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Periodic PropertiesPeriodic Properties
of the Elementsof the Elements
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Development of PeriodicDevelopment of Periodic
TableTable
Elements in thesame group
generally have
similar chemical
properties.
Properties are not
identical, however.
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Development of PeriodicDevelopment of Periodic
TableTable
Dmitri Mendeleevand Lothar Meyer
independently
came to the
same conclusionabout how
elements should
be grouped.
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Mendeleev insisted elements be listed in the same
family based on chemical and physical properties.
This forced him to leave several blank spaces in his
table.
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Development of PeriodicDevelopment of Periodic
TableTable
Mendeleev, for instance, predicted thediscovery of germanium (which he called eka-
silicon) as an element with an atomic weight
between that of zinc and arsenic, but with
chemical properties similar to those of silicon.
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1913, two years after Rutherford proposed hisNuclear Model of the atom, an Englishphysicist named Henry Mosely developed the
concept of atomic numbers.
Mosely determined that the frequency of X-rays emitted as different elements werebombarded with high energy electrons.
He found that each element produces uniquefrequencies
He found that frequency increased as atomicnumber increased
He arranged frequencies in order byassigning whole number values called atomic
number.
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This method clarified some problems with early
versions of the periodic table which was based on
atomic weight.
Ex. Atomic weight of Ar (18
) is greater than K (19
)
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Periodic TrendsPeriodic Trends
In this chapter, we will rationalize
observed trends inSizes of atoms and ions.
Ionization energy.
Electron affinity.
Electronegativity
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Effective Nuclear ChargeEffective Nuclear Charge
Many properties of atoms depend on:
Electron configuration
How strongly outer electrons are
attracted to the nucleus.
Force of attraction between an electron
and nucleus depends on:
the magnitude of the net nuclear charge
acting on the electron
Average distance between the nucleus
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Effective Nuclear ChargeEffective Nuclear ChargeIn a many-electron
atom, electrons areboth attracted to the
nucleus and repelled by
other electrons.
The nuclear charge that
an electron experiences
depends on both
factors.
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Effective Nuclear ChargeEffective Nuclear ChargeThe effective nuclear
charge, Zeff, is found
this way:
Zeff= Z S
where Z is the atomic
number and S is ascreening constant,
usually close to the
number of inner
electrons.
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In a many electron atom:
Each electron is simultaneouslyattracted to the nucleus
Repelled by other electrons
We can estimate the net attraction
of each electron to the nucleus by
considering how it interacts with theaverage environment created by
the nucleus and the other electrons
in the atom.
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through an electric field created bythe nucleus
And the electron density of otherelectrons.
In other words:
The nucleus attracts valenceelectrons and inner shell electronsrepel them.
We want to know: What is the netresult.
It is called effective nuclear charge.
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What is the effective nuclar charge for Na?What is the effective nuclar charge for Na?Nuclear charge is 11
Screening constant 10
We expect the 3s electron to
experience an effective nuclear
charge of +1
This is why 2p electrons are higher
in energy than 2s elecrons
2s electrons are less effectively
shielded by the nucleus than 2p
2s have higher Zeff than 2p
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Sizes of AtomsSizes of Atoms
The bonding atomic
radius is defined as
one-half of the
distance betweencovalently bonded
nuclei.
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Sizes of AtomsSizes of AtomsBonding atomic radiustends to
decrease from left toright across a row
due to increasing Zeff.
increase from top tobottom of a column
due to increasing value
of n
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Sizes of IonsSizes of IonsIonic size depends
upon:
Nuclear charge.
Number of
electrons.
Orbitals in which
electrons reside.
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Sizes of IonsSizes of IonsCations are
smaller than theirparent atoms.
The outermost
electron is removed
and repulsions arereduced.
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Sizes of IonsSizes of Ions
Anions are larger
than their parentatoms.
Electrons are added
and repulsions areincreased.
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Sizes of IonsSizes of Ions
Ions increase in size
as you go down a
column.
Due to increasing
value of n.
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Sizes of IonsSizes of Ions
In an isoelectronic series, ions have the samenumber of electrons.
Ionic size decreases with an increasing
nuclear charge.
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Ionization EnergyIonization EnergyAmount of energy required to
remove an electron from the ground
state of a gaseous atom or ion.
First ionization energy is that
energy required to remove first
electron.Second ionization energy is that
energy required to remove second
electron, etc.
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Ionization EnergyIonization EnergyIn an isoelectronic series, ions have the samenumber of electrons.
Ionic size decreases with an increasing nuclear
charge.
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Trends in First IonizationTrends in First Ionization
EnergiesEnergiesAs one goes down a
column, less energy isrequired to remove
the first electron.
For atoms in the same
group, Zeff is essentiallythe same, but the
valence electrons are
farther from the
nucleus.
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Trends in First IonizationTrends in First Ionization
EnergiesEnergies
Generally, as one
goes across a row, it
gets harder to
remove an electron.
As you go from left to
right, Zeff increases.
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Trends in First IonizationTrends in First Ionization
EnergiesEnergies
However, there are
two apparent
discontinuities in thistrend.
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Trends in First IonizationTrends in First Ionization
EnergiesEnergiesT e first o rset een ro s II
and III .
Ele tron re o ed fro-or ital rat er t an s-
or ital
Ele tron fart er fron le s
all a o nt of
re lsion s ele trons.
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Trends in First IonizationTrends in First Ionization
EnergiesEnergies
The second occurs
between Groups VAand VIA.
Electron removed comes
from doubly occupied
orbital.
Repulsion from other
electron in orbital helps
in its removal.
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Electron AffinityElectron Affinity
Energy change accompanying addition of electron togaseous atom:
Cl + e
qqCl
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Trends in Electron AffinityTrends in Electron Affinity
In general, electron
affinity becomes
more exothermic as
you go from left toright across a row.
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Trends in Electron AffinityTrends in Electron Affinity
There are
again,
however, two
discontinuitiesin this trend.
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Trends in Electron AffinityTrends in Electron Affinity
The first occurs
between Groups IA
and IIA.
Added electron must
go in p-orbital, not s-
orbital.
Electron is farther
from nucleus and
feels repulsion from
s-electrons.
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Trends in Electron AffinityTrends in Electron Affinity
The second occurs
between Groups IVAand VA.
Group VA has no
empty orbitals.
Extra electron must go
into occupied orbital,
creating repulsion.
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roper es o e a ,roper es o e a ,
Nonmetals,Nonmetals,
and Metalloidsand Metalloids
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Metals versus NonmetalsMetals versus Nonmetals
Differences between metals and nonmetals
tend to revolve around these properties.
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Metals versus NonmetalsMetals versus Nonmetals
Metals tend to form cations.
Nonmetals tend to form anions.
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MetalsMetals
Tend to be lustrous,
malleable, ductile,
and good conductors
of heat andelectricity.
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MetalsMetals
Compounds formedbetween metals and
nonmetals tend to be
ionic.
Metal oxides tend to
be basic.
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NonmetalsNonmetals
Dull, brittle
substances that arepoor conductors ofheat and electricity.
Tend to gain
electrons inreactions with metalsto acquire noble gasconfiguration.
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NonmetalsNonmetals
Substancescontaining only
nonmetals are
molecular
compounds.Most nonmetal
oxides are acidic.
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MetalloidsMetalloids
Have some
characteristics ofmetals, some of
nonmetals.
For instance, siliconlooks shiny, but is
brittle and fairly poor
conductor.
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Group TrendsGroup Trends
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Alkali MetalsAlkali Metals
Soft, metallic solids.
Name comes from
Arabic word forashes.
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Alkali MetalsAlkali Metals
Found only as compounds in nature.
Have low densities and melting points.
Also have low ionization energies.
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Alkali MetalsAlkali Metals
Their reactions with water are famously exothermic.
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Alkali MetalsAlkali Metals
Alkali metals (except Li) react with oxygen to
form peroxides.
K, Rb, and Cs also form superoxides:
K + O2 qqKO2
Produce bright colors when placed in flame.
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Alkaline Earth MetalsAlkaline Earth Metals
Have higher densities and melting points thanalkali metals.
Have low ionization energies, but not as low
as alkali metals.
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Alkaline Earth MetalsAlkaline Earth Metals
Be does not react
with water, Mgreacts only with
steam, but others
react readily with
water.Reactivity tends to
increase as go down
group.
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Group 6AGroup 6A
Oxygen, sulfur, and selenium are nonmetals.
Tellurium is a metalloid.
The radioactive polonium is a metal.
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OxygenOxygenTwo allotropes:O2
O3, ozone
Three anions:
O2, oxide
O22, peroxide
O21, superoxide
Tends to take electrons
from other elements
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SulfurSulfur
Weaker oxidizing
agent than oxygen.
Most stable allotrope
is S8, a ringed
molecule.
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Group VIIA: HalogensGroup VIIA: Halogens
Prototypical nonmetals
Name comes from the Greek halos and
gennao: salt formers
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Group VIIA: HalogensGroup VIIA: Halogens
Large, negative electron
affinities
Therefore, tend to oxidize
other elements easily
React directly with metals
to form metal halides
Chlorine added to water
supplies to serve as
disinfectant
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Group VIIIA: Noble GasesGroup VIIIA: Noble Gases
Astronomical ionization energies
Positive electron affinities
Therefore, relatively unreactive
Monatomic gases
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Group VIIIA: NobleGroup VIIIA: Noble
GasesGasesXe forms threecompounds:
XeF2
XeF4 (at right)
XeF6
Kr forms only one stablecompound:
KrF2
Th t bl HA F