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ch22. Chemistry of the Nonmetals.doc

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Shadi Dalili

22 Chemistry of the NonmetalsoMain group elements are grouped together because their valence electrons occupy only s or p orbitals221 Insulated NanowiresoCarbon nanotubeslong thin hollow cylinders of carbon atomsoScientists could make similar tubes from boron nitride contains BN units that are isoelectronic with carbon in the that each BN unit contains eight valence electrons or four per atomjust like carbonoSize and electronegativity of a carbon atom are also almost equal to the average of those properties for a boron atom and a nitrogen atomBN can form a number of structures similar to those formed by carbon including nanotubesoImportant difference between boron nitride nanotubes and carbon nanotubes conductivityC conducts electricity BN acts as insulatorsoCould combine a conducting C nanotube within an insulating BN nanotube forming an insulated conducting wire222 The Main Group Elements Bonding and PropertiesAtomic Size and Types of BondsoEffective nuclear charge becomes greater as we move to the right across a row resulting in smaller radii and increasing ionization energy and electronegativityoTend to form anions in ionic compounds easily reduced gaining electrons to completely fill their p orbitals to attain noble gas configurationsoAct in reactions as oxidizing agents oxidize other substances while being reducedoSmallest halogens and elements in the oxygen group are the strongest oxidizing agents in the p blockoElements neat the centre of the p block have fewer p electrons and do not usually fill the p orbitals by forming anions instead they share electrons forming covalent bondsoElements on the far left of the p block have only one p electron and form cations in ionic compounds and electrondeficient species species with an incomplete octet in covalent compoundsoLeft of diagonal line elements are metals that form cations and metallic compoundsoRight of diagonal elements are nonmetals that form anions and covalent compoundsoMetallic elements such as thallium and lead have very low electronegativities 18 and 19 nonmetallic elements such as oxygen and fluorine have highest electronegativities 35 and 401223 Silicates The Most Abundant Matter in Earths CrustoMost abundant elements in Earths crust are oxygen 4550 by mass and silicon about 28 by mass Al Fe Ca Na Mg K Ti HoSilicatescovalent atomic solids that contain silicon oxygen and various metal atoms ex rocks clays and soils contain silicatesoStructures of silicates determine their propertiesQuartz and GlassoSilicon and oxygen form a network covalent structure in which a silicon atom bonds to four oxygen atoms by single covalent sigma bonds forming tetrahedral shape with silicon atom in the middle and four oxygen atoms at the corners forms single bonds because silicon atom is too large to allow substantial overlap between the p orbitals on the two atomsoSilicon atom in this structure obtains a complete octet each oxygen atom is one electron short of an octeteach O atom forms a second covalent bond to a different Si atom forming the 3D structure of quartz has a formula unit of SiO generally 2called silica each Si atom is in a tetrahedron surrounded by four O atoms each O atom acts as a bridge connecting the corners of two tetrahedronsoIf cooled quickly after melting silica it does not crystallize back into the quartz structure but Si and O atoms form a randomly ordered or amorphous structure called glass common glass is amorphous SiO2oCarbon often bonds to oxygen with a double bond one sigma and one pi bondAluminosilicatesoAlumiosilicates are a family of compounds in which aluminum atoms substitute for silicon atoms in some of the lattice sites of the silica structureoAluminum ion has only three valence electrons in contrast to the four valence electrons of Si a SiO unit becomes AlO upon 22substitution oNegative charge is balanced by a positive counterion ex Naprovide necessary balancing in albite NaAlSiO or NaAlO382SiOin which onefourth of Si atoms are substituted by Al atoms 23 ex anorthite CaAlSiO has Al atoms replacing half of the Si 2282atoms and the charge is balanced by Ca ionsIndividual Silicate Units Silicate Chains and Silicate SheetsoIn many silicates the oxygen atoms are not connected to two silicon atoms to form the neutral compound that is found in quartzoInstead O atoms gain electrons from metal atoms and form 4poyatomic anions such as SiO positively charged metal ions 4then bond to the silicon oxide In these minerals the SiO4 tetrahedrons occur singly in chains or in sheetso When occurs singly not bonded to other tetrahedrons it forms 4 the SiOpolyatomic anion which has four extra electrons that 42
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