selenium. It was discovered in 1782 and named after the Latin word for earth, tellus. Tellurium is toxic: in humans, it is partly metabolized into dimethyl telluride, (CH3)2Te, a gas with a garlic odor exhaled in the breath of victims of tellurium exposure or poisoning. It is often found as compounds of gold (tellurides), such as calaverite and krennerite. The city of Telluride Colorado was named in hope that settlers would find a gold strike of telluride, which never materialized.



Iodine is a halogen; its name comes from the Greek word iodes, which means violet, because iodine gas is violet (purple) colored. Solid iodine is blue-black and shiny. At room temperature and pressure the solid sublimates directly into the purple gas, so the liquid form is not seen. Iodine is used as a disinfectant in various forms, most commonly as tincture of iodine, which is used to treat cuts.



Xenon is a rare, colorless, odorless, dense noble gas. Up until 1962, it was believed that noble (inert) gases would not react. However, in that year Neil Bartlett allowed xenon gas to mix with a red gas called platinum hexafluoride, and he discovered that a compound of xenon was formed: xenon hexafluoroplatinate (XePtF6), the world’s first noble gas compound.



Cesium is a soft, silvery colored alkali metal. It’s melting point is 28.5 °C (83.3 °F), so it can exist as a liquid near room temperature. The current time standard for the United States is based on a cesium atomic frequency standard at the National Institute of Standards and Technology in Boulder, Colorado. In 1967 a standard second was adopted based on the frequency of a transition in the Cs-133 atom:

1 second = 9,192, 631,770 cycles of the standard Cs-133 transition. Prior to 1964 the international standard second had been based upon the orbital period of the Earth, but the cesium clock period was found to be much more stable than the Earth’s orbit.



Barium is a soft, silvery metallic element chemically resembling calcium but more reactive. Barium sulfate is used to increase the contrast in x-ray imaging of the digestive system. It works by coating the esophagus, stomach, or intestine with a material that is not absorbed into the body so that diseased or damaged areas can be clearly seen by x-ray examination or CT scan.



Hafnium is another element whose existence was predicted by Mendeleev, who stated that the unknown element would have the properties of zirconium and titanium. It was discovered in 1923 in an ore of zirconium, and was hard to isolate because its chemical properties are very similar to those of zirconium. It was named Hafnium to honor the city where it was discovered: Copenhagen, whose Latin name is Hafnia. It is a silvery gray transition metal. Its large neutron capture cross-section makes it a good material for neutron absorption in control rods in nuclear power plants.


Tantalum is a rare, hard, blue-grey, shiny transition metal that is very resistant to corrosion. The element was named for the Greek mythological character Tantalus, who was punished by the gods by being forced to stand in knee-deep water, unable to reach the fruit hanging above his head, thus eternally tantalized by it. One of the main uses of tantalum is in the production of electronic components. Because tantalum can be used to coat other metals with a very thin layer, a high capacitance can be achieved in a small volume. This makes tantalum capacitors attractive for portable electronics such as cell phones, making it a valuable metal. Tantalum is considered a conflict resource: the ore from which it is obtained is found in Central Africa and the proceeds from its sale are used to fund warfare.


Tungsten is a silvery-white lustrous metal that is extremely hard and dense. The name comes from the Swedish words “tung” and “sten,” meaning heavy stone. The symbol for tungsten is W, because the original name of the element was wolfram: it is found in the ore wolframite. Tungsten has the highest melting point of any metal: 3422°C or 6192° F. It is used widely as the filament in incandescent light bulbs and in halogen tungsten lamps, where the bulbs are coated with a halogen like bromine or iodine to prevent the tungsten filament from decaying. These lamps are more energy efficient than the standard incandescent light bulbs.


Rhenium is an extremely rare, silvery-white, heavy, third-row transition metal in group 7 of the periodic table. The melting point of rhenium is 3186°C, the third highest of any element, and it has the second highest boiling point at melting point and second-highest boiling point of any element at 5630°C. Rhenium is chemically similar to manganese and technetium. It was discovered in 1908 and named after the river Rhine. Because of its high melting point, nickel based alloys of rhenium are used in the combustion chambers, turbine blades and exhaust nozzles of jet engines. These alloys contain up to 6% rhenium, making jet engine construction the largest use of this element.


Osmium is hard, brittle, bluish-white transition metal in the platinum group. It is the densest naturally occurring element, with a density of 22.59 g/cc. (Indium is a close second.) It is alloyed with other metals and used where extreme hardness and durability are required, such as in pen nibs and electrical contacts. Osmium tetroxide is toxic and has a characteristic unpleasant odor, so the discoverer of Osmium, Smithson Tennant, named it for the Greek word “osme” meaning smell.


Iridium is a very, brittle, silvery-white transition metal of the platinum group. It is considered the second densest element of the periodic table, after osmium. It was discovered by Smithson Tennant, who named it after the Greek goddess Iris, personification of the rainbow, because of the colors of its salts. Iridium is one of the rarest elements of the earth’s crust, but is found in higher concentrations in certain (chondritic) meteorites and asteroids. The discovery of unusually high concentrations of iridium in the clay layer at the Cretaceous-Paleogene boundary gave evidence for the Alvarez hypothesis that the extinction of the dinosaurs was caused by the impact of a massive asteroid 66 million years ago.



Platinum is a precious, valuable metal: it is soft, silvery-white, and dense with a beautiful lustrous sheen. It is noble metal and, like gold, it does not oxidize in air even at high temperatures and is unaffected by common acids. It is used for jewelry, and as a catalyst for chemical reactions. The most important use of platinum is in catalytic converters for cars, facilitating the complete combustion of unburned hydrocarbons.



Gold is one of the “noble metals,” a group that resists oxidation and corrosion. Other noble metals are: ruthenium, rhodium, palladium, silver, osmium, iridium and platinum. Humans have known and treasured gold since prehistoric times. It was used for decoration and as a form of money. It was also regarded as a perfect substance, since it was rare, beautiful, never tarnished, and was unaffected by all acids except aqua regia (a mixture of hydrochloric acid and nitric acid.) It is a soft, yellow metal with a beautiful lustrous sheen. It is the most malleable and ductile of all the elements, and it is an excellent conductor of heat and electricity. Gold has a very high density, 19.32 g/cm3: if a basketball were made of solid gold, it would weight 300 pounds. (But it is not the densest element: osmium and iridium are denser.)



Mercury or quicksilver has been known since ancient times. Mercury is a dense, silvery-white, poisonous metal with a mirror-like appearance, and it is the only metal that is liquid at room temperature. It was named for the planet Mercury by alchemists, who likened it to the fastest moving planet. Mercury is used in barometers and manometers because of its high density, and in thermometers because it has a high rate of nearly linear thermal expansion. Various compounds of mercury are used in medicine, dentistry, cosmetics and also in agriculture to make fungicides. Mercury and most of its compounds are highly toxic. The pure metal is absorbed easily by inhalation, ingestion or through the skin.


Thallium is a soft, gray post-transition metal that resembles tin. It was discovered by William Crookes and Claude-Auguste Lamy in 1861, and named thallium because of the green line in its atomic spectrum. (Thallium comes from the Greek word for a green twig.) Soluble thallium salts are toxic and were used in the past in rat poison and insecticides. It has also been used as a murder weapon, and was called “the poisoner’s poison.”


Lead is a soft, dense, malleable and corrosion resistant metal. It has been known since ancient times and was used by the ancient Romans to make water pipes. The symbol for lead is Pb, which comes from the Latin word plumbum, meaning waterworks. (This is also where the work plumbing comes from. ) Lead’s high density makes it useful as a shield against X-ray and gamma ray radiation: before your dentist takes X-rays, you will be covered with a lead-lined apron. Throughout history — before the scientific advancements of the 20th century revealed its potent toxicity — lead was widely used in a variety of products, including cosmetics, paint, glaze, solder, pipes and gasoline. Lead continues to pose a real public health danger today. For example, children living in older homes can still breathe in or ingest dust or paint chips from peeling lead paint on the walls.
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Bismuth is a brittle silvery-white metal, which gains a pink tinge when the surface oxidizes. It has been known since ancient times, although it was often confused with lead and tin, which have similar properties. Bismuth compounds are used in cosmetics, pigments and a few pharmaceuticals. The best known is bismuth subsalicylate, which is used to treat diarrhea: it is the active ingredient in Pepto-Bismol.


Polonium is a rare, silvery-gray, radioactive, low-melting metalloid. The most common isotope, polonium-210 is a strong alpha emitter. Marie and Pierre Curie discovered polonium while they were investigating the cause of pitchblende (uranium oxide) radioactivity. Pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than the uranium and thorium combined. This spurred the Curies to search for additional radioactive elements. They first separated out polonium from pitchblende in July 1898, and five months later, also isolated radium. In accordance with the Curies’ wishes, polonium is named after Poland, where Marie Curie was born and grew up.


Astatine is the rarest naturally occurring element in the earth’s crust: at any time there is less than an ounce on the whole earth. It is highly radioactive and occurs as the decay product of various heavier elements. The most stable isotope is astatine-210 with a half-life of 8.1 hours. It was first synthesized in 1940 by the bombardment of bismuth with alpha particles, and it was named from the Greek word for unstable.


Radon is a noble gas, therefore it is chemically inert: it is colorless, odorless, and also radioactive. It was discovered in 1900 by Friedrich Dorn, a German chemist, who was studying the decay chain of radium. Radon undergoes alpha decay, and since it is a gas, it is easily inhaled and is harmful to living tissue. In addition, it decays into longer-lived radioactive elements which can collect on dust particles and be inhaled as well. Radon often seeps into homes through the decay of heavier elements, and is a danger to homeowners, who are advised to install radon detectors to avoid health hazards.


Francium is the largest of the alkali metals, and it should therefore be the most reactive. In theory, its reaction with water would be far more violent than that of sodium or potassium or cesium, and should result in a large explosion. However, it is not available in sufficient quantities to perform this reaction. It is the second most rare element on earth: less than thirty grams of francium exist on earth at any one time. Francium is radioactive: its isotopes, with mass numbers ranging from 200 to 232, undergo alpha or beta decay. The longest-lived isotope has a half life of only 22 minutes.


Radium is one of the alkali earth metals: it was discovered by Marie and Pierre Curie in 1898, the same year that they discovered polonium. They extracted about one gram of radium from eight tons of pitchblende ore. Radium is highly radioactive and its decay product, radon, is also radioactive. A distinguishing characteristic of radium is that the radiation causes the metal and its compounds to glow in the dark, a phenomenon called radio-luminescence. Because of this property, radium used to be used in luminous paints, for example, in clock and watch dials. It is now considered too hazardous for such uses. The hazard was discovered when the female workers, who applied the radium paint using small paint brushes that they sharpened by licking, developed cancers of the mouth. Eventually over 100 workers died from the effects of radium poisoning.