Characteristics of Some Elements
Here are the names, symbols, atomic numbers and some simple physical characteristics of the first twenty elements plus some selected others. This is not an intensive treatment and is given for general interest only.

Hydrogen, H, atomic #1 -  A colourless, odourless, gaseous element. It is the lightest element and the most abundant in the universe. Present in the atmosphere and in all organic compounds. It comes in three isotopes, hydrogen-1, deuterium-2, and an artificial isotope tritium-3. It is found in nature as a diatomic gas, H2. Hydrogen reacts with most elements almost as if it were a metal. It acts like a metal but isn't a solid like a metal because of its low mass. It was discovered by Henry Cavendish in 1776.
Helium, He, atomic #2 - A colourless, odourless, gaseous, nonmetallic element belonging to the noble gases group of elements. Helium has the lowest boiling point of all the known substances and can be solidified only under pressure. It is used to provide an inert environment for welding and semiconductor manufacture, as a refrigerant for superconductors, and as a dilutent in breathing apparatus. It is also used to fill balloons, everything from party size to the Goodyear blimp. It is totally inert chemically and has no known compounds. It was discovered in the solar spectrum in 1868 by Lockyer.
Lithium, Li, atomic #3 - A soft, silvery metal, the first truely metallic member of Group I in the periodic table. It is a rare element and is never found uncombined by itself.  It is used in batteries and in fusion research. Lithium salts are used in psychomedicine. The element reacts with oxygen and water violently and is kept under parrafin oil for this reason. On heating it reacts with nitrogen and hydrogen.
Beryllium, Be, atomic #4 - A grey, metallic element of Group II. Used to manufacture Be-Cu alloys which are used in nuclear reactors as reflectors and moderators. Beryllium oxide is used in ceramics and in nuclear reactors. Beryllium and its compounds are toxic and can cause serious lung disease such as berylliosis. It resists oxidation because it forms an oxide layer. The element was isolated independantly by F. Wohler and A.A. Bussey in 1828.
Boron, B, atomic #5 - An element of group III. Very hard (9.3 on the Mohrs scale) and a poor conductor of electricity. It is never found free in nature. It is used in semiconductors and in filaments for specialized aerospace applications. Boron is used in flares to give a green colour. Boron-10 is used in nuclear reactors as part of the control rods and shields. It was first discovered in 1080 by Sir Humprey Davy, J.L. Gay-Lussac and L.J. Thenard.
Carbon, C, atomic #6 - A nonmetallic element belonging to group IV. Comes in three main forms, diamond, graphite and carbon (charcoal). Diamond is an extremely hard and highly refractive crystal. Graphite is a soft, greyish-black slippery substance. Graphite is a good conductor of heat and electricity and is used in electrical contacts, high-temperature equipment and as a solid lubricant. Graphite mixed with clay is the "lead" in pencils. Carbon has the unique ability to bond to itself in long chains. For this reason all life on Earth is carbon-based.
Nitrogen, N, atomic #7 - A colourless, odourless, gaseous element belonging to Group V. It makes up 78% of the atmosphere and is an essential component in proteins and nucleic acids in living organisms. It is obtained from the air from fractional distillation of liquid air. It is also used in the manufacture of ammonia. The gas is diatomic, N2, and is used to provide an inert atmosphere in welding and metallurgy.  It is relativey inert but will react with hydrogen at high temperatures and pressures. It will react with oxygen when excited with electricity or lightning. It was discovered in 1772 by D. Rutherford.
Oxygen, O, atomic #8 - A colourless, odourless, gaseous element belonging to Group VI. It is the most abundant element in the Earth's crust (49.2%) and is present in the atmosphere (20% by volume). Atmosphere oxygen is of vital importance for all organisms that carry out aerobic respiration. For industrial purposes it is obtained by the fractional distillation of liquid air. It is used in metallurgical processes to produce high temperature flames and in breathing apparatus. The most common form is O2 but it can be found in an allotropic form called ozone, O3, in the high atmosphere and around electrical circuits. The clear crisp smell of air after a thunderstorm is actually the smell of ozone created by the lightning.
Fluorine, F, atomic #9 - A poisonous, pale, yellow, gaseous element belonging to group VII. It is used in the synthesis of organic fluorine compounds. Chemically it is the most reactive and electronegative non-metallic element. It is a highly dangerous compound, causing severe chemical burns on contact with the skin. The element was identified by Scheele in 1771 and was first isolated by Moissan in 1886.
Neon, Ne, atomic #10 - A colourless, gaseous element belonging to Group VII, the noble gases. Neon is a rare gas (0.0018% of the atmosphere) and is obtained by the fractional distillation of liquid air. It is used in discharge tubes and neon lamps, in which it has a characteristic red glow. It forms fluoride compounds but only under intense pressure and temperatures. For all intents and purposes it is an unreactive element. It was discovered in 1898 by Sir William Ramsey and M.W. Travers.
Sodium, Na, atomic #11 - A soft, silvery, reactive element belonging to Group I. Sodium was known long ago as 'natrium' hence its symbol is a reflection of this. Sodium occurs as the chloride in sea water and in the mineral "halite". The metal is used as a reducing agent in chemical reactions and also as a coolant in some nuclear reactors. Chemically, it is highly reactive, oxidizing in air and reacting violently with water. It is stored in the lab under paraffin oil for this reason. Sodium is a major element required for life by all organisms. It was first discovered and isolated by Humphry Davy in 1807.
Magnesium, Mg, atomic #12 - A silvery, metallic element belonging to Group II. The element is found in a number of minerals. It also is present in seawater and is an essential element of life. Magnesium is used in a number of light alloys. Chemically, it is very reactive. In air it forms a protective oxide coating but when ignited it burns with an intense white flame. It also reacts with the halogens, sulphur and nitrogen. Magnesium was first isolated in 1828 by Bussy.
Aluminum, Al, atomic #13 - A silvery, white, lustrous, metallic element belonging to Group III. The metal itself is highly reactive but is protected by a thin transparent layer of oxide. The metal is extracted from bauxite ore, Al2O3, by electrolysis. Pure aluminum is soft and ductile but its strength can be increased by alloying it with copper, manganese, zinc, silicon and magnesium. Its lightness, and strength when alloyed along with its corrosion resistance and electrical conductivity (62% that of copper) make it suitable for a variety of uses. These include vehicle and aircraft construction, window and door frames and overhead power cables. Although it is the third most abundant element in the Earth's crust (8.1% by weight) it was not isolated until 1825 by H.C. Oersted.
Silicon, Si, #14 - A metalloid element belonging to Group IV. Silicon is the second most abundant element in the Earth's crust (25.7% by weight)  occurring in various forms of silicon oxide. Chemically silicon is less reactive than carbon. The element combines with oxygen only when red hot and is dissolved by molten alkali. The major ingredient of computer technology is the silicon chip, upon which wires are engraved on an ultra thin layer of silicon crystal. The element was identified by Lavoisier in 1787 and was first isolated in 1823 by Berzelius.
Phosphorus, P, atomic #15 - A nonmetallic element belonging to Group V. There are white (yellow), red , black and violet allotropes of this element. The element is highly reactive and will spontaneously ignite with oxygen from the air. It is stored under water for this reason. Phosphorus is an essential element for living organisms. Phosphorus burns on the skin are known for their deep penetration, intense pain and the length of time required for healing.  Phosphorus was discovered by Brandt in 1669 when he boiled buckets of putrified urine.
Sulphur, S, atomic #16 - A yellow, nonmetallic element belonging to group VI. The element occurs in many sulphide and sulphate minerals and native sulphur is also found in Italy and the USA. Sulphur has a variety of allotrophic forms, the S8 ring being the most common. At higher temperatures the rings break and form long strands numbering in the millions of S atoms long. It can be used as a plant fungicide and as an ingredient in the vulcanization of rubber. The element is also used to produce sulphuric acid.
Chlorine, Cl, atomic #17 - A halogen element of Group VII. It is a poisonous greenish-yellow gas and occurs widely in nature as sodium chloride in seawater and as the mineral halite, NaCl. It also occurs as "carnallite" which is a mixture of potassium chloride and magnesium chloride and as "sylvite" which is pure potassium chloride. It has many applications including the chlorination of drinking water and the manufacture of a large number of organic compounds. It reacts very well with many other elements and is a strong oxidizing agent. It was discovered in 1774 by Karl Scheele and Humphry Davy confirmed it as an element in 1810.
Argon, Ar, atomic #18 - A single atom element of the Noble Gas group. It is present in the air (0.93%). Argon is separated from air by fractional distillation. It is slightly soluble in water, colourless, and odourless. It's uses include inert gas atmospheres for welding and special-metal manufacturing such as Titanium and Zirconium. When it is mixed with nitrogen in an 80% Ar-20% N2 ratio it is used to fill electric incandescent light bulbs. The element is inert chemically and has no true compounds. Lord Rayleigh and Sir William Ramsey identified argon in 1894.
Potassium, K, atomic #19 - Known to the ancients as "kalium" its symbol reflects this older name. Potassium is a very soft silvery metallic element belonging to Group I. The element occurs in seawater and in a number of minerals. The metal has a few uses but potassium salts are used in a wide range of applications. Potassium is an essential element for living organisms. Chemically it is highly reactive, resembling sodium in its behavior and compounds. It also forms an orange coloured superoxide, KO2. Potassium was discovered by Sir Humphry Davy in 1807.
Calcium, Ca, atomic #20 - A soft, grey, metallic element belonging to Group II. Calcium compounds are common in the Earth's crust. Limestone, gypsum and marble are essentially calcium compounds. It is used as a reducing agent in the extraction of thorium, zirconium and uranium. Calcium is an essential element for living organisms, being needed for the normal growth and development of bones and muscle.
Iron, Fe, atomic #26 - An element that ushered in the Iron Age. It was known as "ferrum" hence it's elemental symbol. A silvery, malleable and ductile metallic transition element. The main sources are the ores, haematite (Fe2O3), magnetite (Fe3O4) and pyrite (FeS2).  The metal ore is smelted in a blast furnace to give impure pig iron, which is further processed to give cast iron, wrought iron, and various types of steel. It is the fourth most abundant element is the Earth's crust. It is required as a trace element by living organisms. Iron is quite reactive, being oxidized by moist air, it displaces hydrogen from acids and combines easily with nometallic elements.
Nickel, Ni, atomic #28 - A malleable, ductile, silvery, metallic, transition element. Nickel is found in many ores and may make up to 20% of a meteorite's mass. Nickel metal is used in special steels and is an effective catalyst. It is remarkedly stable in air and is therefore an ideal coinage metal. Nickel was discovered by A.F. Cronstedt in 1751.
Copper, Cu, atomic #29 - Copper has been known since ancient times as "cuprum". This name has been known for about 3000 years and has been linked with the island of Cryprus. The metal is malleable and ductile and it is an excellent conductor of electricity and heat. Native copper appears in isolated pockets in some parts of the world. Copper metal is used to make electric cables and wires. Its alloys, brass (copper-zinc) and phosphor bronze (copper-tin) are used extensively. Water does not attack copper but in moist atmospheres it slowly forms a characteristic green surface layer called a patina of copper oxide. The metal will not react with dilute sulphuric or hydrochloric acid, but it will react with nitric acid. Copper solutions are noted for their blue appearance.
Zinc, Zn, atomic #30 - A blue-white, metallic, transition element. The metal is used in galvanizing and in a number of alloys. Chemically it is a reactive metal, combining with oxygen and other nonmetals and reacting with dilute hydrochloric acid to form hydrogen gas.
Bromine, Br, atomic #35 - A halogen element of group VII. It is a red volatile liquid at room temperature and has a reddish-brown vapour. Chemically it is intermediate in reactivity between chlorine and iodine. The liquid is harmful to human tissue and the vapour irritates the eyes and throat. The element was discovered in 1826 by Antoine Balard and is produced commercially by extraction from seawater.
Silver, Ag, atomic #47 - A white, lustrous, soft, metallic transition element. It occurs as the element in a native state and in combined form as the minerals "argentite" (Ag2S) and "horn silver" (AgCl). It is also present in the ores of lead and copper and is extracted as a by-product. The element is used in jewelery, tableware, and silver compounds are used in photography. Chemically, silver is less reactive than copper. A dark silver sulphide forms when silver tarnishes in air because of the presence of sulphur compounds. It was known in Roman times as 'argentum' hence it symbol.
Tin, Sn, atomic #50 - A silvery, malleable, metallic element belonging to Group IV. It is found in ores only, never in the native state. The metal is used as a thin protective coating for steel plate and is a constituent of a number of alloys such as phosphor bronze, gun metal, solder, Babbit metal, and pewter. Chemically it is quite reactive. It was known in the past as "stannum" hence its symbol.
Antimony, Sb, atomic #51 - An element belonging to Group V. The stable form of antimony is a bluish-white metal. Yellow and black non-metallic allotrophic forms are also formed. The main source is stibnite (Sb2S3). The main use of the metal is as an alloying agent in lead-accumulator plates, type metals, bearing alloys, solders, Britannia metal, and pewter. Its compounds are used in flame-proofing, paints, ceramics, enamels, glass, dyestuffs, and rubber technology. The element will burn in air. It was first reported by Tholden in 1450 as "stibium".
Iodine, I, atomic #53 - A dark violet, non-metallic element belonging to Group VII. The element is insoluble in water but soluble in ethanol and other organic solvents. When heated it gives a violet vapour that sublimes. Iodine is a required trace element by living organisms. It is concentrated in the thyroid glands as a constituent of the thyroid hormones. The element is present in sea water and was once extracted from seaweed. It is used in medicine as a mild antiseptic (dissolved in ethanol as tincture of iodine), and in the manufacture of iodine compounds. Chemically, it is less reactive than the other halogens. It was discovered in 1812 by Courtois.
Gold, Au, atomic #79 - A soft yellow malleable metallic transition element. It is found as the free metal in gravel or in quartz veins, and is also present in some lead and copper sulphide ores. It also occurs combined with silver in "telluride sylvanite" (Au,Ag)Te2. It is used in jewelery, dentistry, and electronic devices. Chemically, it is unreactive, being unaffected by oxygen. It reacts with chlorine at 200oC to form gold (III) chloride. It was formally named by the Romans as "aurum" hence its symbol.
Mercury, Hg, atomic #80 - A heavy, silvery, liquid metallic element belonging to the Zinc group. It is the only metal which is a fluid at room temperature. It was known in ancient times as "hydroargentum" which means liquid silver. The main ore is a sulphide called "cinnabar" (HgS), which can be decomposed to its elements very easily with heat.  Mercury is used in thermometers, barometers, thermostats, and other scientific apparatus, and in dental amalgams as fillings.
Lead, Pb, atomic #82 - A heavy, dull, grey, soft, ductile, metallic element belongs to Group IV.  The main ore is the sulphide "galena" (PbS) along with "litharge" (PbO). Lead has a variety of uses including building construction, lead plate accumulators, bullets and shot, and is a constituent of such alloys as solder, pewter, bearing metals, type metals, and fusible alloys. Lead was used to make pipes for the Roman aquaducts. The lead, known as "plumbum" was worked on by artisans who knew how to melt, mold and seal lead sheets into pipes. The artisans were known as "plumbers".
Uranium, U, atomic #92 - A white, radioactive, metallic element belonging to the actinoids. It occurs as uranite ore (U2O3), from which the metal is extracted. Only 0.71% of the mined uranium is suitable for fission in a nuclear reactor. As a fuel and as a weapon uranium has achieved enormous technical and political importance once it was first used for these purposes. It was discovered by M.H. Klaproth in 1789.