Periodic Table of the Elements
The periodic table was invented by the Russian chemist Dmitri Mendeleyev (1834-1907), which he published in Principles of Chemistry in 1869. His periodic table arranged the 63 known chemical elements in order of their atomic mass. Today, the elements in the periodic table are ordered by their atomic number. The 18 columns of the table are called the Chemical Groups, increasing in density and radioactivity and decreasing in reactivity. The 7 rows are called the Periods, increasing in atomic numbers from the electropositive left to the electronegative right. Elements in the same horizontal period have the same number of electron shells. Some of the elements, the Lanthanides and the Actinides, are broken out on their own.
Groups are elements with the same placement of electrons in their outer sublevels. Some of the groups are:
|Group IA||Alkaline metals|
|Group IIA||Alkaline Earth metals|
|Group IIIA||Light metals|
|Group IVA||Carbon family|
|Group VA||Nitrogen family|
|Group VIA||Chalcogen (Oxygen) family|
|Group VIIIA||Noble Gases|
Elements in the periodic table are either Metals or Non-metals. Non-metals include the Metalloids, the solids carbon, phosphorus, sulfur, selenium, bromine, iodine, nitrogen, and oxygen; the Halogens (gases) and the inert gases, or Noble Gases.
Metalloids (or semi-metals) have physical and chemical properties that fall between metals and non-metals. These include: boron, silicon, germanium, arsenic, antimony, tellurium, and polonium. These brittle elements often have the appearance of metals, but differ greatly in their electrical properties. The electrical conductivities of silicon and germanium lie between those of metals and those of non-metal insulators. These elements, known as semiconductors, become more conductive at high temperatures, a trait not shared by true metals.
Halogens ("salt formers") are fluorine, chlorine, bromine, iodine, and astatine. All halogens occur in nature except for astatine. Halogens are active, nonmetallic elements that readily combine with metals to form compounds known as salts.
Noble gases are elements that are gaseous at room temperature and pressure, and are called noble because they rarely bond with other elements. This results from the the fact that their electronic configurations consist of completely-filled sublevels. The noble gases are: helium, neon, argon, krypton, xenon, and radon.
Metals are the largest group of chemical elements, comprising 75% of the known elements. The only groups containing no metals are the Halogens (Group 17), and the Noble Gases (Group 18). Metals have the ability to conduct heat and electricity and can, in most cases, be worked. Those elements that can be drawn into wire are called ductile metals, and those that can be hammered into sheets are called malleable. Most metals exhibit luster, a high reflectivity of light when in polished form. Many metals possess a simple cubic or hexagonal crystalline structure at the atomic level. Metal oxides react with water to form basic alkaline solutions. Metals also combine with non-metals to form ionic compounds.
Lanthanides (or lanthanoids), follow lanthanum in Group 3. This group, also known as the rare earths, consists of the following elements: cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, homium, erbium, thulium, ytterbium, and lutetium. The rare earths, except for promethium which is radioactive, are not actually all that rare. The lanthanoids occur in many minerals but chiefly in monazite. Pure lanthanoids are silvery metals with high melting points. All but samarium, europium, and ytterbium tarnish slowly in air; the exceptions being much more reactivite toward oxygen or moisture. The outermost electrons of the lanthanoids fill the 4f subshell.
Actinides are the 14 elements that follow actinium in group 3. All the actinides are radioactive, and most, particularly the transuranic elements, are produced artificially. The actinides, which fill the 5f subshell, exhibit properties similar to the lanthanoids respective to the number of electrons which fill their outermost shell. Curium, for example, possesses chemistry similar to that of gadolinium. The actinides as a group are strongly electropositive, shiny, hard metals that tarnish in air.
Transactinides are the elements beyond the final member of the actinide series, lawrencium. All are radioactive, artificially-produced elements with half-lives of less than five seconds, except for the comparatively long-lived isotopes of the recently discovered element 114. The transactinides exhibit chemical properties in line with the less dense members of their respective groups.
The Discovery of the Elements
Elements known before the Common Era (BCE) include the metals copper, silver, and gold, which were used to make coins. These three elements, which occur in native form, were the first metals that ancient humans came across. Mercury was probably first found as droplets in the mining of mercury sulfide, or cinnabar, which ancient Egyptians used as a red pigment. Classical Greeks used mercury for medicine. Other metals known in that time include tin, lead, zinc, and iron. Native iron, occuring only rarely at the earth's surface, is the principal constituent of many meteorites and the source of early iron samples. Early metalsmiths first obtained tin, zinc, and lead from the ores of other metals, alloying tin with copper to make bronze and zinc (in the form of its oxide or carbonate) to make brass.
Ancient cultures also recognized the nonmetals sulfur and carbon. The first person to make a mark on a surface with a charred stick knew carbon it its amorphous (shapeless) form; the discovery of diamonds (possibly in India's Golconda region along the Krista River) occurred many millennia later. Sulfur occurs in its native elemental form in Sicily, which is probably where the ancient Mediterranean cultures first obtained it.
The metalloid arsenic was another element known to the classical Greeks. They used it to harden copper, and Aristotle thought the element was some kind of sulfur.
Medieval discoveries include the metalloids arsenic and antimony. Alchemist Albertus Magnus isolated arsenic in the year 1250, although the Greeks had used the substance in classical times. The 13th century Latin alchemical writings attributed to Gerber also mention antimony.
From classical times through the European Renaissance, no elements beyond those known to ancient civilizations were discovered. Although Europeans in the Age of the Reformation (1492 to 1635) learned of the element bismuth, no record exists of its discovery (it was not isolated as a metal until 1808).
Discoveries during the European Enlightment (1636 to 1779) begin with the German alchemist Hennig Brandt's discovery of the nonmetal phosphorus in 1669. Subsequent discoveries of this age include cobalt and platinum in 1735, nickel (1751), hydrogen (1766), nitrogen (1772), oxygen and manganese (1774), and molybdenum (1778).
Elements discovered during the world's Industrial Age (1780 to 1923) include chlorine (1780), tellurium (1782), tungsten (1783), zirconium and uranium (1789), titanium (1791), chromium (1797), beryllium (1798), vanadium and niobium (1801), tantalum (1802), rhodium, palladium, cerium, osmium and iridium (1803), sodium and potassium (1807), boron, magnesium, calcium, strontium and barium (1808), iodine (1811), lithium and cadmium (1817), selenium (1818), silicon (1824), aluminum (1825), and bromine (1826), ruthernium (1828), thorium (1829), lanthanum (1839), terbium and erbium (1843), cesium (1860), rubidium and thallium (1861), indium (1863), helium (1868), gallium (1875), holmium (1878), scandium, samarium and thulium (1879), gadolinium (1880), praseodymium and neodymium (1885), fluorine, germanium and dysprosium (1886), polonium (1889), argon (1894), europium (1896), neon, krypton, xenon, and radium (1898), actiniuim (1899), radon (1900), yttrium, ytterbium, and lutetium (1907), protactinium (1917), and hafnium (1923).
During the Silicon Age (1924 to present), elements discovered include rhenium (1925), technetium (1937), francium (1939), astatine, neptunium, and plutonium (1940), americium and curium (1944), promethium (1945), berkelium and californium (1949), einsteinium and fermium (1952), mendelevium (1955), nobelium (1958), lawrencium (1961) rutherfordium (1965), Hahnium (1970), seaborgium (1974), Nielsbohrium (1981), meitnerium (1982), hassium (1984), ununnilium and unununium (1994), ununbium (1996), and elements 114, 116, and 118 (1999).
The Elements Around Us
Most elements can be found in either the Earth's land, air, water, or within it's crust. Elements found on the land are: carbon, sulfur, iron, copper, palladium, silver, tellurium, platinum, gold, and mercury. The air, when dry and measured at sea level, is composed of 78.08% nitrogen, 20.05% oxygen, 0.93% argon, 0.03% carbon dioxide, and lesser quantities of helium, neon, krypton, and xenon. The proportion of lighter gases increases at higher altitudes. Even the driest air contains some water vapor, therefore hydrogen is present.
The Earth's seawater contains hydrogen and oxygen in the form of water. Sodium, chlorine, magnesium, sulfur, calcium, potassium, carbon, and bromine are in the 3.5% dissolved solids that occur in seawater. These are primarily the salts sodium chloride (77.8%) and magnesium chloride (10.9%). Other compounds include magnesium sulfide (4.7%), calcium sulfate (3.6%), potassium sulfate (2.5%), calcium carbonate (0.3%), magnesium bromide (0.2%), and other trace compounds. In fact, even gold exists in seawater, but there is no economical way to extract it.
The Earth's crust contains nearly every element from hydrogen (1) to plutonium (94), with the exception of technetium (43), a synthetic element. Its primary elements include: oxygen (45.6%), silicon (27.3%), aluminum (8.4%), iron (6.2%), calcium (4.7%), magnesium (2.8%), sodium (2.3%), potassium (1.8%), hydrogen (1.5%), and titanium (0.6%).
Some elements are toxic to us. These elements are: beryllium, fluorine, chlorine, bromine, chromium, vanadium, arsenic, bromine, antimony, iodine, neodymium, mercury, thallium, lead, polonium, actinium, and the actinides.
Synthetic elements are elements that do not exist in nature and must be created by man. These elements are: technetium (43), and all the transuranic elements from americium (95) through ununbium (112), along with elements 114, 116, and 118. Surprisingly, neptunium (93) and plutonium (94) occur naturally in minute quantities in uranium ores, but are usually produced artificially.