ch 7 periodic properties

8/10/2019 CH 7 Periodic Properties

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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


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


However, there aretwo apparent

discontinuities in thistrend.


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PeriodicProperties

of theElements


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


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


There areagain,

however, twodiscontinuitiesin this trend.


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PeriodicProperties

of theElements


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


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

ch 7 periodic properties

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