periodicity honors2

8/8/2019 Periodicity Honors2

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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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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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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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Ions increase in size

as you go down a

column.

Due to increasing

value of n.


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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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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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EnergiesEnergies

However, there are

two apparent

discontinuities in thistrend.


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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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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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There are

again,

however, two

discontinuitiesin this trend.


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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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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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Found only as compounds in nature.

Have low densities and melting points.

Also have low ionization energies.


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Their reactions with water are famously exothermic.


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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

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