Period (periodic table)
A period in the periodic table is a row of chemical elements. All elements in a row have the same number of electron shells; each next element in a period is less metallic than its predecessor. Arranged this way, groups of elements in the same column have similar chemical and physical properties, reflecting the periodic law. For example, the alkali metals lie in the first column and share similar properties, such as high reactivity and the tendency to lose one electron to arrive at a noble-gas electronic configuration; as of 2016, a total of 118 elements have been confirmed. Modern quantum mechanics explains these periodic trends in properties in terms of electron shells; as atomic number increases, shells fill with electrons in the order shown at right. The filling of each shell corresponds to a row in the table. In the s-block and p-block of the periodic table, elements within the same period do not exhibit trends and similarities in properties. However, in the d-block, trends across periods become significant, in the f-block elements show a high degree of similarity across periods.
There are seven complete periods in the periodic table, comprising the 118 known elements. Any new elements will be placed into an eighth period; the first period contains the least elements than any other, with only two and helium. They therefore do not follow the octet rule. Chemically, helium behaves like a noble gas, thus is taken to be part of the group 18 elements. However, in terms of its nuclear structure it belongs to the s block, is therefore sometimes classified as a group 2 element, or both 2 and 18. Hydrogen loses and gains an electron, so behaves chemically as both a group 1 and a group 17 element. Hydrogen is the most abundant of the chemical elements, constituting 75% of the universe's elemental mass. Ionized hydrogen is just a proton. Stars in the main sequence are composed of hydrogen in its plasma state. Elemental hydrogen is rare on Earth, is industrially produced from hydrocarbons such as methane. Hydrogen is present in water and most organic compounds. Helium exists only as a gas except in extreme conditions.
It is the second-most abundant in the universe. Most helium was formed during the Big Bang, but new helium is created through nuclear fusion of hydrogen in stars. On Earth, helium is rare, only occurring as a byproduct of the natural decay of some radioactive elements. Such'radiogenic' helium is trapped within natural gas in concentrations of up to seven percent by volume. Period 2 elements involve the 2p orbitals, they include the biologically most essential elements besides hydrogen: carbon and oxygen. Lithium is the least dense solid element. In its non-ionized state it is one of the most reactive elements, so is only found in compounds, it is the heaviest primordial element forged in large quantities during the Big Bang. Beryllium has one of the highest melting points of all the light metals. Small amounts of beryllium were synthesised during the Big Bang, although most of it decayed or reacted further within stars to create larger nuclei, like carbon, nitrogen or oxygen. Beryllium is classified by the International Agency for Research on Cancer as a group 1 carcinogen.
Between 1% and 15% of people are sensitive to beryllium and may develop an inflammatory reaction in their respiratory system and skin, called chronic beryllium disease. Boron does not occur as a free element, but in compounds such as borates, it is an essential plant micronutrient, required for cell wall strength and development, cell division and fruit development, sugar transport and hormone development, though high levels are toxic. Carbon is the fourth-most abundant element in the universe by mass after hydrogen and oxygen and is the second-most abundant element in the human body by mass after oxygen, the third-most abundant by number of atoms. There are an infinite number of compounds that contain carbon due to carbon's ability to form long stable chains of C—C bonds. All organic compounds, those essential for life, contain at least one atom of carbon. Nitrogen is found as inert diatomic gas, N2, which makes up 78% of the Earth's atmosphere by volume, it is an essential component of proteins and therefore of life.
Oxygen comprising 21% of the atmosphere by volume and is required for respiration by all animals, as well as being the principal component of water. Oxygen is the third-most abundant element in the universe, oxygen compounds dominate the Earth's crust. Fluorine is the most reactive element in its non-ionized state, so is never found that way in nature. Neon is a noble gas used in neon lighting. All period three elements have at least one stable isotope. All but the noble gas argon are essential to basic biology. Sodium is an alkali metal, it is present in Earth's oceans in large quantities in the form of sodium chloride. Magnesium is an alkaline earth metal. Magnesium ions are found in chlorophyll. Aluminium is a post-transition metal, it is the most abundant metal in the Earth's crust. Silicon is a metalloid, it is a semiconductor. Silicon dioxide is the principal constituent of sand; as Carbon is to Biology, Silicon is to Geology. Phosphorus is a
Timeline of chemical element discoveries
The discovery of the 118 chemical elements known to exist as of 2019 is presented in chronological order. The elements are listed in the order in which each was first defined as the pure element, as the exact date of discovery of most elements cannot be determined. There are plans to synthesise more elements, it is not known how many elements are possible; each element's name, atomic number, year of first report, name of the discoverer, notes related to the discovery are listed. History of the periodic table Periodic table Extended periodic table The Mystery of Matter: Search for the Elements Transfermium Wars History of the Origin of the Chemical Elements and Their Discoverers Last updated by Boris Pritychenko on March 30, 2004 History of Elements of the Periodic Table Timeline of Element Discoveries The Historyscoper Discovery of the Elements - The Movie - YouTube The History Of Metals Timeline. A timeline showing the discovery of metals and the development of metallurgy. —Eric Scerri, 2007, The periodic table: Its story and its significance, Oxford University Press, New York, ISBN 9780195305739
The alkali metals are a group in the periodic table consisting of the chemical elements lithium, potassium, rubidium and francium. This group lies in the s-block of the periodic table of elements as all alkali metals have their outermost electron in an s-orbital: this shared electron configuration results in their having similar characteristic properties. Indeed, the alkali metals provide the best example of group trends in properties in the periodic table, with elements exhibiting well-characterised homologous behaviour; the alkali metals are all shiny, soft reactive metals at standard temperature and pressure and lose their outermost electron to form cations with charge +1. They can all be cut with a knife due to their softness, exposing a shiny surface that tarnishes in air due to oxidation by atmospheric moisture and oxygen; because of their high reactivity, they must be stored under oil to prevent reaction with air, are found only in salts and never as the free elements. Caesium, the fifth alkali metal, is the most reactive of all the metals.
In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, excluding hydrogen, nominally a group 1 element but not considered to be an alkali metal as it exhibits behaviour comparable to that of the alkali metals. All the alkali metals react with water, with the heavier alkali metals reacting more vigorously than the lighter ones. All of the discovered alkali metals occur in nature as their compounds: in order of abundance, sodium is the most abundant, followed by potassium, rubidium and francium, rare due to its high radioactivity. Experiments have been conducted to attempt the synthesis of ununennium, to be the next member of the group, but they have all met with failure. However, ununennium may not be an alkali metal due to relativistic effects, which are predicted to have a large influence on the chemical properties of superheavy elements. Most alkali metals have many different applications. One of the best-known applications of the pure elements is the use of rubidium and caesium in atomic clocks, of which caesium atomic clocks are the most accurate and precise representation of time.
A common application of the compounds of sodium is the sodium-vapour lamp, which emits light efficiently. Table salt, or sodium chloride, has been used since antiquity. Lithium finds use as an anode in lithium batteries. Sodium and potassium are essential elements, having major biological roles as electrolytes, although the other alkali metals are not essential, they have various effects on the body, both beneficial and harmful. Sodium compounds have been known since ancient times. While potash has been used since ancient times, it was not understood for most of its history to be a fundamentally different substance from sodium mineral salts. Georg Ernst Stahl obtained experimental evidence which led him to suggest the fundamental difference of sodium and potassium salts in 1702, Henri-Louis Duhamel du Monceau was able to prove this difference in 1736; the exact chemical composition of potassium and sodium compounds, the status as chemical element of potassium and sodium, was not known and thus Antoine Lavoisier did not include either alkali in his list of chemical elements in 1789.
Pure potassium was first isolated in 1807 in England by Sir Humphry Davy, who derived it from caustic potash by the use of electrolysis of the molten salt with the newly invented voltaic pile. Previous attempts at electrolysis of the aqueous salt were unsuccessful due to potassium's extreme reactivity. Potassium was the first metal, isolated by electrolysis; that same year, Davy reported extraction of sodium from the similar substance caustic soda by a similar technique, demonstrating the elements, thus the salts, to be different. Petalite was discovered in 1800 by the Brazilian chemist José Bonifácio de Andrada in a mine on the island of Utö, Sweden. However, it was not until 1817 that Johan August Arfwedson working in the laboratory of the chemist Jöns Jacob Berzelius, detected the presence of a new element while analysing petalite ore; this new element was noted by him to form compounds similar to those of sodium and potassium, though its carbonate and hydroxide were less soluble in water and more alkaline than the other alkali metals.
Berzelius gave the unknown material the name "lithion/lithina", from the Greek word λιθoς, to reflect its discovery in a solid mineral, as opposed to potassium, discovered in plant ashes, sodium, known for its high abundance in animal blood. He named the metal inside the material "lithium". Lithium and potassium were part of the discovery of periodicity, as they are among a series of triads of elements in the same group that were noted by Johann Wolfgang Döbereiner in 1850 as having similar properties. Rubidium and caesium were the first elements to be discovered using the spectroscope, invented in 1859 by Robert Bunsen and Gustav Kirchhoff; the next year, they discovered caesiu
A metalloid is a type of chemical element which has properties in between, or that are a mixture of, those of metals and nonmetals. There is neither a standard definition of a metalloid nor complete agreement on the elements appropriately classified as such. Despite the lack of specificity, the term remains in use in the literature of chemistry; the six recognised metalloids are boron, germanium, arsenic and tellurium. Five elements are less so classified: carbon, selenium and astatine. On a standard periodic table, all eleven elements are located in a diagonal region of the p-block extending from boron at the upper left to astatine at lower right; some periodic tables include a dividing line between metals and nonmetals and the metalloids may be found close to this line. Typical metalloids have a metallic appearance, but they are brittle and only fair conductors of electricity. Chemically, they behave as nonmetals, they can form alloys with metals. Most of their other physical properties and chemical properties are intermediate in nature.
Metalloids are too brittle to have any structural uses. They and their compounds are used in alloys, biological agents, flame retardants, optical storage and optoelectronics, pyrotechnics and electronics; the electrical properties of silicon and germanium enabled the establishment of the semiconductor industry in the 1950s and the development of solid-state electronics from the early 1960s. The term metalloid referred to nonmetals, its more recent meaning, as a category of elements with intermediate or hybrid properties, became widespread in 1940–1960. Metalloids are sometimes called semimetals, a practice, discouraged, as the term semimetal has a different meaning in physics than in chemistry. In physics, it refers to the electronic band structure of a substance. A metalloid is an element that possesses properties of both metals and non metals, and, therefore hard to classify as either a metal or a nonmetal; this is a generic definition that draws on metalloid attributes cited in the literature.
Difficulty of categorisation is a key attribute. Most elements have a mixture of metallic and nonmetallic properties, can be classified according to which set of properties is more pronounced. Only the elements at or near the margins, lacking a sufficiently clear preponderance of either metallic or nonmetallic properties, are classified as metalloids. Boron, germanium, arsenic and tellurium are recognised as metalloids. Depending on the author, one or more from selenium, polonium, or astatine are sometimes added to the list. Boron sometimes is excluded, with silicon. Sometimes tellurium is not regarded as a metalloid; the inclusion of antimony and astatine as metalloids has been questioned. Other elements are classified as metalloids; these elements include hydrogen, nitrogen, sulfur, gallium, iodine, lead and radon. The term metalloid has been used for elements that exhibit metallic lustre and electrical conductivity, that are amphoteric, such as arsenic, vanadium, molybdenum, tin and aluminium.
The p-block metals, nonmetals that can form alloys with metals or modify their properties have occasionally been considered as metalloids. No accepted definition of a metalloid exists, nor any division of the periodic table into metals and nonmetals. Classifying an element as a metalloid has been described by Sharp as "arbitrary"; the number and identities of metalloids depend on. Emsley recognised four metalloids. On average, seven elements are included in such lists. A single quantitative criterion such as electronegativity is used, metalloids having electronegativity values from 1.8 or 1.9 to 2.2. Further examples include packing the Goldhammer-Herzfeld criterion ratio; the recognised metalloids have packing efficiencies of between 34% and 41%. The Goldhammer-Herzfeld ratio equal to the cube of the atomic radius divided by the molar volume, is a simple measure of how metallic an element is, the recognised metalloids having ratios from around 0.85 to 1.1 and averaging 1.0. Other authors have relied on, for example, bulk coordination number.
Jones, writing on the role of classification in science, observed that " are defined by more than two attributes". Masterton and Slowinski used three criteria to describe the six elements recognised as metalloids: metalloids have ionization energies around 200 kcal/mol and electronegativity values close to 2.0. They said that metalloids are semiconductors, though antimony and arsenic have electrical conductivities approaching those of metals. Selenium and polonium are suspected as not in this scheme. Metalloids lie on either side of the dividing line between nonmetals; this can be found, on some periodic tables. Elements to the lower left of the line display increasing metallic behaviour.
Group 6 element
Group 6, numbered by IUPAC style, is a group of elements in the periodic table. Its members are chromium, molybdenum and seaborgium; these are all transition metals and chromium and tungsten are refractory metals. The period 8 elements of group 6 are to be either unpenthexium or unpentoctium; this may not be possible. Neither unpenthexium nor unpentoctium have been synthesized, it is unlikely that this will happen in the near future. Like other groups, the members of this family show patterns in its electron configuration the outermost shells resulting in trends in chemical behavior: "Group 6" is the new IUPAC name for this group. Group 6 must not be confused with the group with the old-style group crossed names of either VIA or VIB; that group is now called group 16. Chromium was first reported on July 26, 1761, when Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains of Russia, which he named "Siberian red lead,", found out in less than 10 years to be a bright yellow pigment.
Though misidentified as a lead compound with selenium and iron components, the mineral was crocoite with a formula of PbCrO4. Studying the mineral in 1797, Louis Nicolas Vauquelin produced chromium trioxide by mixing crocoite with hydrochloric acid metallic chromium by heating the oxide in a charcoal oven a year later, he was able to detect traces of chromium in precious gemstones, such as ruby or emerald. Molybdenite—the principal ore from which molybdenum is now extracted—was known as molybdena, confused with and implemented as though it were graphite. Like graphite, molybdenite can be used to blacken a surface or as a solid lubricant; when molybdena was distinguishable from graphite, it was still confused with a galena, which took its name from Ancient Greek Μόλυβδος molybdos, meaning lead. It was not until 1778 that Swedish chemist Carl Wilhelm Scheele realized that molybdena was neither graphite nor lead, he and other chemists correctly assumed that it was the ore of a distinct new element, named molybdenum for the mineral in which it was discovered.
Peter Jacob Hjelm isolated molybdenum by using carbon and linseed oil in 1781. Regarding tungsten, in 1781 Carl Wilhelm Scheele discovered that a new acid, tungstic acid, could be made from scheelite. Scheele and Torbern Bergman suggested that it might be possible to obtain a new metal by reducing this acid. In 1783, José and Fausto Elhuyar found an acid made from wolframite, identical to tungstic acid; that year, in Spain, the brothers succeeded in isolating tungsten by reduction of this acid with charcoal, they are credited with the discovery of the element. During the 1800s, chromium was used as a component of paints and in tanning salts. At first, crocoite from Russia was the main source, but in 1827, a larger chromite deposit was discovered near Baltimore, United States; this made the United States the largest producer of chromium products until 1848 when large deposits of chromite where found near Bursa, Turkey. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924.
For about a century after its isolation, molybdenum had no industrial use, owing to its relative scarcity, difficulty extracting the pure metal, the immaturity of the metallurgical subfield. Early molybdenum steel alloys showed great promise in their increased hardness, but efforts were hampered by inconsistent results and a tendency toward brittleness and recrystallization. In 1906, William D. Coolidge filed a patent for rendering molybdenum ductile, leading to its use as a heating element for high-temperature furnaces and as a support for tungsten-filament light bulbs. In 1913, Frank E. Elmore developed a flotation process to recover molybdenite from ores. During the first World War, demand for molybdenum spiked; some British tanks were protected by 75 mm manganese steel plating, but this proved to be ineffective. The manganese steel plates were replaced with 25 mm molybdenum-steel plating allowing for higher speed, greater maneuverability, better protection. After the war, demand plummeted until metallurgical advances allowed extensive development of peacetime applications.
In World War II, molybdenum again saw strategic importance as a substitute for tungsten in steel alloys. In World War II, tungsten played a significant role in background political dealings. Portugal, as the main European source of the element, was put under pressure from both sides, because of its deposits of wolframite ore at Panasqueira. Tungsten's resistance to high temperatures and its strengthening of alloys made it an important raw material for the arms industry. Unlike other groups, the members of this family do not show patterns in its electron configuration, as two lighter members of the group are exceptions from the Aufbau principle: Most of the chemistry has been observed only for the first three members of the group; the chemistry of seaborgium is not established and therefore the rest of the section deals only with its upper neighbors in the periodic table. The elements in the group, like those of groups 7—11, have high melting points, form volatile compounds in higher oxidat
The chalcogens are the chemical elements in group 16 of the periodic table. This group is known as the oxygen family, it consists of the elements oxygen, selenium and the radioactive element polonium. The chemically uncharacterized synthetic element livermorium is predicted to be a chalcogen as well. Oxygen is treated separately from the other chalcogens, sometimes excluded from the scope of the term "chalcogen" altogether, due to its different chemical behavior from sulfur, selenium and polonium; the word "chalcogen" is derived from a combination of the Greek word khalkόs principally meaning copper, the Latinised Greek word genēs, meaning born or produced. Sulfur has been known since antiquity, oxygen was recognized as an element in the 18th century. Selenium and polonium were discovered in the 19th century, livermorium in 2000. All of the chalcogens have six valence electrons, leaving them two electrons short of a full outer shell, their most common oxidation states are −2, +2, +4, +6. They have low atomic radii the lighter ones.
Lighter chalcogens are nontoxic in their elemental form, are critical to life, while the heavier chalcogens are toxic. All of the chalcogens have some role as a nutrient or a toxin; the lighter chalcogens, such as oxygen and sulfur, are toxic and helpful in their pure form. Selenium is an important nutrient but is commonly toxic. Tellurium has unpleasant effects, polonium is always harmful, both in its chemical toxicity and its radioactivity. Sulfur has more than 20 allotropes, oxygen has nine, selenium has at least five, polonium has two, only one crystal structure of tellurium has so far been discovered. There are numerous organic chalcogen compounds. Not counting oxygen, organic sulfur compounds are the most common, followed by organic selenium compounds and organic tellurium compounds; this trend occurs with chalcogen pnictides and compounds containing chalcogens and carbon group elements. Oxygen is extracted from air and sulfur is extracted from oil and natural gas. Selenium and tellurium are produced as byproducts of copper refining.
Polonium and livermorium are most available in particle accelerators. The primary use of elemental oxygen is in steelmaking. Sulfur is converted into sulfuric acid, used in the chemical industry. Selenium's most common application is glassmaking. Tellurium compounds are used in optical disks, electronic devices, solar cells; some of polonium's applications are due to its radioactivity. Chalcogens show similar patterns in electron configuration in the outermost shells, where they all have the same number of valence electrons, resulting in similar trends in chemical behavior: All chalcogens have six valence electrons. All of the solid, stable chalcogens do not conduct heat well. Electronegativity decreases towards the chalcogens with higher atomic numbers. Density and boiling points, atomic and ionic radii tend to increase towards the chalcogens with higher atomic numbers. Out of the six known chalcogens, one has an atomic number equal to a nuclear magic number, which means that their atomic nuclei tend to have increased stability towards radioactive decay.
Oxygen has three stable isotopes, 14 unstable ones. Sulfur has four stable isotopes, 20 radioactive ones, one isomer. Selenium has six observationally stable or nearly stable isotopes, 26 radioactive isotopes, 9 isomers. Tellurium has eight stable or nearly stable isotopes, 31 unstable ones, 17 isomers. Polonium has 42 isotopes, it has an additional 28 isomers. In addition to the stable isotopes, some radioactive chalcogen isotopes occur in nature, either because they are decay products, such as 210Po, because they are primordial, such as 82Se, because of cosmic ray spallation, or via nuclear fission of uranium. Livermorium isotopes; the most stable livermorium isotope is 293Lv. Among the lighter chalcogens, the most neutron-poor isotopes undergo proton emission, the moderately neutron-poor isotopes undergo electron capture or β+ decay, the moderately neutron-rich isotopes undergo β− decay, the most neutron rich isotopes undergo neutron emission; the middle chalcogens have similar decay tendencies as the lighter chalcogens, but their isotopes do not undergo proton emission and some of the most neutron-starved isotopes of tellurium undergo alpha decay.
Polonium's isotopes tend to decay with beta decay. Isotopes with nuclear spins are more common among the chalcogens selenium and tellurium than they are with sulfur. Oxygen's most common allotrope is diatomic oxygen, or O2, a reactive paramagnetic molecule, ubiquitous to aerobic organisms and has a blue color in its liquid state. Another allotrope is O3, or ozone, three oxygen atoms bonded together in a bent formation. There is an allotrope called tetraoxygen, or O4, six allotropes of solid oxygen including "red oxygen", which has the formula O8. Sulfur has over 20 known allotropes, more than any other element except carbon; the most common allotropes are in the form of eight-atom rings, but other molecular allotropes that contain as few as two atoms or as many as 20 are known. Other notable sulfur allotropes include monoclinic sulfur. Rhombic sulfur is the more stable of the two allotropes. Monoclinic sulfur takes the form of long ne