g - Wits Structural Chemistry

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g - Wits Structural Chemistry

Illustrating Z eff for Mg10:38 7


QUESTIONCalculate the effective nuclear charge ( Z eff ) forNa.Z eff = Z-S= 11-10= +110:38 8


Sizes of Atoms and IonsATOMIC RADIUS• Consider a simple diatomicmolecule.• The ……………………. thetwo nuclei is called the bonddistance.• If atoms which make up the………………………. thesame, then half the bonddistance is called the covalentradius of the atom.9


Sizes of Atoms and IonsPeriodic Trends in Atomic Radii• As a consequence of the ordering in the periodic table,properties of elements vary periodically.• Atomic size …………………. consistently through theperiodic table.• As we move down a group, the atoms become larger.• As we move across a period, atoms become smaller.There are two factors at work:• principal quantum number, n, and• the effective nuclear charge, Z eff .10


Sizes of Atoms and IonsPeriodic Trends in Atomic Radii• Down a group: As n increases the atomic radiusincreases.Across the periodic table:-the number of …………………… electrons remainsconstant.- the effective nuclear charge increases.Hence an increased ……………………between the nucleusand the10:38 12outermost electrons. This attraction causes the atomic


Sizes of Atoms and IonsPeriodic Trends in Atomic RadiiAs a general rule, A valence shell electron is:• Strongly shielded by electrons in inner shells,which are closer to the nucleus• Less ……………….. shielded by other electrons in thesame shell s > p > d > f• Weakly shielded by other electrons in the samesubshell, Which are at the same ………………. from thenucleus13


QUESTIONArrange the following in order of increasingatomic radii.a. Mg, Cl & AlAnswerMg> Al > Cl14


Sizes of Atoms and IonsTrends in the Sizes of Ions• Ion size is the …………….. between ions in an ioniccompound.• Ion size also depends on nuclear charge, number ofelectrons, and orbitals that contain the valence electrons.• Cations: ……………. than the parent ion.• Anions: ……………than the parent ion.15


10:38 16


Sizes of Atoms and IonsTrends in the Sizes of Ions• For ions of the same charge, ion size …………….. downa group.• All the members of an …………………….. have thesame number of electrons.• As nuclear charge increases in an isoelectronic series theions become ………………………O 2- > F - > Na +10:38 17


QUESTIONArrange the following isoelectronicseries in increasing order of size:Al 3+ ,O 2- , F - , Na + & Mg 2+AnswerO 2- > F - > Na + > Mg 2+ > Al 3+ 18


Relevance of ionic size• Zinc containing enzyme carbonic anhydrase inred blood cells facilitates the speed of keybiological reactions.• CO 2 (aq) + H 2 O(l) HCO 3- (aq) + H + (aq)• Ionic size important but not only factor whenconsidering reaction.• Cd 2+ binds preferentially over Zn 2+ with atoms ofthe protein however the reaction of CO 2 withwater is not facilitated.• Dramatic reversal of biological properties with10:38Cd 2+ 19cannot be fully explained.


Ionization Energy• The first ionization energy, I 1 , is the amount of energyrequired to remove an electron from a gaseous atom:Na(g) Na + (g) + e - .• The second ionization energy, I 2 , is the energy required toremove an electron from a gaseous ion:Na + (g) Na 2+ (g) + e - .• The larger ionization energy, the more difficult it is toremove the electron.21


Ionization Energy22


Ionization EnergyVariations in Successive IonizationEnergies• There is a sharp increase in ionization energy when acore electron is removed.23


Ionization EnergyPeriodic Trends in Ionization Energies• Ionization energy …………………….. a group andincreases across a period.• Outermost electron is more readily……………………… as we go down a group as theatom gets bigger.• Ionization energy generally increases across a period.• Z eff increases, hence it becomes more difficult to remove anelectron.• Two exceptions: removing the first p electron andremoving the fourth p electron.24


Ionization EnergyPeriodic Trends in Ionization Energies• The s electrons are more effective at shielding than pelectrons.• When a second electron is placed in relatively stable halffilled p orbitals, the electron-electron repulsionincreases. Thus there is a ………………. in ionizationenergy.25


Ionization EnergyElectron Configuration of Ions• Cations: electrons …………….. from orbital withhighest principle quantum number, n, first:Li (1s 2 2s 1 ) Li + (1s 2 )Fe ([Ar]3d 6 4s 2 ) Fe 3+ ([Ar]3d 5 )• Anions: electrons …………….. to the orbital withhighest n:F (1s 2 2s 2 2p 5 ) F - (1s 2 2s 2 2p 6 )28


QUESTIONArrange the following in order of decreasingionisation energy.K, Na, Rb & MgAnswerMg>Na> K>Rb29


Electron Affinities• Electron affinity is the …………………. of ionizationenergy.• Electron affinity is the energy change when a gaseousatom ………….. an electron to form a gaseous ion:Cl(g) + e - Cl - (g)• Electron affinity can either be ………………….(as theabove example) or endothermic:Ar(g) + e - Ar - (g)30


Electron Affinities• Look at electron configurations to determine whetherelectron affinity is positive or negative.• The extra electron in Ar needs to be placed in the 4s orbitalwhich is significantly higher in energy than the 3p orbital.31


Electron Affinities32


Metals, Nonmetals, and Metalloids33


Lecture 2Metals, Nonmetals, andMetalloidsMetals• Metallic character refers to the extent to which theelement exhibits the physical and chemical propertiesof metals (shiny and lustrous, malleable and ductile,good conductor of electricity, oxides form basic ionicsolids, and tend to form cations in aqueous solution).• Metals have ……………..ionization energies.• Most neutral metals are oxidized rather than reduced.34


Non MetalsMetals, Nonmetals, andMetalloids• Properties of non metals (dull, brittle, poor conductors ofelectricity, lower melting points then metal, non metallicoxides acidic, etc.)MetalloidsMetalloids have properties that are intermediate betweenmetals and nonmetals.Example: Si has a metallic luster but it is brittle.Metalloids have found fame in the semiconductorindustry. 35


Metals, Nonmetals, andMetalloids• When metals are ……………… they tend to formcharacteristics cations.• All group 1A metals form M + ions.• All group 2A metals form M 2+ ions.• Most transition metals have variable charges.Metals36


Metals, Nonmetals, andMetalloidsMetals37


Metals, Nonmetals, andMetalloidsMetals• Most metal oxides are basic: (STOPPED) 28/3Metal oxide + water metal hydroxideNa 2 O(s) + H 2 O(l) 2NaOH(aq)Nonmetals• Nonmetals are more diverse in their behavior than metals.• When nonmetals react with metals, nonmetals tend togain electrons:metal + nonmetal salt2Al(s) + 3Br 2 (l) 2AlBr 3 (s)38


Metals, Nonmetals, andMetalloids• Most nonmetal oxides are ……………nonmetal oxide + water acidP 4 O 10 (s) + H 2 O(l) 4H 3 PO 4 (aq)NonmetalsMetalloids• Metalloids have properties that are intermediate betweenmetals and nonmetals.39


• Alkali metals are all soft.Group Trends for theActive MetalsGroup 1A: The Alkali Metals• Chemistry dominated by the loss of their single selectron:M M + + e -• Reactivity increases as we move ……………the group.• Alkali metals react with water to form MOH andhydrogen gas:2M(s) + 2H 2 O(l) 2MOH(aq) + H 2 (g)40


Group Trends for theActive MetalsGroup 1A: The Alkali Metals• Alkali metal produce different oxides when reacting withO 2 :4Li(s) + O 2 (g) 2Li 2 O(s)2Na(s) + O 2 (g) Na 2 O 2 (s)K(s) + O 2 (g) KO 2 (s)(oxide)(peroxide)(superoxide)• Alkali metals emit characteristic colors when placed in ahigh temperature flame.• The s electron is excited by the flame and emits energywhen it returns to the ground state.41


Group Trends for theActive MetalsGroup 1A: The Alkali MetalsLi line: 2p 2stransitionNa line (589 nm):3p 3s transitionK line: 4p 4stransition42


Metallic BondingElectron-Sea Model of Metallic Bonding• We use a delocalized model for electrons in a metal.– The metal nuclei are seen to exist in a sea of electrons.– No electrons are ………………between any two metalatoms.– Therefore, the electrons can flow freely through themetal.– Without any definite bonds, the metals are easy todeform (and are malleable and ductile).43


Group Trends for theActive MetalsGroup 1A: The Alkali Metals45


Group Trends for theActive MetalsGroup 2A: The Alkaline Earth Metals46


• Account for the melting point of theelements as you proceed:a. down group 1b. down group 2c. in a period from group 1 to group 247


Group Trends for theActive MetalsGroup 2A: The Alkaline Earth Metals• Alkaline earth metals are …………… and more densethan the alkali metals.• The chemistry is dominated by the …………….. of two selectrons:M M 2+ + 2e - .Mg(s) + Cl 2 (g) MgCl 2 (s)2Mg(s) + O 2 (g) 2MgO(s)• Be does not react with water. Mg will only react withsteam. Ca onwards:Ca(s) + 2H 2 O(l) Ca(OH) 2 (aq) + H 2 (g)48


Group Trends for Selected• Hydrogen is a unique element.Nonmetals• Most often occurs as a colorless diatomic gas, H 2 .Hydrogen• It can either gain another electron to form the hydrideion, H - , or lose its electron to become H + :2Na(s) + H 2 (g) 2NaH(s)2H 2 (g) + O 2 (g) 2H 2 O(g)• H + is a proton.• The aqueous chemistry of hydrogen is dominated byH + (aq).49


Group Trends for SelectedNonmetalsGroup 6A: The Oxygen Group50


Group Trends for SelectedNonmetalsGroup 6A: The Oxygen Group• As we move down the group the ……………. characterincreases (O 2 is a gas, Te is a metalloid, Po is a metal).• There are two important forms of oxygen: O 2 and ozone,O 3 . Ozone can be prepared from oxygen:3O 2 (g) 2O 3 (g) H = +284.6 kJ.• Ozone is pungent and toxic.51


Group Trends for SelectedNonmetalsGroup 6A: The Oxygen Group• Oxygen (or dioxygen, O 2 ) is a potent oxidizing agentsince the O 2- ion has a noble gas configuration.• There are two oxidation states for oxygen: 2- (e.g. H 2 O)and 1- (e.g. H 2 O 2 ).• Sulfur is another important member of this group.• Most common form of sulfur is yellow S 8 .• Sulfur tends to form S 2- in compounds (sulfides).52


Group Trends for SelectedNonmetalsGroup 7A: The Halogens53


Group Trends for SelectedNonmetalsGroup 7A: The Halogens• The chemistry of the ……………. is dominated bygaining an electron to form an anion:X 2 + 2e - 2X - .• Fluorine is one of the most reactive substances known:2F 2 (g) + 2H 2 O(l) 4HF(aq) + O 2 (g) H = -758.7 kJ.• All halogens consists of diatomic molecules, X 2 .54


Group Trends for SelectedNonmetalsGroup 7A: The Halogens• Chlorine is the most industrially useful halogen. It isproduced by the electrolysis of brine (NaCl):2NaCl(aq) + 2H 2 O(l) 2NaOH(aq) + H 2 (g) + Cl 2 (g).• The reaction between chorine and water produceshypochlorous acid (HOCl) which disinfects pool water:Cl 2 (g) + H 2 O(l) HCl(aq) + HOCl(aq).• Hydrogen compounds of the halogens are all strong acidswith the exception of HF.55


Group Trends for SelectedNonmetalsGroup 8A: The Noble Gases56


Group Trends for SelectedNonmetalsGroup 8A: The Noble Gases• These are all ………………… and monatomic.• They are notoriously unreactive because they havecompletely filled s and p sub-shells.• In 1962 the first compound of the noble gases wasprepared: XeF 2 , XeF 4 , and XeF 6 .• To date the only other noble gas compounds known areKrF 2 and HArF.57


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