Chemistry - an illustrated guide to science (794128), страница 8
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Thus, each canbe thought of as having a full outershell of electrons. The single bond isshown as H-H.2 The ammonia molecule● A n itrogennitrogenatom3 hydrogenatomsammoniamolecule (NH3)4 The methane moleculeatom has five electrons inits outer shell and needs another threeelectrons to complete the shell.
Inam m on ia, three hydrogen atoms eachdonate one electron to form three N-Hbonds. The nitrogen atom now hascontrol of eight electrons and has acomplete outer shell, while eachhydrogen atom has control of twoelectrons and also has a completeouter shell.3 The methane moleculeatom has four electrons inits outer shell and needs another fourelectrons to complete the shell.
Inm ethan e, four hydrogen atoms eachdonate one electron to form four C-Hbonds. The carbon atom now hascontrol of eight electrons and has acomplete outer shell, while eachhydrogen atom has control of twoelectrons and also has a completeouter shell.carbon atom4 hydrogenatomsamolecule (CH4 )© Diagram Visual Information Ltd.● A carbon44ELEMENTS AND COMPOUNDSKey wordsammonium ioncoordinatebondingcovalent bondcovalentcompoundhydronium ionlone pairChemical combination:coordinate bonding1 Ammonium ionHCoordinate bondingbon din g is a particularform of covalent bonding in which oneatom provides both electrons that thetwo atoms share.+H● Coordin ateNH+H1 Ammonium ionis a non-bonding or lon e pair ofelectrons on the nitrogen atom of anammonia molecule. Nitrogen uses thislone pair to form a coordinate bondwith a hydrogen ion, forming theam m on iu m ion , NH4 + .••NHHH● ThereH2 Hydronium ionH2 Hydronium ionhydron iu m ion , H3O+ , forms in asimilar way.+H● TheAl 3+ ion is very small and carries ahigh charge.
It attracts electrons sostrongly that aluminum chloride is acovalen t com pou n d. It exists as Al 2Cl 6molecules in which two AlCl 3molecules are linked by coordinatebonds formed by the donation of lonepairs of electrons from two chlorineatoms.H+HHClClClAlAl● The© Diagram Visual Information Ltd.H3 Aluminum chloride4 Ionic compounds withcovalent characterions in a sodium chloride latticeare perfectly spherical. Thus the bondsin this compound are said to beperfectly ionic.● But in an ionic compound consistingof a small, highly charged positive ionand a large negative ion such aslithium iodide, the positive ion attractselectron charge away from thenegative ion.
The result is that thenegative ion becomes distorted, andelectron density becomesconcentrated between the ions,creating a bond similar to a covalen tbon d. Compounds like lithium iodideare said to be ionic with covalentcharacter.O•● The•••3 Aluminum chlorideOClCl••Cl4 Ionic compounds with covalent characterNa+Cl –Li +I–45Mixtures and solutionsCHANGES IN MATTERKey wordsProducing a solutionemulsionionic compoundmixturesolublesolutionsuspension1 Solutions● A solu tionwatersaltclear liquidProducing a suspensionis a homogeneous m ixtu reof substances.
Particles in solutions arevery small and cannot be seen. Theparticles may be atoms, ions, ormolecules, and their diameters aretypically less than 5 nm. Salt dissolvesin water to form a clear colorlesssolution.● Many, but not, all ion ic com pou n dsare solu ble in water.● A small proportion of organiccompounds are soluble in water.However, organic compounds aregenerally more soluble in organicsolvents such as hexane and ethanol.2 Suspensions● A su spen sionwaterflourcloudy mixture with solidparticles in the liquidis a heterogeneousmixture of two components. Theparticles will settle out over a period oftime. Suspended particles havediameters that are typically 1,000 nmor more.● When flour is mixed with water, itforms a white suspension.
Tinyparticles of flour are suspended in thewater. The flour particles can befiltered off from the suspension.3 Emulsionsem u lsion is a colloidal dispersionof small droplets of one liquid inanother (See page 46).● When oil and water are mixed, theyform an emulsion. Oil is less densethan water and forms the upper layer.● Tiny oil droplets are suspended in thewater.
After a while, the oil dropletsjoin together, and two layers areformed.● The mixture of oil and water can beseparated using a separating funnel.wateroilcloudy mixtureforming an emulsionwhen shakenliquids separatewhen leftto stand© Diagram Visual Information Ltd.● AnProducing an emulsion46CHANGES IN MATTERKey wordsaerosolcolloidemulsionfiltrationfoamgelsolsolutionsuspensionColloids1 In AirColloids● A colloidis a substance made ofparticles whose size is intermediatebetween those in solu tion s andsu spen sion s.● The particles in a suspension have adiameter of typically 1,000 nm ormore.
The particles in a suspensionwill settle over a period of time.● The particles in a colloid areapproximately 500 nm or less indiameter and do not settle onstanding.● The particles in a colloid cannot beseparated from the dispersion mediumby ordinary techniques like filtrationand centrifugation.● A colloid consists of a dispersingmedium and dispersed substance.These terms are analogous to theterms solute and solvent.● Colloids are classified according to theoriginal phases of their constituents.The main types are: aerosols, foam s,em u lsion s, sols, and gels.● Aerosols are extremely small solid orliquid particles suspended in air oranother gas.● Foams form when a gas is suspendedin a liquid or a solid.● Emulsions form when small particlesof a liquid are suspended in anotherliquid.● Sols form when solid particles aresuspended in a liquid.● Gels are solid particles arranged as afine network in a liquid to form a jelly.aerosol1 Colloid typeaerosolgasliquidclouds, fog, insecticide sprayaerosolgassoliddust, smoke2 In liquidspaint1 Colloid typeExamples2 Phase of3 Phase ofdispersing medium dispersed substancefoamliquidgasfroth, whipped creamemulsionliquidliquidmilk, salad dressingsolliquidsolidmilk of magnesia, paint3 In solidscork© Diagram Visual Information Ltd.Examples2 Phase of3 Phase ofdispersing medium dispersed substance1 Colloid typepolystyreneExamples2 Phase of3 Phase ofdispersing medium dispersed substancesolid foamsolidgascork, polyurethanegelsolidliquidagar, geletine, jellysolid solsolidsolidalloys47Simple and fractionaldistillationSimple distillation of sea waterboiling point (°C)cwaterfCHANGES IN MATTERKey wordsboiling pointdistillationfractionaldistillationmixture1001,420salt1 Simple distillation● Distillationis a process in which am ixtu re of materials is heated toseparate the components.● Simple distillation is used when theboilin g poin ts of the components arewidely separated.● In the diagram, salt water is placed in around-bottom flask.
Water boils at100°C and becomes water vapor.● A condenser consists of an inner tubesurrounded by a jacket of cold water.This jacket ensures that the inner tuberemains cool.● The vapor passes into the condenser,where it is cooled and changes backinto liquid.● The water runs out of the condenserand is collected in a second flask.● The salt remains in the roundbottomed flask.edgbFractional distillation of ethanolboiling point (°C)cfwater100ethanol7878°C79°C80°Cidejhbabcdefghijsea waterheat sourcethermometercondenser with cold watercold water incold water outdistillate of pure watersolution of alcohol and waterfractionating column of glass beadsdistillate of ethanol2 Fractional distillation● Fraction aldistillation is used toseparate components whose boilingpoints are similar.● Ethanol boils at 78°C and turns tovapor. Because the boiling point ofwater is only 100°C, a significantamount of water also becomes vaporas a result of evaporation.● The fractionating column containsglass beads, which provide a largesurface area for vapor to condense andthe resulting liquid to subsequentlyboil.● As the vapor mixture moves up thefractionating column, it condenses andthen boils again to become vapor.Each time, the proportion of ethanolin the mixture increases.© Diagram Visual Information Ltd.a48CHANGES IN MATTERKey wordsfiltrateimmiscibleinsolublemixtureresiduesolublesolventSeparating solutions1 Separating a mixture of two solids1 Separating a mixture oftwo solids● Sugaris solu ble in ethanol, while salt isin solu ble.
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