Chapter 7 Periodic Properties of the Elements

Chapter 7Chapter 7Periodic Periodic

PropertiesPropertiesof the of the

ElementsElements

Development of Periodic Development of Periodic TableTable

Elements in the Elements in the same group same group generally have generally have similar chemical similar chemical properties.properties.

Properties are Properties are not identical, not identical, however.however.


Dmitri Dmitri Mendeleev and Mendeleev and Lothar Meyer Lothar Meyer independently independently came to the came to the same same conclusion conclusion about how about how elements elements should be should be grouped.grouped.


Mendeleev, for instance, predicted the Mendeleev, for instance, predicted the discovery of germanium (which he called discovery of germanium (which he called eka-silicon) as an element with an eka-silicon) as an element with an atomic weight between that of zinc and atomic weight between that of zinc and arsenic, but with chemical properties arsenic, but with chemical properties similar to those of silicon.similar to those of silicon.

Periodic TrendsPeriodic Trends

In this chapter, we will In this chapter, we will rationalize observed trends inrationalize observed trends in Sizes of atoms and ions.Sizes of atoms and ions. Ionization energy.Ionization energy. Electron affinity.Electron affinity.

Effective Nuclear ChargeEffective Nuclear Charge

In a many-electron In a many-electron atom, electrons are atom, electrons are both attracted to the both attracted to the nucleus and repelled nucleus and repelled by other electrons.by other electrons.

The nuclear charge The nuclear charge that an electron that an electron experiences depends experiences depends on both factors.on both factors.

Effective Nuclear ChargeEffective Nuclear Charge

The effective The effective nuclear charge, nuclear charge, ZZeffeff, , is found this way:is found this way:

ZZeffeff = = ZZ −− SS

where where ZZ is the is the atomic number and atomic number and SS is a screening is a screening constant, usually constant, usually close to the number close to the number of inner electrons.of inner electrons.

Sizes of AtomsSizes of Atoms

The bonding The bonding atomic radius is atomic radius is defined as one-defined as one-half of the half of the distance between distance between covalently covalently bonded nuclei.bonded nuclei.

Sizes of AtomsSizes of Atoms

Bonding atomic Bonding atomic radius tends to… radius tends to… ……decrease from decrease from

left to right left to right across a rowacross a rowdue to increasing due to increasing ZZeffeff..

……increase from top increase from top to bottom of a to bottom of a columncolumndue to increasing due to increasing

value of value of nn

Sizes of IonsSizes of Ions

Ionic size Ionic size depends upon:depends upon: Nuclear Nuclear

charge.charge. Number of Number of

electrons.electrons. Orbitals in Orbitals in

which which electrons electrons reside.reside.


Cations are Cations are smaller than smaller than their parent their parent atoms.atoms. The outermost The outermost

electron is electron is removed and removed and repulsions are repulsions are reduced.reduced.


Anions are Anions are larger than larger than their parent their parent atoms.atoms. Electrons are Electrons are

added and added and repulsions are repulsions are increased.increased.


Ions increase in Ions increase in size as you go size as you go down a column.down a column. Due to increasing Due to increasing

value of value of nn..


In an In an isoelectronic seriesisoelectronic series, ions have , ions have the same number of electrons.the same number of electrons.

Ionic size decreases with an Ionic size decreases with an increasing nuclear charge.increasing nuclear charge.

Ionization EnergyIonization Energy

Amount of energy required to Amount of energy required to remove an electron from the remove an electron from the ground state of a gaseous atom or ground state of a gaseous atom or ion.ion. First ionization energy is that energy First ionization energy is that energy

required to remove first electron.required to remove first electron. Second ionization energy is that Second ionization energy is that

energy required to remove second energy required to remove second electron, etc.electron, etc.

Ionization EnergyIonization Energy

It requires more energy to remove It requires more energy to remove each successive electron.each successive electron.

When all valence electrons have been When all valence electrons have been removed, the ionization energy takes removed, the ionization energy takes a quantum leap.a quantum leap.

Trends in First Ionization Trends in First Ionization EnergiesEnergies

As one goes down As one goes down a column, less a column, less energy is required energy is required to remove the first to remove the first electron.electron. For atoms in the For atoms in the

same group, same group, ZZeffeff is is essentially the essentially the same, but the same, but the valence electrons valence electrons are farther from the are farther from the nucleus.nucleus.


Generally, as one Generally, as one goes across a goes across a row, it gets row, it gets harder to remove harder to remove an electron.an electron. As you go from As you go from

left to right, left to right, ZZeffeff increases.increases.


However, there However, there are two apparent are two apparent discontinuities in discontinuities in this trend.this trend.


The first occurs The first occurs between Groups between Groups IIA and IIIA.IIA and IIIA.

Electron removed Electron removed from from pp-orbital -orbital rather than rather than ss--orbitalorbital Electron farther Electron farther

from nucleusfrom nucleus Small amount of Small amount of

repulsion by repulsion by ss electrons.electrons.


The second occurs The second occurs between Groups between Groups VA and VIA.VA and VIA. Electron removed Electron removed

comes from doubly comes from doubly occupied orbital.occupied orbital.

Repulsion from Repulsion from other electron in other electron in orbital helps in its orbital helps in its removal.removal.

Electron AffinityElectron Affinity

Energy change accompanying addition Energy change accompanying addition of electron to gaseous atom:of electron to gaseous atom:

Cl + eCl + e−− Cl Cl−−

Trends in Electron Trends in Electron AffinityAffinity

In general, In general, electron affinity electron affinity becomes more becomes more exothermic as exothermic as you go from left you go from left to right across a to right across a row.row.


There are There are again, again, however, two however, two discontinuitidiscontinuities in this es in this trend.trend.


The first occurs The first occurs between Groups between Groups IA and IIA.IA and IIA. Added electron Added electron

must go in must go in pp--orbital, not orbital, not ss--orbital.orbital.

Electron is farther Electron is farther from nucleus and from nucleus and feels repulsion feels repulsion from from ss-electrons.-electrons.


The second occurs The second occurs between Groups between Groups IVA and VA.IVA and VA. Group VA has no Group VA has no

empty orbitals.empty orbitals. Extra electron Extra electron

must go into must go into occupied orbital, occupied orbital, creating repulsion.creating repulsion.

Next TimeNext Time

We complete Chapter 7We complete Chapter 7

Properties of Metal, Properties of Metal, Nonmetals,Nonmetals,

and Metalloidsand Metalloids

Metals versus NonmetalsMetals versus Nonmetals

Differences between metals and Differences between metals and nonmetals tend to revolve around nonmetals tend to revolve around these properties.these properties.

Metals versus NonmetalsMetals versus Nonmetals

Metals tend to form cations.Metals tend to form cations. Nonmetals tend to form anions.Nonmetals tend to form anions.

MetalsMetals

Tend to be Tend to be lustrous, lustrous, malleable, malleable, ductile, and good ductile, and good conductors of conductors of heat and heat and electricity.electricity.

MetalsMetals

Compounds Compounds formed between formed between metals and metals and nonmetals tend nonmetals tend to be ionic.to be ionic.

Metal oxides Metal oxides tend to be basic.tend to be basic.

NonmetalsNonmetals Dull, brittle Dull, brittle

substances that substances that are poor are poor conductors of conductors of heat and heat and electricity.electricity.

Tend to gain Tend to gain electrons in electrons in reactions with reactions with metals to acquire metals to acquire noble gas noble gas configuration.configuration.

NonmetalsNonmetals

Substances Substances containing only containing only nonmetals are nonmetals are molecular molecular compounds.compounds.

Most nonmetal Most nonmetal oxides are acidic.oxides are acidic.

MetalloidsMetalloids

Have some Have some characteristics of characteristics of metals, some of metals, some of nonmetals.nonmetals.

For instance, For instance, silicon looks silicon looks shiny, but is shiny, but is brittle and fairly brittle and fairly poor conductor.poor conductor.

Group Group TrendsTrends

Alkali MetalsAlkali Metals

Soft, metallic Soft, metallic solids.solids.

Name comes Name comes from Arabic word from Arabic word for ashes.for ashes.


Found only as compounds in nature.Found only as compounds in nature. Have low densities and melting Have low densities and melting

points.points. Also have low ionization energies.Also have low ionization energies.


Their reactions with water are famously Their reactions with water are famously exothermic.exothermic.


Alkali metals (except Li) react with Alkali metals (except Li) react with oxygen to form peroxides.oxygen to form peroxides.

K, Rb, and Cs also form superoxides:K, Rb, and Cs also form superoxides:

K + OK + O22 KO KO22

Produce bright colors when placed in Produce bright colors when placed in flame.flame.

Alkaline Earth MetalsAlkaline Earth Metals

Have higher densities and melting Have higher densities and melting points than alkali metals.points than alkali metals.

Have low ionization energies, but not Have low ionization energies, but not as low as alkali metals.as low as alkali metals.

Alkaline Earth MetalsAlkaline Earth Metals

BeBe does not react does not react with water, Mg with water, Mg reacts only with reacts only with steam, but others steam, but others react readily with react readily with water.water.

Reactivity tends Reactivity tends to increase as go to increase as go down group.down group.

Group 6AGroup 6A

Oxygen, sulfur, and selenium are Oxygen, sulfur, and selenium are nonmetals.nonmetals.

Tellurium is a metalloid.Tellurium is a metalloid. The radioactive polonium is a metal.The radioactive polonium is a metal.

OxygenOxygen

Two allotropes:Two allotropes: OO22

OO33, ozone, ozone Three anions:Three anions:

OO22−−, oxide, oxide OO22

22−−, peroxide, peroxide OO22

11−−, superoxide, superoxide Tends to take Tends to take

electrons from other electrons from other elements (oxidation)elements (oxidation)

SulfurSulfur

Weaker oxidizing Weaker oxidizing agent than agent than oxygen.oxygen.

Most stable Most stable allotrope is Sallotrope is S88, a , a ringed molecule.ringed molecule.

Group VIIA: HalogensGroup VIIA: Halogens

Prototypical nonmetalsPrototypical nonmetals Name comes from the Greek Name comes from the Greek haloshalos

and and gennaogennao: “salt formers”: “salt formers”

Group VIIA: HalogensGroup VIIA: Halogens

Large, negative Large, negative electron affinitieselectron affinities Therefore, tend to Therefore, tend to

oxidize other elements oxidize other elements easilyeasily

React directly with React directly with metals to form metal metals to form metal halideshalides

Chlorine added to Chlorine added to water supplies to serve water supplies to serve as disinfectantas disinfectant

Group VIIIA: Noble Group VIIIA: Noble GasesGases

Astronomical ionization energiesAstronomical ionization energies Positive electron affinitiesPositive electron affinities

Therefore, relatively unreactiveTherefore, relatively unreactive Monatomic gasesMonatomic gases

Group VIIIA: Noble Group VIIIA: Noble GasesGases

Xe forms three Xe forms three compounds:compounds: XeFXeF22

XeFXeF44 (at right) (at right) XeFXeF66

Kr forms only one Kr forms only one stable compound:stable compound: KrFKrF22

The unstable HArF The unstable HArF was synthesized in was synthesized in 2000.2000.

Chapter 7 Periodic Properties of the Elements

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