Periodic TrendsPeriodic Trends

OBJECTIVES:OBJECTIVES: Interpret Interpret periodic trendsperiodic trends in atomic radii, in atomic radii,

ionic radii, ionization energies, and ionic radii, ionization energies, and electronegativities.electronegativities.

Trends in Atomic SizeTrends in Atomic Size First problem: Where do you start First problem: Where do you start

measuring from?measuring from? The electron cloud doesn’t have a The electron cloud doesn’t have a

definite edge.definite edge. We get around this by measuring We get around this by measuring

more than 1 atom at a time.more than 1 atom at a time.

Atomic SizeAtomic Size

Atomic RadiusAtomic Radius = half the distance between two nuclei of a = half the distance between two nuclei of a diatomic molecule. diatomic molecule. DiatomicDiatomic means “two atoms” or a two means “two atoms” or a two atom moleculeatom molecule

}Radius

Trends in Atomic SizeTrends in Atomic Size

Influenced by three factors:Influenced by three factors:1. 1. Energy LevelEnergy Level Higher energy level is further away.Higher energy level is further away.

2. 2. Charge on NucleusCharge on Nucleus More charge pulls electrons in closer.More charge pulls electrons in closer.

3. 3. Shielding EffectShielding EffectElectrons between nucleus and Electrons between nucleus and

valance electronsvalance electrons

Group TrendsGroup Trends

As we go As we go downdown a a group...group...

each atom adds each atom adds another energy another energy level,level,

so the atoms get so the atoms get bigger.bigger.

HLi

Na

K

Rb

Periodic TrendsPeriodic Trends As you go As you go acrossacross a period, the radius a period, the radius

gets smaller.gets smaller. Electrons are in Electrons are in same energy levelsame energy level.. More More nuclear chargenuclear charge.. Outermost electrons are closer.Outermost electrons are closer.

Na Mg Al Si P S Cl Ar

Atomic Radius TrendsAtomic Radius Trends

OverallOverall

Atomic Number

Ato

mic

Rad

ius

(nm

)

H

Li

Ne

Ar

10

Na

K

Kr

Rb

Periodic Trend in Atomic Periodic Trend in Atomic RadiusRadius

Ion FormationIon Formation

An atom can easily lose or gain An atom can easily lose or gain electrons. The resulting ion is an electrons. The resulting ion is an atom that has an imbalance of charge atom that has an imbalance of charge or carries either a positive charge (a or carries either a positive charge (a loss of electrons) or a negative loss of electrons) or a negative charge (a gain of electrons).charge (a gain of electrons).

A positive ion is called a A positive ion is called a cationcation..A negative ion is called an A negative ion is called an anionanion..

Trends in Ionization Trends in Ionization EnergyEnergy

The amount of energy required to The amount of energy required to completely remove an electron from a completely remove an electron from a gaseous atom.gaseous atom.

Removing one electron makes a 1+ Removing one electron makes a 1+ ion.ion.

The energy required to The energy required to remove the remove the first electronfirst electron is called the is called the first first ionization energy.ionization energy.

Ionization EnergyIonization Energy TheThe second second ionization energy is the ionization energy is the

energy required to remove the energy required to remove the second electron.second electron.

Always greater than first IE.Always greater than first IE. The The thirdthird IE is the energy required to IE is the energy required to

remove a third electron.remove a third electron. Greater than 1st or 2nd IE.Greater than 1st or 2nd IE.

Ionization EnergyIonization Energy

How many valance electrons does Magnesium have? How many is it willing to give up?

First Ionization EnergyFirst Ionization Energy

Second Ionization EnergySecond Ionization Energy

Third ionization EnergyThird ionization Energy

Mg3+

1s22s22p5

What Determines IEWhat Determines IE The greater the nuclear charge, the The greater the nuclear charge, the

greater IE.greater IE. Greater distance from nucleus Greater distance from nucleus

decreases IEdecreases IE Filled and half-filled sublevel have Filled and half-filled sublevel have

lower energy, so achieving them is lower energy, so achieving them is easier, lower IE.easier, lower IE.

Shielding effect on valence electronsShielding effect on valence electrons

Group TrendsGroup Trends

As you go down a group, first IE As you go down a group, first IE decreases because...decreases because...

the valence electron is further awaythe valence electron is further away and there is more shielding.and there is more shielding.

Periodic TrendsPeriodic Trends As you go across a period, As you go across a period, All the atoms in the same period have the All the atoms in the same period have the

same highest energy level.same highest energy level. They have similar shielding.They have similar shielding. But, increasing nuclear charge reduces But, increasing nuclear charge reduces

atomic radius (bringing valence closer)atomic radius (bringing valence closer) so IE generally increases from left to right.so IE generally increases from left to right. Exceptions are at full and 1/2 full Exceptions are at full and 1/2 full

sublevels.sublevels.

Firs

t Ion

izat

ion

ener

gy

Atomic number

He

He has a greater He has a greater IE than H.IE than H.

same shielding same shielding greater nuclear greater nuclear

chargecharge

H

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He

Li has lower IE than H

more shielding further away outweighs greater

nuclear charge Li

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He

Be has higher IE than Li

same shielding greater nuclear

charge

Li

Be

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He B has lower IE

than Be same shielding greater nuclear

charge By removing an

electron we leave the s orbital full

Li

Be

B

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He

Li

Be

B

C

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He

Li

Be

B

C

N

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He

Li

Be

B

C

N

O

Breaks the Breaks the pattern, because pattern, because removing an removing an electron leaves electron leaves 1/2 filled p orbital1/2 filled p orbital

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He

Li

Be

B

C

N

O

F

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He

Li

Be

B

C

N

O

F

Ne

Ne has a lower Ne has a lower IE than HeIE than He

Both are full,Both are full, Ne has more Ne has more

shieldingshielding Greater Greater

distancedistance

Firs

t Ion

izat

ion

ener

gy

Atomic number

H

He

Li

Be

B

C

N

O

F

Ne Na has a lower

IE than Li Both are s1

Na has more shielding

Greater distance

Na

Firs

t Ion

izat

ion

ener

gy

Atomic number

Driving ForceDriving Force Full Energy Levels require lots of Full Energy Levels require lots of

energy to remove their electrons.energy to remove their electrons. Noble Gases have full energy level.Noble Gases have full energy level. Atoms behave in ways to achieve Atoms behave in ways to achieve

noble gas configuration.noble gas configuration.

Trends in Ionic SizeTrends in Ionic Size Cations form by Cations form by losinglosing electrons. electrons. Cations are smaller than the atom Cations are smaller than the atom

they come from.they come from. Metals form cations.Metals form cations. Cations of representative elements Cations of representative elements

have noble gas configuration.have noble gas configuration.

Na Na+1

Ionic SizeIonic Size Anions form by Anions form by gaininggaining electrons. electrons. Anions are bigger than the atom they Anions are bigger than the atom they

come from.come from. Nonmetals form anions.Nonmetals form anions. Anions of representative elements Anions of representative elements

have noble gas configuration.have noble gas configuration.

Cl Cl-1

Ion Group TrendsIon Group Trends Adding energy levelAdding energy level Ions get bigger as you Ions get bigger as you

go down.go down.

Li1+

Na1+

K1+

Rb1+

Cs1+

Ion Periodic TrendsIon Periodic Trends Across the period, nuclear charge Across the period, nuclear charge

increases so they get smaller.increases so they get smaller. Energy level changes between anions Energy level changes between anions

and cations.and cations.

Li1+

Be2+

B3+

C4+

N3-O2- F1-

Ionic Radius TrendsIonic Radius Trends

Size of Isoelectronic IonsSize of Isoelectronic Ions Iso- means the sameIso- means the same Iso electronic ions have the same # of Iso electronic ions have the same # of

electronselectrons AlAl3+3+ Mg Mg2+ 2+ NaNa1+ 1+ Ne FNe F1- 1- OO2- 2- and Nand N3-3- all have 10 electronsall have 10 electrons all have the configuration: 1sall have the configuration: 1s222s2s222p2p66

Size of Isoelectronic IonsSize of Isoelectronic Ions Positive ions that have more protons Positive ions that have more protons

would be smaller.would be smaller.

Al3+

Mg2+

Na1+ Ne F1- O2- N3-

ElectronegativityElectronegativity is a measure of the ability of anis a measure of the ability of an

atom in a molecule to attracatom in a molecule to attract t

electrons to itself.electrons to itself.

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

Concept proposed byConcept proposed byLinus PaulingLinus Pauling1901-19941901-1994

ElectronegativityElectronegativity The tendency for an atom to The tendency for an atom to attract attract

electrons to itself when it iselectrons to itself when it is chemically chemically combined combined with another element.with another element.

How fair is the sharing?How fair is the sharing? Large electronegativity means the atom Large electronegativity means the atom

pulls the electrons toward it.pulls the electrons toward it. Atoms with large electron affinity have Atoms with large electron affinity have

larger electronegativity.larger electronegativity.

ElectronegativityElectronegativity Relative ability of atoms to attract electrons of bond.Relative ability of atoms to attract electrons of bond.

At1.9

I2.2

Br2.7

Cl2.8

Po1.8

Te2.0

Se2.5

S2.4

Bi1.7

Sb1.8

As2.2

P2.1

Pb1.5

Sn1.7

Ge2.0

Si1.7

F4.1

O3.5

N3.1

Tl1.4

Na1.0

Cs0.9

Rb0.9

K0.9

Ba1.0

Mg1.2

Sr1.0

Ca1.0

In1.5

Ga1.8

Al1.5

H2.2

Li1.0

Be1.5

B2.0

C2.5

ElectronegativityElectronegativity

Ionization Energy, Electronegativity, and Electron Affinity INCREASE

Atomic Size Increases

Ionic size increases