Chapter 8Periodic RelationshipsAmong the Elements
Development of Periodic Table• Elements in the samegroup generally havesimilar chemicalproperties.• Properties are notidentical, however.
Development of Periodic TableDmitri Mendeleevand Lothar Meyerindependentlycame to the sameconclusion abouthow elementsshould be grouped.
Development of Periodic TableMendeleev, for instance, predicted the discovery ofgermanium (which he called eka-silicon) as anelement with an atomic weight between that of zincand arsenic, but with chemical properties similar tothose of silicon.
Periodic Trends• In this chapter, we will rationalize observedtrends in– Sizes of atoms and ions.– Ionization energy.– Electron affinity.• To do so, however, we must first understand……..
Effective Nuclear Charge• In a many-electron atom,electrons are bothattracted to the nucleusand repelled by otherelectrons.• The nuclear charge thatan electron experiencesdepends on both factors.
Effective Nuclear ChargeThe effective nuclearcharge, Zeff, is found thisway:Zeff = Z − σwhere Z is the atomicnumber and σ (sigma) isa shielding or screeningconstant, usually close tothe number of innerelectrons.
Sizes of AtomsThe bonding atomicradius is defined as one-half of the distancebetween covalentlybonded nuclei.
Sizes of AtomsBonding atomic radiustends to……decrease from left toright across a rowdue to increasing Zeff.…increase from top tobottom of a columndue to increasing value ofn
Sizes of Ions• Ionic size dependsupon:– Nuclear charge.– Number ofelectrons.– Orbitals in whichelectrons reside.
Sizes of Ions• Cations are smallerthan their parentatoms.– The outermostelectron is removedand repulsions arereduced.
Sizes of Ions• Anions are largerthan their parentatoms.– Electrons are addedand repulsions areincreased.
Sizes of Ions• Ions increase in size asyou go down a column.– Due to increasing valueof n.
Sizes of Ions• In an isoelectronic series, ions have the samenumber of electrons.• Ionic size decreases with an increasing nuclearcharge.
Ionization Energy• Amount of energy required to remove anelectron from the ground state of a gaseousatom or ion.– First ionization energy is that energy required toremove first electron.– Second ionization energy is that energy requiredto remove second electron, etc.
Ionization Energy• It requires more energy to remove each successiveelectron.• When all valence electrons have been removed, theionization energy takes a quantum leap.
Trends in First Ionization Energies• As one goes down acolumn, less energy isrequired to remove thefirst electron.– For atoms in the samegroup, Zeff is essentiallythe same, but thevalence electrons arefarther from thenucleus.
Trends in First Ionization Energies• Generally, as one goesacross a row, it getsharder to remove anelectron.– As you go from left toright, Zeff increases.
Trends in First Ionization EnergiesHowever, there are twoapparent discontinuitiesin this trend.
Trends in First Ionization Energies• The first occurs betweenGroups IIA and IIIA.• Electron removed fromp-orbital rather than s-orbital– Electron farther fromnucleus– Small amount of repulsionby s electrons.
Trends in First Ionization Energies• The second occursbetween Groups VA andVIA.– Electron removed comesfrom doubly occupiedorbital.– Repulsion from otherelectron in orbital helps inits removal.
Electron AffinityEnergy change accompanying addition ofelectron to gaseous atom:Cl + e−→ Cl−
Trends in Electron AffinityIn general, electronaffinity becomes moreexothermic as you gofrom left to rightacross a row.
Trends in Electron AffinityThere are again,however, twodiscontinuities inthis trend.
Trends in Electron Affinity• The first occursbetween Groups IAand IIA.– Added electron mustgo in p-orbital, not s-orbital.– Electron is fartherfrom nucleus andfeels repulsion from s-electrons.
Trends in Electron Affinity• The second occursbetween Groups IVAand VA.– Group VA has no emptyorbitals.– Extra electron must gointo occupied orbital,creating repulsion.
Alkali Metals• Soft, metallic solids.• Name comes fromArabic word for ashes.
Alkali Metals• Found only as compounds in nature.• Have low densities and melting points.• Also have low ionization energies.
Alkali MetalsTheir reactions with water are famously exothermic.
Alkali Metals• Alkali metals (except Li) react with oxygen to formperoxides.• K, Rb, and Cs also form superoxides:K + O2 → KO2• Produce bright colors when placed in flame.
Alkaline Earth Metals• Have higher densities and melting points thanalkali metals.• Have low ionization energies, but not as low asalkali metals.
Alkaline Earth Metals• Be does not react withwater, Mg reacts onlywith steam, but othersreact readily withwater.• Reactivity tends toincrease as go downgroup.
Group 6A• Oxygen, sulfur, and selenium are nonmetals.• Tellurium is a metalloid.• The radioactive polonium is a metal.
Oxygen• Two allotropes:– O2– O3, ozone• Three anions:– O2−, oxide– O22−, peroxide– O21−, superoxide• Tends to take electronsfrom other elements(oxidation)
Sulfur• Weaker oxidizingagent than oxygen.• Most stable allotropeis S8, a ringedmolecule.
Group VIIA: Halogens• Prototypical nonmetals• Name comes from the Greek halos and gennao:“salt formers”
Group VIIA: Halogens• Large, negative electronaffinities– Therefore, tend to oxidizeother elements easily• React directly with metals toform metal halides• Chlorine added to watersupplies to serve asdisinfectant
Group VIIIA: Noble Gases• Astronomical ionization energies• Positive electron affinities– Therefore, relatively unreactive• Monatomic gases
Group VIIIA: Noble Gases• Xe forms threecompounds:– XeF2– XeF4 (at right)– XeF6• Kr forms only one stablecompound:– KrF2• The unstable HArF wassynthesized in 2000.