periodic trends 1

1

PERIODICITY AND ELECTRON PERIODICITY AND ELECTRON CONFIGURATIONSCONFIGURATIONS

2

Effective Nuclear Charge, Effective Nuclear Charge, Z*Z*

Effective Nuclear Charge, Effective Nuclear Charge, Z*Z*

• Z* is the nuclear charge experienced by Z* is the nuclear charge experienced by the outermost electrons.the outermost electrons.

• Explains why E(2s) < E(2p)Explains why E(2s) < E(2p)

• Z* increases across a period owing to Z* increases across a period owing to incomplete shielding by inner electrons.incomplete shielding by inner electrons.

• Estimate Z* by -->Estimate Z* by --> [ [ Z - (no. inner electrons) Z - (no. inner electrons) ]]

• Charge felt by 2s e- in Li Charge felt by 2s e- in Li Z* = 3 - 2 = 1 Z* = 3 - 2 = 1

• Be Be Z* = 4 - 2 = 2Z* = 4 - 2 = 2

• B B Z* = 5 - 2 = 3Z* = 5 - 2 = 3 and so on!and so on!

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Effective Effective Nuclear Nuclear ChargeCharge

Electron cloud for 1s electrons

Figure 8.6

4

EffectiveEffective Nuclear Charge, Z* Nuclear Charge, Z*

• Atom Z* Experienced by Electrons in Valence Orbitals

• Li +1.28

• Be -------

• B +2.58

• C +3.22

• N +3.85

• O +4.49

• F +5.13

Increase in Increase in Z* across a Z* across a periodperiod

5General Periodic General Periodic TrendsTrends

• Atomic and ionic sizeAtomic and ionic size

• Ionization energyIonization energy

• Electron affinityElectron affinity

Higher effective nuclear charge.Electrons held more tightly

Smaller orbitals.Electrons held moretightly.

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Atomic Atomic SizeSize

Atomic Atomic SizeSize

• Size goes UPSize goes UP on going on going down a group.down a group.

• Because electrons are Because electrons are added farther from the added farther from the nucleus, there is less nucleus, there is less attraction.attraction.

• Size goes DOWNSize goes DOWN on going on going across a period.across a period.

• Size goes UPSize goes UP on going on going down a group.down a group.

• Because electrons are Because electrons are added farther from the added farther from the nucleus, there is less nucleus, there is less attraction.attraction.

• Size goes DOWNSize goes DOWN on going on going across a period.across a period.

7Atomic Atomic RadiiRadii

Figure 8.9Figure 8.9

8

Trends in Atomic SizeTrends in Atomic SizeSee Figures 8.9 & 8.10See Figures 8.9 & 8.10

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40

Li

Na

K

Kr

He

NeAr

2nd period

3rd period 1st transitionseries

Radius (pm)

Atomic Number

0

50

100

150

200

250

0 5 10 15 20 25 30 35 40

Li

Na

K

Kr

He

NeAr

2nd period

3rd period 1st transitionseries

Radius (pm)

Atomic Number

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Ion SizesIon SizesIon SizesIon Sizes

Li,152 pm3e and 3p

Li+, 60 pm2e and 3 p

+Does the size goDoes the size goup or down up or down when losing an when losing an electron to form electron to form a cation?a cation?

Does the size goDoes the size goup or down up or down when losing an when losing an electron to form electron to form a cation?a cation?

Idea Check! What do you think?

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• CATIONS are SMALLER than the CATIONS are SMALLER than the atoms from which they come.atoms from which they come.• The electron/proton attraction The electron/proton attraction

has gone UP and so size has gone UP and so size DECREASES.DECREASES.

Li,152 pm3e and 3p

Li +, 78 pm2e and 3 p

+Forming Forming a cation.a cation.Forming Forming a cation.a cation.

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F,64 pm9e and 9p

F- , 136 pm10 e and 9 p

-Does the size go up or Does the size go up or down when gaining an down when gaining an electron to form an electron to form an anion?anion?

Does the size go up or Does the size go up or down when gaining an down when gaining an electron to form an electron to form an anion?anion?

Idea Check!

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• ANIONS are LARGER than the atoms ANIONS are LARGER than the atoms from which they come.from which they come.

• The electron/proton attraction has The electron/proton attraction has gone DOWN and so size INCREASES.gone DOWN and so size INCREASES.

• Trends in ion sizes are the same as Trends in ion sizes are the same as atom sizes. atom sizes.

Forming Forming an anion.an anion.Forming Forming an anion.an anion.F, 71 pm

9e and 9pF-, 133 pm10 e and 9 p

-

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Trends in Ion SizesTrends in Ion Sizes

Figure 8.13Figure 8.13

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Redox Reactions

Redox Reactions

Why do metals lose Why do metals lose

electrons in their electrons in their

reactions? reactions?

Why does Mg form MgWhy does Mg form Mg2+2+

ions and not Mgions and not Mg3+3+??

Why do nonmetals take Why do nonmetals take

on electrons?on electrons?

Why do metals lose Why do metals lose

electrons in their electrons in their

reactions? reactions?

Why does Mg form MgWhy does Mg form Mg2+2+

ions and not Mgions and not Mg3+3+??

Why do nonmetals take Why do nonmetals take

on electrons?on electrons?

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Ionization EnergyIonization EnergySee Screen 8.12See Screen 8.12


IE = energy required to remove an electron IE = energy required to remove an electron from an atom in the gas phase.from an atom in the gas phase.

Mg (g) + 738 kJ ---> MgMg (g) + 738 kJ ---> Mg++ (g) + e- (g) + e-

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Mg (g) + 735 kJ ---> MgMg (g) + 735 kJ ---> Mg++ (g) + e- (g) + e-

MgMg+ + (g) + 1451 kJ ---> Mg(g) + 1451 kJ ---> Mg2+2+ (g) + e- (g) + e-

MgMg2+2+ (g) + 7733 kJ ---> Mg (g) + 7733 kJ ---> Mg3+3+ (g) + e- (g) + e-

Energy cost is very high to dip into a Energy cost is very high to dip into a shell of lower n. shell of lower n. This is why ox. no. = Group no.This is why ox. no. = Group no.



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Trends in Ionization EnergyTrends in Ionization Energy

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 350

500

1000

1500

2000

2500

1st Ionization energy (kJ/mol)

Atomic NumberH Li Na K

HeNe

ArKr

18Trends in Ionization Trends in Ionization

EnergyEnergyTrends in Ionization Trends in Ionization

EnergyEnergy

• IE increases across a period IE increases across a period because Z* increases.because Z* increases.

• Metals lose electrons more Metals lose electrons more easily than nonmetals.easily than nonmetals.

• Metals are good reducing Metals are good reducing agents.agents.

• Nonmetals lose electrons with Nonmetals lose electrons with difficulty.difficulty.

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Trends in Ionization Trends in Ionization EnergyEnergy

Trends in Ionization Trends in Ionization EnergyEnergy

• IE decreases down a group IE decreases down a group

• Because size increases.Because size increases.

• Reducing ability generally Reducing ability generally increases down the periodic increases down the periodic table. table.

• See reactions of Li, Na, KSee reactions of Li, Na, K

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Electron AffinityElectron Affinity

A few elements GAIN electrons to A few elements GAIN electrons to form anions.form anions.

Electron affinity is the energy Electron affinity is the energy change when an electron is added:change when an electron is added:

A(g) + e- ---> AA(g) + e- ---> A--(g) E.A. = ∆E(g) E.A. = ∆E

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Electron Affinity of OxygenElectron Affinity of Oxygen

∆∆E is E is EXOEXOthermic thermic because O has because O has an affinity for an an affinity for an e-.e-.

[He] O atom

EA = - 141 kJ

+ electron

O [He] - ion

22

Electron Affinity of Electron Affinity of NitrogenNitrogen

∆∆E is E is zero zero for Nfor N- -

due to electron-due to electron-electron electron repulsions.repulsions.

EA = 0 kJ

[He] N atom

[He] N- ion

+ electron

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• Affinity for electron Affinity for electron increases across a increases across a period (EA becomes period (EA becomes more negative).more negative).

• Affinity decreases down Affinity decreases down a group (EA becomes a group (EA becomes less negative).less negative).

Atom EAAtom EAFF -328 kJ-328 kJClCl -349 kJ-349 kJBrBr -325 kJ-325 kJII -295 kJ-295 kJ

Atom EAAtom EAFF -328 kJ-328 kJClCl -349 kJ-349 kJBrBr -325 kJ-325 kJII -295 kJ-295 kJ

Trends in Electron AffinityTrends in Electron Affinity

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Trends in Electron AffinityTrends in Electron Affinity

periodic trends 1

Technology

electronelectron repulsions

electron o

electron configurations

kj trends

size increases

atomic size size

e energy cost

atom z