Aperiod 2 element is one of thechemical elements in the second row (orperiod) of theperiodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behavior of the elements as theiratomic number increases; a new row is started when chemical behavior begins to repeat, creatingcolumns of elements with similar properties.
Calculated atomic radii of period 2 elements in picometers.
Period 2 is the first period in the periodic table from whichperiodic trends can be drawn.Period 1, which only contains two elements (hydrogen andhelium), is too small to draw any conclusive trends from it, especially because the two elements behave nothing like other s-block elements.[1][2] Period 2 has much more conclusive trends. For all elements in period 2, as the atomic number increases, theatomic radius of the elements decreases, theelectronegativity increases, and theionization energy increases.[3]
Period 2 only has twometals (lithium and beryllium) of eight elements, less than for any subsequent period both by number and by proportion. It also has the most number of nonmetals, namely five, among all periods. The elements in period 2 often have the most extreme properties in their respective groups; for example, fluorine is the most reactivehalogen, neon is the most inertnoble gas,[4] and lithium is the least reactivealkali metal.[5]
Lithium (Li) is analkali metal with atomic number 3, occurring naturally intwo isotopes:6Li and7Li. The two make up all natural occurrence of lithium on Earth, althoughfurther isotopes have been synthesized. Inionic compounds, lithium loses anelectron to become positively charged, forming thecation Li+. Lithium is the first alkali metal in the periodic table,[note 1] and the first metal of any kind in the periodic table.[note 2] Atstandard temperature and pressure, lithium is a soft, silver-white, highly reactivemetal. With adensity of 0.564 g⋅cm−3, lithium is the lightest metal and the least dense solid element.[6]
Lithium is one of the few elementssynthesized in theBig Bang.Lithium is the 31st most abundant element on earth,[7] occurring in concentrations of between 20 and 70 ppm by weight,[8] but due to its high reactivity it is only found naturally incompounds.[8]
Beryllium (Be) is the chemical element with atomic number 4, occurring in the form of9Be. At standard temperature and pressure, beryllium is a strong, steel-grey, light-weight,brittle,bivalentalkaline earth metal, with a density of 1.85 g⋅cm−3.[12] It also has one of the highestmelting points of all thelight metals. Beryllium's most commonisotope is9Be, which contains 4 protons and 5 neutrons. It makes up almost 100% of all naturally occurring beryllium and is its only stable isotope; howeverother isotopes have been synthesised. In ionic compounds, beryllium loses its twovalence electrons to form the cation, Be2+.
Due to its stiffness, light weight, and dimensional stability over a wide temperature range, beryllium metal is used in as a structural material in aircraft, missiles andcommunication satellites.[12] It is used as an alloying agent inberyllium copper, which is used to make electrical components due to its high electrical and heat conductivity.[13] Sheets of beryllium are used inX-ray detectors to filter outvisible light and let only X-rays through.[12] It is used as aneutron moderator innuclear reactors because light nuclei are more effective at slowing down neutrons than heavy nuclei.[12] Beryllium's low weight and high rigidity also make it useful in the construction oftweeters inloudspeakers.[14]
Beryllium and beryllium compounds are classified by theInternational Agency for Research on Cancer asGroup 1 carcinogens; they are carcinogenic to both animals and humans.[15] Chronicberylliosis is apulmonary andsystemicgranulomatous disease caused by exposure to beryllium. Between 1% – 15% of people are sensitive to beryllium and may develop an inflammatory reaction in theirrespiratory system andskin, called chronic beryllium disease orberylliosis. The body'simmune system recognises the beryllium as foreign particles and mounts an attack against them, usually in the lungs where they are breathed in. This can cause fever, fatigue, weakness, night sweats and difficulty in breathing.[16]
Boron (B) is the chemical element with atomic number 5, occurring as10B and11B. At standard temperature and pressure, boron is atrivalentmetalloid that has several differentallotropes.Amorphous boron is a brown powder formed as a product of many chemical reactions.Crystalline boron is a very hard, black material with a high melting point and exists in manypolymorphs: Tworhombohedral forms, α-boron and β-boron containing 12 and 106.7 atoms in the rhombohedral unit cell respectively, and 50-atomtetragonal boron are the most common. Boron has a density of 2.34−3.[17] Boron's most commonisotope is11B at 80.22%, which contains 5 protons and 6 neutrons. The other common isotope is10B at 19.78%, which contains 5 protons and 5 neutrons.[18] These are the only stable isotopes of boron; howeverother isotopes have been synthesised. Boron forms covalent bonds with othernonmetals and hasoxidation states of 1, 2, 3 and 4.[19][20][21]Boron does not occur naturally as a free element, but in compounds such asborates. The most common sources of boron aretourmaline,borax, Na2B4O5(OH)4·8H2O, andkernite, Na2B4O5(OH)4·2H2O.[17] it is difficult to obtain pure boron. It can be made through themagnesiumreduction ofboron trioxide, B2O3. This oxide is made by meltingboric acid, B(OH)3, which in turn is obtained from borax. Small amounts of pure boron can be made by thethermal decomposition of boron bromide, BBr3, in hydrogen gas over hottantalum wire, which acts as acatalyst.[17] The most commercially important sources of boron are:sodium tetraborate pentahydrate, Na2B4O7 · 5H2O, which is used in large amounts in making insulatingfiberglass andsodium perboratebleach;boron carbide, aceramic material, is used to make armour materials, especially inbulletproof vests for soldiers and police officers;orthoboric acid, H3BO3 or boric acid, used in the production of textilefiberglass andflat panel displays; sodium tetraborate decahydrate, Na2B4O7 · 10H2O or borax, used in the production of adhesives; and the isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons.[18]
Boron is an essential plantmicronutrient, required for cell wall strength and development, cell division, seed and fruit development, sugar transport and hormone development.[22] However, high soil concentrations of over 1.0ppm can cause necrosis in leaves and poor growth. Levels as low as 0.8 ppm can cause these symptoms to appear in plants particularly boron-sensitive. Most plants, even those tolerant of boron in the soil, will show symptoms of boron toxicity when boron levels are higher than 1.8 ppm.[18] In animals, boron is anultratrace element; in human diets, daily intake ranges from 2.1 to 4.3 mg boron/kg body weight (bw)/day.[23] It is also used as a supplement for the prevention and treatment of osteoporosis and arthritis.[24]
Diamond and graphite, two differentallotropes of carbon
Carbon is the chemical element with atomic number 6, occurring as12C,13C and14C.[25] At standard temperature and pressure, carbon is a solid, occurring inmany different allotropes, the most common of which aregraphite,diamond, thefullerenes andamorphous carbon.[25] Graphite is a soft,hexagonal crystalline, opaque blacksemimetal with very goodconductive andthermodynamically stable properties. Diamond however is a highlytransparentcolourlesscubic crystal with poor conductive properties, is thehardest known naturally occurring mineral and has the highestrefractive index of allgemstones. In contrast to thecrystal lattice structure of diamond and graphite, thefullerenes aremolecules, named afterRichard Buckminster Fuller whose architecture the molecules resemble. There are several different fullerenes, the most widely known being the "buckeyball" C60. Little is known about the fullerenes and they are a current subject of research.[25] There is also amorphous carbon, which is carbon without any crystalline structure.[26] Inmineralogy, the term is used to refer tosoot andcoal, although these are not truly amorphous as they contain small amounts of graphite or diamond.[27][28] Carbon's most common isotope at 98.9% is12C, with six protons and six neutrons.[29]13C is also stable, with six protons and seven neutrons, at 1.1%.[29] Trace amounts of14C also occur naturally but thisisotope is radioactive and decays with a half-life of 5730 years; it is used forradiocarbon dating.[30] Otherisotopes of carbon have also been synthesised. Carbon forms covalent bonds with other non-metals with an oxidation state of −4, −2, +2 or +4.[25]
Carbon is the fourth most abundant element in the universe by mass afterhydrogen,helium and oxygen[31] and is the secondmost abundant element in the human body by mass after oxygen,[32] the third most abundant by number of atoms.[33] There are an almost infinite number of compounds that contain carbon due to carbon's ability to form long stable chains of C — C bonds.[34][35] The simplest carbon-containing molecules are thehydrocarbons, which contain carbon and hydrogen,[34] although they sometimes contain other elements infunctional groups. Hydrocarbons are used asfossil fuels and to manufactureplastics andpetrochemicals. Allorganic compounds, those essential for life, contain at least one atom of carbon.[34][35] When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds[35] includingsugars,lignans,chitins,alcohols,fats, and aromaticesters,carotenoids andterpenes. Withnitrogen it formsalkaloids, and with the addition of sulfur also it formsantibiotics,amino acids, andrubber products. With the addition of phosphorus to these other elements, it formsDNA andRNA, the chemical-code carriers of life, andadenosine triphosphate (ATP), the most important energy-transfer molecule in all living cells.[35]
Nitrogen is the chemical element with atomic number 7, the symbolN andatomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostlyinertdiatomic gas atstandard conditions, constituting 78.08% by volume ofEarth's atmosphere. The element nitrogen was discovered as a separable component of air, by Scottish physicianDaniel Rutherford, in 1772.[36] It occurs naturally in form of two isotopes: nitrogen-14 and nitrogen-15.[37]
Many industrially important compounds, such asammonia,nitric acid, organic nitrates (propellants andexplosives), andcyanides, contain nitrogen. The extremely strong bond in elemental nitrogen dominates nitrogen chemistry, causing difficulty for both organisms and industry in breaking the bond to convert theN 2 molecule into usefulcompounds, but at the same time causing release of large amounts of often useful energy when the compounds burn, explode, or decay back into nitrogen gas.
Nitrogen occurs in all living organisms, and thenitrogen cycle describes movement of the element from air into thebiosphere and organic compounds, then back into the atmosphere. Synthetically producednitrates are key ingredients of industrialfertilizers, and also key pollutants in causing theeutrophication of water systems. Nitrogen is a constituent element ofamino acids and thus ofproteins, and ofnucleic acids (DNA andRNA). It resides in thechemical structure of almost allneurotransmitters, and is a defining component ofalkaloids, biological molecules produced by many organisms.[38]
Oxygen is the chemical element with atomic number 8, occurring mostly as16O, but also17O and18O.
Oxygen is the third-most common element by mass in the universe (although there are more carbon atoms, each carbon atom is lighter). It is highly electronegative and non-metallic, usually diatomic, gas down to very low temperatures. Only fluorine is more reactive among non-metallic elements. It is two electrons short of a full octet and readily takes electrons from other elements. It reacts violently withalkali metals andwhite phosphorus at room temperature and less violently with alkali earth metals heavier than magnesium. At higher temperatures it burns most other metals and many non-metals (including hydrogen, carbon, and sulfur). Many oxides are extremely stable substances difficult to decompose—likewater,carbon dioxide,alumina,silica, and iron oxides (the latter often appearing asrust). Oxygen is part of substances best described as some salts of metals and oxygen-containing acids (thus nitrates, sulfates, phosphates, silicates, and carbonates.
Oxygen is essential to all life. Plants andphytoplankton photosynthesize water and carbon dioxide and water, both oxides, in the presence of sunlight to formsugars with the release of oxygen. The sugars are then turned into such substances as cellulose and (with nitrogen and often sulfur) proteins and other essential substances of life. Animals especially but also fungi and bacteria ultimately depend upon photosynthesizing plants and phytoplankton for food and oxygen.
Fire uses oxygen to oxidize compounds typically of carbon and hydrogen to water and carbon dioxide (although other elements may be involved) whether in uncontrolled conflagrations that destroy buildings and forests or the controlled fire within engines or that supply electrical energy from turbines, heat for keeping buildings warm, or the motive force that drives vehicles.
Oxygen forms roughly 21% of the Earth's atmosphere; all of this oxygen is the result of photosynthesis. Pure oxygen has use in medical treatment of people who have respiratory difficulties.Excess oxygen is toxic.
Oxygen was originally associated with the formation of acids—until some acids were shown to not have oxygen in them. Oxygen is named for its formation of acids, especially with non-metals. Some oxides of some non-metals are extremely acidic, likesulfur trioxide, which formssulfuric acid on contact with water. Most oxides with metals are alkaline, some extremely so, likepotassium oxide. Some metallic oxides are amphoteric, like aluminum oxide, which means that they can react with both acids and bases.
Although oxygen is normally a diatomic gas, oxygen can form an allotrope known asozone. Ozone is a triatomic gas even more reactive than oxygen. Unlike regular diatomic oxygen, ozone is a toxic material generally considered a pollutant. In the upper atmosphere, some oxygen forms ozone which has the property of absorbing dangerous ultraviolet rays within theozone layer. Land life was impossible before the formation of an ozone layer.
Fluorine is the chemical element with atomic number 9. It occurs naturally in its only stable form19F.[39]
Fluorine is a pale-yellow, diatomic gas under normal conditions and down to very low temperatures. Short one electron of the highly stable octet in each atom, fluorine molecules are unstable enough that they easily snap, with loose fluorine atoms tending to grab single electrons from just about any other element. Fluorine is the most reactive of all elements, and it even attacks many oxides to replace oxygen with fluorine. Fluorine even attacks silica, one of the favored materials for transporting strong acids, and burns asbestos. It attackscommon salt, one of the most stable compounds, with the release of chlorine. It never appears uncombined in nature and almost never stays uncombined for long. It burns hydrogen simultaneously if either is liquid or gaseous—even at temperatures close to absolute zero.[40] It is extremely difficult to isolate from any compounds, let alone keep uncombined.
Fluorine gas is extremely dangerous because it attacks almost all organic material, including live flesh. Many of the binary compounds that it forms (called fluorides) are themselves highly toxic, including soluble fluorides and especiallyhydrogen fluoride. Fluorine forms very strong bonds with many elements. With sulfur it can form the extremely stable and chemically inertsulfur hexafluoride; with carbon it can form the remarkable materialTeflon that is a stable and non-combustible solid with a high melting point and a very low coefficient of friction that makes it an excellent liner for cooking pans and raincoats. Fluorine-carbon compounds include some unique plastics.it is also used as a reactant in the making of toothpaste.
Neon is the chemical element with atomic number 10, occurring naturally as three stable isotopes:20Ne,21Ne and22Ne.[41]
Neon is amonatomic gas. With a complete octet of outer electrons, it is highly resistant to electron removal and does not readily accept electrons, making it effectively inert. It is classified as one of thenoble gases.
Although relatively scarce on Earth, neon is the fifth most abundant element in the universe, formed during thealpha process in stars. On Earth it is obtained byfractional distillation of liquid air, where it is present at about 18 ppm by volume in the atmosphere. Neon has no known biological role.
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