Chemistry - an illustrated guide to science (794128), страница 7
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Theformula mass of an ionic compound isthe sum of the atomic masses of theions in their simplest ratio.● Sodium chloride consists of an ioniclattice in which the ions are present inthe ratio 1:1. Therefore, the formula ofsodium chloride is taken to be NaCl .Atomic mass of sodium = 23;chlorine = 35.5.Formula mass of sodium chloridechlorinesodium= 23 + 35.5 = 58.5Na = 23.0 0Cl = 35.45© Diagram Visual Information Ltd.● Ion ic38ELEMENTS AND COMPOUNDSKey wordsanionbondcationcoordinationnumberionic crystalionlatticeStructure of some ioniccrystals1 Simple cubic structure (CsCl)cationsanionsIonic crystals● Inan ion ic crystal, each ion issurrounded by a number of oppositelycharged ions in a lattice structure.● There are several types of ionicstructures.Simple: The atoms form grids.Body centered: One atom sits in thecenter of each cube.Face centered: One atom sits in each“face” of the cube.● The lattice structure is determined bytwo factors:1.
the ratio of the number of cation s(positively charged ions) to an ion s(negatively charged ions)2. the ratio of the radii of the ions.● In general, the higher the value of theradius ratio the higher thecoordin ation n u m ber of the lattice.The coordination number is thenumber of atoms, ions, or moleculesto which bon ds can be formed.2 Face-centered cubic structure (NaCl)cationsanions1 Simple cubic structure(CsCl)● Incesium chloride, the radius ratio is0.94 (due to the large cesium ion).The coordination is 8:8. Each ion issurrounded by 8 oppositely chargedions.2 Face-centered cubicstructure (NaCl)radius ratio in the sodiumchloride lattice is 0.57.
Thecoordination is 6:6. Each ion issurrounded by 6 oppositely chargedions.© Diagram Visual Information Ltd.● The3 Body-centered cubicstructure (CaF2)● Incalcium fluoride the radius ratio is0.75. The coordination is 8:4. Eachcalcium ion is surrounded by 8fluoride ions, while each fluoride ion issurrounded by 4 calcium ions.3 Body-centered cubic structure (CaF2 )cationsanions39Crystal structure ofmetals: lattice structure1 Hexagonal close packingELEMENTS AND COMPOUNDSKey wordsbody centeredcubic packingcrystalface-centeredcubic closepackinghexagonal closepackinglatticeunit cellMetallic crystals● Likeall other crystals, metallic crystalsare composed of u n it cells, sets ofatoms, ions, or molecules in orderlythree dimensional arrangements calledlattices.1 Hexagonal close packing● When2 Face-centered cubic close packingarranged in a single layer, themost efficient method of packing theions is in the form of a hexagon inwhich each ion is surrounded by sixother ions.● In hexagon al close packin g, a secondlayer is positioned so that each ion inthe second layer is in contact withthree ions in the first layer.
The thirdlayer is placed directly above the first,and the fourth layer directly above thesecond, etc. This arrangement issometimes represented as ABABAB.2 Face-centered cubicclose packing● Herethe third layer does not sitdirectly above either the first orsecond layers.
The pattern is repeatedafter three layers, giving rise to anABCABCABC arrangement.3 Body-centered cubicpacking● Herethe layers are formed from ionsarranged in squares. The second layeris positioned so that each sphere inthe second layer is in contact with fourspheres in the first layer. The thirdlayer sits directly above the first layer,giving rise to an ABABAB arrangement.© Diagram Visual Information Ltd.3 Body-centered cubic packing40ELEMENTS AND COMPOUNDSKey wordsbody-centeredcubic packingcoordinationnumberface-centeredcubic closepackinghexagonal closepackingCrystal structure ofmetals: efficient packing1 Efficient packingHexagonal close packing1 Efficient packing● Bothhexagon al close packin g andface-cen tered cu bic close packin g maybe considered as efficient packingsince the spheres occupy 74 percentof the available space.
In botharrangements, each sphere is incontact with 12 others, and is said tohave a coordin ation n u m ber of 12.2 Less efficient packing● Body-cen teredcu bic packin g is lessefficient than hexagonal and facecentered cubic close packing. Spheresoccupy only 68 percent of the availablespace. Each sphere is in contact witheight others (four in the layer aboveand four in the layer below) and,therefore, has a coordination numberof eight.Face-centered cubic close packingMetals showing hexagonalclose packing● Cobalt● Magnesium● Titanium● ZincMetals showing facecentered cubic closepacking● Aluminum● Calcium● Copper● Lead© Diagram Visual Information Ltd.● NickelMetals showing bodycentered cubic packing● Group1 metals● Barium● Chromium● Iron● Vanadium2 Less efficient packingBody-centered cubic packing41Chemical combination:ionic bondingELEMENTS AND COMPOUNDSKey wordsanionbondcationchlorideionic bonding1 Formation of sodium chloride (NaCl)noble gasesoxideshellIonic bonding● IonicSodium atom (Na)bon ds are formed by theattraction of opposite charges.● In ion ic bon din g, the atoms in acompound gain, lose, or shareelectrons so the number of electronsin their outer shell is the same as thenearest n oble gas on the periodictable.● Non-metals gain electrons to givenegatively charged ions (an ion s).● Metal atoms loose electrons to givepositively charged ions (cation s).Chlorine atom (Cl)1 Formation of sodiumchloride (NaCl)● A sodiumChlorine ion (Cl–)Sodium ion (Na+ )Na+NaCl –ClSodium chloride(NaCl)atom has one electron in itsouter shell.
The easiest way it canattain a complete outer shell is bylosing this electron to form a sodiumion, Na+ .● A chlorine atom has seven electronsin its outer shell. The easiest way itcan attain a complete outer shell isby gaining one more electron toform a chloride ion, Cl - .2 Formation ofmagnesium oxide(MgO)● A magnesiumatom has twoelectrons in its outer shell. It losesthese electrons to form amagnesium ion, Mg2+ .● An oxygen atom has six electrons in itsouter shell. It gains two electrons toform an oxide ion, O2- .2 Formation of magnesium oxide (MgO)MgMagnesium ion (Mg2+ )OOxygen ion (O2–)O2–Magnesium oxideNa (sodium atom)Na+ (sodium ion)2.8.12.8Cl (chlorine atom)Cl - (chloride ion)Mg (magnesium atom)Mg2+ (magnesium ion)O (oxygen atom)O2- (oxide ion)2.8.72.8.82.8.22.82.62.8© Diagram Visual Information Ltd.Electronic configuration42ELEMENTS AND COMPOUNDSKey wordscarbonateionlimiting formnitrateradicalresonancestructuresulfateChemical combination:ionic radicals1 Carbonate ionRadicalsis a group of atoms thatcannot be represented by onestructural formula.
It can passunchanged through a series ofchemical reactions. Radicals includethe carbon ate ion, CO32- , the n itrateion, NO3- , and the su lfate ion, SO4 2- .–O● A radicalOOCO–O–OCCO –O–OO2–COOresonancestructurelimitingforms1 Carbonate ion● Thecarbon atom is bonded to threeoxygen atoms. By transferringelectrons, it is possible to write threelim itin g form s for this ion .
(Limitingforms are the possibilities for thedistribution of electrons in a moleculeor ion.)● Electrons are continually beingtransferred in the ion. Thus its exactform is constantly changing. The ion isbest represented as a reson an cestru ctu re (the average of the limitingforms) in which dotted lines indicatethat the charge on the ion, 2-, isspread over all three of thecarbon–oxygen bonds.2 Nitrate ionOO–ONO–OON–ONOOONOlimitingforms–Oresonancestructure2 Nitrate ion● Thenitrogen atom is bonded to threeoxygen atoms. This ion has threelimiting forms.● It is best represented as a resonancestructure in which dotted linesindicate that the charge on the ion, 1-,is spread over all three of thenitrogen–oxygen bonds.3 Sulfate ion© Diagram Visual Information Ltd.● Thesulfur atom is bonded to fouroxygen atoms.
This ion has threelimiting forms.● The ion’s exact form is constantlychanging. It is best represented as aresonance structure in which dottedlines indicate that the charge on theion, 2-, is spread over all four of thesulfur–oxygen bonds.3 Sulfate ion–O–O–OSOOOO–OSSOOO–limitingformsO–OOSOO–OSOOOresonancestructure2–43Chemical combination:covalent bonding1 The hydrogen moleculeELEMENTS AND COMPOUNDSKey wordsammoniabondcarbonhydrogenionic compoundmethanenitrogenshellCovalent bonding● Atomshydrogenmolecule (H2)2 hydrogenatomsgain stability by having acomplete outer shell of electrons.
Inion ic com pou n ds, this is achieved bythe transfer of electrons. In covalentbonding, atoms share electrons.1 The hydrogen molecule3 The ammonia molecule● A hydrogenatom has one electron inits outer shell. In a hydrogenmolecule, two hydrogen atoms eachdonate this electron to form a bon d.Each hydrogen atom can be thoughtof as having control of the pair ofelectrons in the bond.
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