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PeriodicProperties
of theElements
Chapter 7
Periodic Propertiesof the Elements
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PeriodicProperties
of theElements
Effective Nuclear Charge
In a many-electronatom, electrons are
both attracted to thenucleus and repelledby other electrons.
The nuclear chargethat an electronexperiences dependson both factors.
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PeriodicProperties
of theElements
Effective Nuclear Charge
The effective nuclearcharge, Z eff , is found
this way:Z eff = Z S
where Z is the atomicnumber and S is ascreening constant,usually close to thenumber of inner
electrons.
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PeriodicProperties
of theElements
Sizes of Atoms
The bonding atomicradius is defined as
one-half of thedistance betweencovalently bondednuclei.
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PeriodicProperties
of theElements
Sizes of AtomsBonding atomicradius tends todecrease from left to
right across a rowdue to increasing Z eff .
increase from top to
bottom of a columndue to increasing valueof n
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PeriodicProperties
of theElements
Sizes of Ions Ionic size depends
upon:Nuclear charge.
Number ofelectrons.Orbitals in whichelectrons reside.
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PeriodicProperties
of theElements
Sizes of Ions Cations are
smaller than theirparent atoms.
The outermostelectron isremoved andrepulsions are
reduced.
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PeriodicProperties
of theElements
Sizes of Ions Anions are larger
than their parentatoms.
Electrons areadded andrepulsions areincreased.
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PeriodicProperties
of theElements
Sizes of Ions
Ions increase in sizeas you go down a
column.Due to increasingvalue of n .
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PeriodicProperties
of theElements
Ionization Energy
Amount of energy required to removean electron from the ground state of agaseous atom or ion.
First ionization energy is that energyrequired to remove first electron.
Second ionization energy is that energyrequired to remove second electron, etc.
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PeriodicProperties
of theElements
Ionization Energy
It requires more energy to remove eachsuccessive electron.
When all valence electrons have been removed,the ionization energy takes a quantum leap.
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PeriodicProperties
of theElements
Trends in First Ionization Energies
As one goes down acolumn, less energy
is required to removethe first electron.For atoms in the samegroup, Z eff is
essentially the same,but the valenceelectrons are fartherfrom the nucleus.
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PeriodicProperties
of theElements
Trends in First Ionization Energies
Generally, as onegoes across a row, it
gets harder toremove an electron. As you go from left toright, Z eff increases.
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PeriodicProperties
of theElements
Trends in First Ionization Energies
However, there aretwo apparent
discontinuities in thistrend.
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PeriodicProperties
of theElements
Trends in First Ionization Energies
The first occursbetween Groups IIAand IIIA.
Electron removed from p -orbital rather than s -orbital
Electron farther fromnucleusSmall amount ofrepulsion by s electrons.
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PeriodicProperties
of theElements
Trends in First Ionization Energies
The second occursbetween Groups VA
and VIA.Electron removedcomes from doublyoccupied orbital.
Repulsion from otherelectron in orbital helpsin its removal.
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PeriodicProperties
of theElements
Electron Affinity
Energy change accompanying addition ofelectron to gaseous atom:
Cl + e Cl
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PeriodicProperties
of theElements
Trends in Electron Affinity
In general, electronaffinity becomes
more exothermic asyou go from left toright across a row.
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PeriodicProperties
of theElements
Trends in Electron Affinity
There areagain,
however, twodiscontinuitiesin this trend.
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PeriodicProperties
of theElements
Trends in Electron Affinity
The first occursbetween Groups IA
and IIA. Added electron mustgo in p -orbital, not s -orbital.
Electron is fartherfrom nucleus andfeels repulsion froms -electrons.
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PeriodicProperties
of theElements
Trends in Electron Affinity
The second occursbetween Groups IVA
and VA.Group VA has noempty orbitals.Extra electron must
go into occupiedorbital, creatingrepulsion.
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PeriodicProperties
of theElements
Properties of Metal, Nonmetals,
and Metalloids
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PeriodicProperties
of theElements
Metals versus Nonmetals
Differences between metals and nonmetals
tend to revolve around these properties.
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PeriodicProperties
of theElements
Metals versus Nonmetals
Metals tend to form cations. Nonmetals tend to form anions.
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PeriodicProperties
of theElements
Metals
Tend to be lustrous,malleable, ductile,
and goodconductors of heatand electricity.
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PeriodicProperties
of theElements
Metals
Compounds formedbetween metals and
nonmetals tend tobe ionic. Metal oxides tend to
be basic.
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PeriodicProperties
of theElements
Nonmetals
Dull, brittlesubstances that arepoor conductors ofheat and electricity.
Tend to gainelectrons inreactions withmetals to acquirenoble gasconfiguration.
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PeriodicProperties
of theElements
Nonmetals
Substancescontaining only
nonmetals aremolecularcompounds.
Most nonmetal
oxides are acidic.
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PeriodicProperties
of theElements
Metalloids
Have somecharacteristics of
metals, some ofnonmetals. For instance, silicon
looks shiny, but is
brittle and fairly poorconductor.
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PeriodicProperties
of theElements
Alkali Metals
Found only as compounds in nature. Have low densities and melting points.
Also have low ionization energies.
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PeriodicProperties
of theElements
Alkali Metals
Their reactions with water are famously exothermic.
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PeriodicProperties
of the
Elements
Alkali Metals Alkali metals (except Li) react with oxygen to
form peroxides. K, Rb, and Cs also form superoxides:
K + O 2 KO 2 Produce bright colors when placed in flame.
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PeriodicProperties
of the
Elements
Alkaline Earth Metals
Have higher densities and melting points than
alkali metals. Have low ionization energies, but not as low
as alkali metals.
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PeriodicProperties
of the
Elements
Alkaline Earth Metals
Be does not reactwith water, Mg
reacts only withsteam, but othersreact readily withwater.
Reactivity tends toincrease as go downgroup.
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PeriodicProperties
of the
Elements
Group 6A
Oxygen, sulfur, and selenium are nonmetals.
Tellurium is a metalloid. The radioactive polonium is a metal.
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PeriodicProperties
of the
Elements
Sulfur
Weaker oxidizingagent than oxygen.
Most stableallotrope is S
8, a
ringed molecule.
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PeriodicProperties
of the
Elements
Group VIIA: Halogens
Large, negative electronaffinities
Therefore, tend to oxidizeother elements easily
React directly with metalsto form metal halides
Chlorine added to watersupplies to serve asdisinfectant
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PeriodicProperties
of the
Elements
Group VIIIA: Noble Gases
Astronomical ionization energies
Positive electron affinitiesTherefore, relatively unreactive
Monatomic gases
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PeriodicProperties
of the
Group VIIIA: Noble Gases
Xe forms threecompounds:
XeF 2 XeF 4 (at right)XeF 6
Kr forms only one stablecompound:
KrF2
The unstable HArF was